February 6, 2001 { 10 : 40
DRAFT
1
The BABAR Collaboration
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
B. Aubert, A. Bazan, A. Boucham, D. Boutigny, I. De Bonis, J. Favier, J.-M. Gaillard, A. Jeremie,
Y. Karyotakis, T. Le Flour, J. P. Lees, S. Lieunard, P. Petitpas, P. Robbe, V. Tisserand, K. Zachariadou
Laboratoire de Physique des Particules, F-74941 Annecy-le-Vieux, CEDEX, France
A. Palano
Universita di Bari, Dipartimento di Fisica and INFN, I-70126 Bari, Italy
G. P. Chen, J. C. Chen, N. D. Qi, G. Rong, P. Wang, Y. S. Zhu
Institute of High Energy Physics, Beijing 100039, China
G. Eigen, P. L. Reinertsen, B. Stugu
University of Bergen, N-5007 Bergen, Norway
B. Abbott, G. S. Abrams, L. Amerman, A. W. Borgland, A. B. Breon, D. N. Brown, J. Button-Shafer,
A. R. Clark, S. Dardin, C. Day, S. F. Dow, Q. Fan, I. Gaponenko, M. S. Gill, F. R. Goozen, S. J. Gowdy,
Y. Groysman, C. Hernikl, R. G. Jacobsen, R. C. Jared, R. W. Kadel, J. Kadyk, A. Karcher, L. T. Kerth,
I. Kipnis, S. Kluth, J. F. Kral, R. Lafever, C. LeClerc, M. E. Levi, S. A. Lewis, C. Lionberger, T. Liu, M. Long,
L. Luo, G. Lynch, P. Luft, E. Mandelli, M. Marino, K. Marks, C. Matuk, A. B. Meyer, R. Minor, A. Mokhtarani,
M. Momayezi, M. Nyman, P. J. Oddone, J. Ohnemus, D. Oshatz, S. Patton, M. Pedrali-Noy, A. Perazzo,
C. Peters, W. Pope, M. Pripstein, D. R. Quarrie, J. E. Rasson, N. A. Roe, A. Romosan, M. T. Ronan,
V. G. Shelkov, R. Stone, P. D. Strother,1 A. V. Telnov, H. von der Lippe, T. F. Weber, W. A. Wenzel, G. Zizka
Lawrence Berkeley National Laboratory and University of California, Berkeley, CA 94720, USA
P. G. Bright-Thomas, C. M. Hawkes, A. Kirk, D. J. Knowles, S. W. O'Neale, A. T. Watson, N. K. Watson
University of Birmingham, Birmingham, B15 2TT, UK
T. Deppermann, H. Koch, J. Krug, M. Kunze, B. Lewandowski, K. Peters, H. Schmuecker, M. Steinke
Ruhr Universitat Bochum, Inst. f. Experimentalphysik 1, D-44780 Bochum, Germany
J. C. Andress, N. R. Barlow, W. Bhimji, N. Chevalier, P. J. Clark, W. N. Cottingham, N. De Groot, N. Dyce,
B. Foster, A. Mass, J. D. McFall, D. Wallom, F. F. Wilson
University of Bristol, Bristol BS8 1TH, UK
K. Abe, C. Hearty, J. A. McKenna, D. Thiessen
University of British Columbia, Vancouver, BC, Canada V6T 1Z1
B. Camanzi, T. J. Harrison,2 A. K. McKemey, J. Tinslay
Brunel University, Uxbridge, Middlesex UB8 3PH, UK
E. I. Antohin, V. E. Blinov, A. D. Bukin, D. A. Bukin, A. R. Buzykaev, M. S. Dubrovin, V. B. Golubev,
V. N. Ivanchenko, G. M. Kolachev, A. A. Korol, E. A. Kravchenko, S. F. Mikhailov, A. P. Onuchin,
A. A. Salnikov, S. I. Serednyakov, Yu. I. Skovpen, V. I. Telnov, A. N. Yushkov
Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
J. Booth, A. J. Lankford, M. Mandelkern, S. Pier, D. P. Stoker
University of California at Irvine, Irvine, CA 92697, USA
1 Now at Queen Mary, University of London, London, E1 4NS, UK
2 Now at University of Birmingham, Birmingham, B15 2TT, UK
2
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
DRAFT
February 6, 2001 { 10 : 40
A. Ahsan, K. Arisaka, C. Buchanan, S. Chun
University of California at Los Angeles, Los Angeles, CA 90024, USA
J. G. Branson, R. Faccini,3 D. B. MacFarlane, S. A. Prell, Sh. Rahatlou, G. Raven, V. Sharma
University of California at San Diego, La Jolla, CA 92093, USA
S. Burke, D. Callahan, C. Campagnari, B. Dahmes, D. Hale, P. A. Hart, N. Kuznetsova, S. Kyre, S. L. Levy,
O. Long, A. Lu, J. May, J. D. Richman, W. Verkerke, M. Witherell, S. Yellin
University of California at Santa Barbara, Santa Barbara, CA 93106, USA
J. Beringer, J. DeWitt, D. E. Dorfan, A. M. Eisner, A. Frey, A. A. Grillo, M. Grothe, C. A. Heusch,
R. P. Johnson, W. Kroeger, W. S. Lockman, T. Pulliam, W. Rowe, H. Sadrozinski, T. Schalk, R. E. Schmitz,
B. A. Schumm, A. Seiden, E. N. Spencer, M. Turri, W. Walkowiak, M. Wilder, D. C. Williams
University of California at Santa Cruz, Santa Cruz, CA 95064, USA
E. Chen, G. P. Dubois-Felsmann, A. Dvoretskii, J. E. Hanson, D. G. Hitlin, Yu. G. Kolomensky,4 S. Metzler,
J. Oyang, F. C. Porter, A. Ryd, A. Samuel, M. Weaver, S. Yang, R. Y. Zhu
California Institute of Technology, Pasadena, CA 91125, USA
S. Devmal, T. L. Geld, S. Jayatilleke, S. M. Jayatilleke, G. Mancinelli, B. T. Meadows, M. D. Sokolo
University of Cincinnati, Cincinnati, OH 45221, USA
P. Bloom, B. Broomer, E. Erdos, S. Fahey, W. T. Ford, F. Gaede, W. C. van Hoek, D. R. Johnson,
A. K. Michael, U. Nauenberg, A. Olivas, H. Park, P. Rankin, J. Roy, S. Sen, J. G. Smith, D. L. Wagner
University of Colorado, Boulder, CO 80309, USA
J. Blouw, J. L. Harton, M. Krishnamurthy, A. So er, W. H. Toki, D. W. Warner, R. J. Wilson, J. Zhang
Colorado State University, Fort Collins, CO 80523, USA
T. Brandt, J. Brose, G. Dahlinger, M. Dickopp, R. S. Dubitzky, P. Eckstein, H. Futterschneider, M. L. Kocian,
R. Krause, R. Muller-Pfe erkorn, K. R. Schubert, R. Schwierz, B. Spaan, L. Wilden
Technische Universitat Dresden, D-01062 Dresden, Germany
L. Behr, D. Bernard, G. R. Bonneaud, F. Brochard, J. Cohen-Tanugi, S. Ferrag, G. Fouque, F. Gastaldi,
P. Matricon, P. Mora de Freitas, C. Renard, E. Roussot, S. T'Jampens, C. Thiebaux, G. Vasileiadis, M. Verderi
Ecole Polytechnique, F-91128 Palaiseau, France
A. Anjomshoaa, R. Bernet, F. Di Lodovico, F. Muheim, S. Playfer, J. E. Swain
University of Edinburgh, Edinburgh EH9 3JZ, UK
M. Falbo
Elon College, Elon College, NC 27244
C. Bozzi, S. Dittongo, M. Folegani, L. Piemontese, A. .C. Ramusino
Universita di Ferrara, Dipartimento di Fisica and INFN, I-44100 Ferrara, Italy
3 Jointly appointed with Universita di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy
4 Now at LBNL and University of California, Berkeley, CA 94720, USA
February 6, 2001 { 10 : 40
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
DRAFT
3
E. Treadwell
Florida A&M University, Tallahassee, FL 32307, USA
F. Anulli,5 R. Baldini-Ferroli, A. Calcaterra, R. de Sangro, D. Falciai, G. Finocchiaro, P. Patteri, I. M. Peruzzi,5
M. Piccolo, Y. Xie, A. Zallo
Laboratori Nazionali di Frascati dell'INFN, I-00044 Frascati, Italy
S. Bagnasco, A. Buzzo, R. Contri, G. Crosetti, P. Fabbricatore, S. Farinon, M. Lo Vetere, M. Macri, S. Minutoli,
M. R. Monge, R. Musenich, M. Pallavicini, R. Parodi, S. Passaggio, F. C. Pastore, C. Patrignani, M. G. Pia,
C. Priano, E. Robutti, A. Santroni
Universita di Genova, Dipartimento di Fisica and INFN, I-16146 Genova, Italy
R. Bartoldus, T. Dignan, R. Hamilton, U. Mallik
University of Iowa, Iowa City, IA 52242, USA
J. Cochran, H. B. Crawley, P.-A. Fischer, J. Lamsa, R. McKay, W. T. Meyer, E. I. Rosenberg
Iowa State University, Ames, IA 50011-3160, USA
J. N. Albert, C. Beigbeder, M. Benkebil, D. Breton, R. Cizeron, S. Du, G. Grosdidier, C. Hast, A. Hocker,
V. LePeltier, A. M. Lutz, S. Plaszczynski, M. H. Schune, S. Trincaz-Duvoid, K. Truong, A. Valassi, G. Wormser
Laboratoire de l'Accelerateur Lineaire, F-91898 Orsay Cedex, France
O. Alford, D. Behne, R. M. Bionta, J. Bowman, V. Brigljevic, A. Brooks, V. A. Dacosta, O. Fackler, D. Fujino,
M. Harper, D. J. Lange, M. Mugge, T. G. O'Connor, H. Olson, L. Ott, E. Parker, B. Pedrotti, M. Roeben,
X. Shi, K. van Bibber, T. J. Wenaus, D. M. Wright, C. R. Wuest, B. Yamamoto
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
M. Carroll, P. Cooke, J. R. Fry, E. Gabathuler, R. Gamet, M. George, M. Kay, S. McMahon,6 A. Muir,
D. J. Payne, R. J. Sloane, P. Sutcli e, C. Touramanis, G. Zioulas
University of Liverpool, Liverpool L69 3BX, UK
M. L. Aspinwall, D. A. Bowerman, P. D. Dauncey, I. Eschrich, N. J. W. Gunawardane, R. Martin, J. A. Nash,
D. R. Price, P. Sanders, D. Smith
University of London, Imperial College, London, SW7 2BW, UK
D. E. Azzopardi, J. J. Back, P. Dixon, P. F. Harrison, D. Newman-Coburn,7 R. J. L. Potter, H. W. Shorthouse,
M. I. Williams, P. B. Vidal
Queen Mary, University of London, London, E1 4NS, UK
G. Cowan, S. George, M. G. Green, A. Kurup, C. E. Marker, P. McGrath, T. R. McMahon, F. Salvatore,
I. Scott, G. Vaitsas
University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, UK
D. Brown, C. L. Davis, Y. Li, J. Pavlovich
University of Louisville, Louisville, KY 40292, USA
5 Jointly appointed with Univ. di Perugia, I-06100 Perugia, Italy
6 Now at University of California at Irvine, Irvine, CA 92697, USA
7 Deceased
4
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
DRAFT
February 6, 2001 { 10 : 40
J. Allison, R. J. Barlow, J. T. Boyd, J. Fullwood, F. Jackson, A. Khan,8 G. D. La erty, N. Savvas,
E. T. Simopoulos, R. J. Thompson, J. H. Weatherall
University of Manchester, Manchester M13 9PL, UK
R. Bard, C. Dallapiccola,9 A. Farbin, A. Jawahery, V. Lillard, J. Olsen, D. A. Roberts, J. R. Schieck
University of Maryland, College Park, MD 20742, USA
G. Blaylock, K. T. Flood, S. S. Hertzbach, R. Ko er, C. S. Lin, S. Willocq, J. Wittlin
University of Massachusetts, Amherst, MA 01003, USA
B. Brau, R. Cowan, F. Taylor, R. K. Yamamoto
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
D. I. Britton, R. Fernholz,10 M. Houde, M. Milek, P. M. Patel, J. Trischuk
McGill University, Montreal, QC, Canada H3A 2T8
F. Lanni, F. Palombo
Universita di Milano, Dipartimento di Fisica and INFN, I-20133 Milano, Italy
J. M. Bauer, M. Booke, L. Cremaldi, R. Kroeger, M. Reep, J. Reidy, D. A. Sanders, D. J. Summers
University of Mississippi, University, MS 38677, USA
J. F. Arguin, M. Beaulieu, J. P. Martin, J. Y. Nief, R. Seitz, P. Taras, A. Woch, V. Zacek
Universite de Montreal, Montreal, QC, Canada, H3C 3J7
H. Nicholson, C. S. Sutton
Mount Holyoke College, South Hadley, MA 01075, USA
N. Cavallo, G. De Nardo, F. Fabozzi, C. Gatto, L. Lista, D. Piccolo, C. Sciacca
Universita di Napoli Federico II, Dipartimento di Fisica and INFN, I-80126 Napoli, Italy
N. M. Cason, J. M. LoSecco
University of Notre Dame, Notre Dame, IN 46556, USA
J. R. G. Alsmiller, T. A. Gabriel, T. Handler, J. Heck
Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
M. Iwasaki, N. B. Sinev,
University of Oregon, Eugene, OR 97403, USA
R. Caracciolo, F. Colecchia, F. Dal Corso, F. Galeazzi, M. Marzolla, G. Michelon, M. Morandin, M. Posocco,
M. Rotondo, S. Santi, F. Simonetto, R. Stroili, E. Torassa, C. Voci
Universita di Padova, Dipartimento di Fisica and INFN, I-35131 Padova, Italy
8 Now at University of Edinburgh, Edinburgh EH9 3JZ, UK
9 Now at University of Massachusetts, Amherst, MA 01003, USA
10 Now at Princeton University, Princeton, NJ 08544, USA
February 6, 2001 { 10 : 40
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
DRAFT
5
P. Bailly, M. Benayoun, H. Briand, J. Chauveau, P. David, C. De la Vaissiere, L. Del Buono, J.-F. Genat,
O. Hamon, Ph. Leruste, F. Le Diberder, H. Lebbolo, J. Lory, L. Martin, F. Martinez-Vidal,11 L. Roos, J. Stark,
S. Versille, B. Zhang
Universite Paris VI et VII, F-75252 Paris, Cedex 05, France
P. F. Manfredi, L. Ratti, V. Re, V. Speziali
Universita di Pavia, Dipartimento di Fisica and INFN, I-27100 Pavia, Italy
E. D. Frank, L. Gladney, Q. H. Guo, J. H. Panetta
University of Pennsylvania, Philadelphia, PA 19104, USA
C. Angelini, G. Batignani, S. Bettarini, M. Bondioli, F. Bosi, M. Carpinelli, F. Forti, D. Gagliardi, M. A. Giorgi,
A. Lusiani, P. Mammini, M. Morganti, F. Morsani, N. Neri, A. Profeti, E. Paoloni, M. Rama, G. Rizzo,
F. Sandrelli, G. Simi, G. Triggiani
Universita di Pisa, Scuola Normale Superiore, and INFN, I-56010 Pisa, Italy
M. Haire, D. Judd, K. Paick, L. Turnbull, D. E. Wagoner
Prairie View A&M University, Prairie View, TX 77446, USA
J. Albert, C. Bula, M. H. Kelsey, C. Lu, K. T. McDonald, V. Miftakov, B. Sands, S. F. Scha ner,
A. J. S. Smith, A. Tumanov, E. W. Varnes
Princeton University, Princeton, NJ 08544, USA
F. Bronzini, A. Buccheri, C. Bulfon, G. Cavoto, D. del Re, F. Ferrarotto, F. Ferroni, K. Fratini, E. Lamanna,
E. Leonardi, M. A. Mazzoni, S. Morganti, G. Piredda, F. Safai Tehrani, M. Serra, C. Voena
Universita di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy
R. Waldi
Universitat Rostock, D-18051 Rostock, Germany
P. F. Jacques, M. Kalelkar, R. J. Plano
Rutgers University, New Brunswick, NJ 08903, USA
T. Adye, B. Claxton, J. Dowdell, U. Egede, B. Franek, S. Galagedera, N. I. Geddes, G. P. Gopal, J. Kay,12
J. Lidbury, S. Madani, S. Metcalfe,12 13 G. Markey,12 P. Olley, M. Watt, S. M. Xella
Rutherford Appleton Laboratory, Didcot, Oxon., OX11 0QX, UK
;
R. Aleksan, P. Besson,7 P. Bourgeois, P. Convert, G. De Domenico, A. de Lesquen, S. Emery, A. Gaidot,
S. F. Ganzhur, Z. Georgette, L. Gosset, P. Gran, G. Hamel de Monchenault, S. Herve, M. Karolak,
W. Kozanecki, M. Langer, G. W. London, V. Marques, B. Mayer, P. Micout, J. P. Mols, J. P. Mouly,
Y. Penichot, J. Rolquin, B. Serfass, J. C. Toussaint, M. Usseglio, G. Vasseur, C. Yeche, M. Zito
DAPNIA, Commissariat a l'Energie Atomique/Saclay, F-91191 Gif-sur-Yvette, France
N. Copty, M. V. Purohit, F. X. Yumiceva
University of South Carolina, Columbia, SC 29208, USA
11 Now at Universita di Pisa, I-56010 Pisa, Italy
12 At CLRC Daresbury Laboratory, Daresbury, Warrington, Cheshire, WA4 4AD, UK
13 Now at Stanford Linear Accelerator Center, Stanford, CA 94309, USA
6
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
DRAFT
February 6, 2001 { 10 : 40
I. Adam, A. Adesanya, P. L. Anthony, D. Aston, J. Bartelt, J. Becla, R. Bell, E. Bloom, C. T. Boeheim,
A. M. Boyarski, R. F. Boyce, D. Briggs, F. Bulos, W. Burgess, B. Byers, G. Calderini, R. Chestnut, R. Claus,
M. R. Convery, R. Coombes, L. Cottrell, D. P. Coupal, D. H. Coward, W. W. Craddock, S. DeBarger,
H. DeStaebler, J. Dorfan, M. Doser, W. Dunwoodie, J. E. Dusatko, S. Ecklund, T. H. Fieguth, D. R. Freytag,
T. Glanzman, G. L. Godfrey, G. Haller, A. Hanushevsky, J. Harris, A. Hasan, C. Hee, T. Himel, M. E. Hu er,
T. Hung, W. R. Innes, C. P. Jessop, H. Kawahara, L. Keller, M. E. King, L. Klaisner, H. J. Krebs,
U. Langenegger, W. Langeveld, D. W. G. S. Leith, S. K. Louie, S. Luitz, V. Luth, H. L. Lynch, J. McDonald,
G. Manzin, H. Marsiske, T. Mattison,14 M. McCulloch, M. McDougald, D. McShurley, S. Menke, R. Messner,
S. Metcalfe, M. Morii,15 R. Mount, D. R. Muller, D. Nelson, M. Norby, C. P. O'Grady, L. Olavson, J. Olsen,
F. G. O'Neill, G. Oxoby, P. Paolucci,16 T. Pavel, J. Perl, M. Pertsova, S. Petrak, G. Putallaz, P. E. Raines,
B. N. Ratcli , R. Reif, S. H. Robertson, L. S. Rochester, A. Roodman, J. J. Russel, L. Sapozhnikov,
O. H. Saxton, T. Schietinger, R. H. Schindler, J. Schwiening, G.Sciolla,17 J. T. Seeman, V. V. Serbo, S. Shapiro,
K. Skarpass Sr., A. Snyder, E. Soderstrom, A. Soha, S. M. Spanier, A. Stahl, P. Stiles, D. Su, M. K. Sullivan,
M. Talby, H. A. Tanaka, J. Va'vra, S. R. Wagner, R. Wang, T. Weber, A. J. R. Weinstein, J. L. White,
U. Wienands, W. J. Wisniewski, C. C. Young, N. Yu
Stanford Linear Accelerator Center, Stanford, CA 94309, USA
P. R. Burchat, C. H. Cheng, D. Kirkby, T. I. Meyer, C. Roat
Stanford University, Stanford, CA 94305-4060, USA
R. Henderson, N. Khan
TRIUMF, Vancouver, BC, Canada V6T 2A3
S. Berridge, W. Bugg, H. Cohn, E. Hart, A. W. Weidemann
University of Tennessee, Knoxville, TN 37996, USA
T. Benninger, J. M. Izen, I. Kitayama, X. C. Lou, M. Turcotte
University of Texas at Dallas, Richardson, TX 75083, USA
F. Bianchi, M. Bona, F. Daudo, B. Di Girolamo, D. Gamba, P. Grosso, A. Smol, P. .P. Trapani, D. Zanin
Universita di Torino, Dipartimento di Fisica and INFN, I-10125 Torino, Italy
L. Bosisio, G. Della Ricca, L. Lanceri, A. Pompili, P. Poropat, M. Prest, E. Vallazza, G. Vuagnin
Universita di Trieste, Dipartimento di Fisica and INFN, I-34127 Trieste, Italy
R. S. Panvini
Vanderbilt University, Nashville, TN 37235, USA
A. De Silva,18 R. Kowalewski, J. M. Roney
University of Victoria, Victoria, BC, Canada V8W 3P6
H. R. Band, E. Charles, S. Dasu, P. Elmer, J. R. Johnson, J. Nielsen, W. Orejudos, Y. Pan, R. Prepost,
I. J. Scott, J. Walsh,11 S. L. Wu, Z. Yu, H. Zobernig
University of Wisconsin, Madison, WI 53706, USA
14 Now at University of British Columbia, Vancouver, BC, Canada V6T 1Z1
15 Now at Harvard University, Cambridge, MA 02138
16 Now at Universita di Napoli Federico II, I-80126 Napoli, Italy
17 Now at Massachusetts Institute of Technology, Cambridge, MA 02139, USA
18 Now at TRIUMF, Vancouver, BC, Canada V6T 2A3
February 6, 2001 { 10 : 40
203
204
DRAFT
T. B. Moore, H. Neal
Yale University, New Haven, CT 06511, USA
7
8
DRAFT
February 6, 2001 { 10 : 40
The BABAR Detector
BABAR is the detector for the SLAC PEP-II asymmetric e+e; B Factory operating at the (4S ) resonance.
BABAR was designed to perform comprehensive studies of CP-violation in B-meson decays. The BABAR detector
has many subsystems con gured around a beryllium beam pipe. Charged particle tracks are measured in a multilayer silicon vertex tracker surrounded by a cylindrical wire drift chamber. Electromagnetic showers from electrons
and photons are detected in an array of CsI crystals located just inside the solenoidal coil of a superconducting
magnet. Muons and neutral hadrons are identi ed by arrays of resistive plate chambers inserted into gaps in the
iron ux return of the magnet. Charged hadrons are identi ed by dE=dx measurements in the tracking detectors
and in a quartz ring-imaging Cherenkov detector surrounding the drift chamber. The trigger, data acquisition
and data-monitoring systems are VME- and network-based, and controlled and operated by custom-designed
on-line software. Details of the design and layout of the detector components and their associated electronics and
software are presented together with performance data.
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
1. Introduction
The primary physics goal of the BABAR experiment is the systematic study of CP -violating
asymmetries in the decay of neutral B mesons
to CP eigenstates. Such measurements are designed to test the Standard Model prediction for
CP -violation based on the Cabibbo-KobayashiMaskawa mechanism [1]; but many other decay
modes of the B mesons, charged and neutral, permit searches for this and other sources of CP violation. The secondary goal is to perform precision measurements of decays of bottom and
charm mesons and leptons, and to search for
rare processes that become accessible with the
high luminosity of the PEP-II B Factory [2]. The
design of the detector is optimized for CP studies,
but it is also well suited for these other physics
studies. The scienti c goals of the BABAR experiment were rst presented in the Letter of Intent [3]; detailed physics studies have been documented in the BABAR Physics Book [4] and earlier
workshops [5].
The PEP-II B Factory is an asymmetric e+e;
collider designed to operate at a luminosity of
3 1033 cm;2 s;1 and beyond at a center-of-mass
energy of 10.58 GeV, the mass of the (4S ) resonance. This resonance decays exclusively to
B 0 B 0 and B + B ; pairs and thus provides an ideal
laboratory for the study of B mesons. In PEP-II,
the electron beam of 8.98 GeV collides head-on
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
with the positron beam of 3.12 GeV resulting in
a boost to the (4S ) resonance of
= 0:55.
This boost makes it possible to reconstruct the
decay vertices of the two B mesons and to determine their relative decay times. One can therefore measure the time dependence of decay rates.
The crucial test of CP invariance is a comparison
of these rates for B 0 and B 0 to a CP eigenstate.
Experimentally, this requires events in which one
B meson decays to a CP eigenstate and is fully
reconstructed and the other B meson is tagged
as B 0 or a B 0 by its decay products: a charged
lepton, a charged kaon, or a D decay.
The very small branching ratios of B mesons to
CP modes, typically 10;4 , the need for full reconstruction of nal states with two or more charged
particles and several 0 , plus the tagging of the
second neutral B , place stringent requirements on
the detector:
a large and uniform acceptance, in particular down to small polar angles relative to
the boost direction, to avoid particle losses;
excellent detection eciency for charged
particles down to 60 MeV=c and for photons to 20 MeV;
very good momentum resolution to separate
small signals from background;
excellent energy and angular resolution for
the detection of photons from 0 and radia-
February 6, 2001 { 10 : 40
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
DRAFT
tive B decays in the range from 20 MeV to
4 GeV;
very good vertex resolution, both transverse
and parallel to the beam;
identi cation of electrons and muons, primarily for the detection of semi-leptonic decays used to tag the B avor, and also for
the study of leptonic and rare decays;
identi cation of hadrons over a wide range
of momenta for B avor-tagging, for the
separation of pions from kaons in decay
modes such as B 0 ! K or B 0 !
+ ; , as well as in charm meson and decays;
a highly redundant, selective trigger system
so as to avoid signi cant signal losses and
systematic uncertainties;
low-noise electronics and a data-acquisition
system of high exibility and operational
stability;
high degree of reliability of components and
frequent monitoring and automated calibrations, plus control of the environmental conditions to ensure continuous and stable operation;
an online computing and network system
that can control, process, and store the expected high volume of data; and
detector components that can tolerate signi cant doses of radiation and operate under high-background conditions.
To reach the desired sensitivity for the most interesting measurements, data sets of order 108 B
mesons will be needed. For the peak cross section
at the (4S ) of 1.1 nb, this will require an integrated luminosity of order 100 fb;1 or three years
of reliable and highly ecient operation having a
detector of state-of-the art capabilities at a storage ring operating at design luminosity or above
with few interruptions.
In the following, a brief overview of the principal components of the detector, the trigger, the
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
9
data-acquisition, and the online computing and
control system is given. This is to be followed by
brief descriptions of the PEP-II interaction region, the beam characteristics, and the impact
of the beam generated background on the design
and operation of the detector components. The
following chapters contain detailed presentations
of the design, construction, and performance for
all of the systems that make up the BABAR detector.
REFERENCES
1. N. Cabibbo, Phys. Rev. Lett. 10 (1963) 531;
M. Kobayashi, T. Maskawa, Prog. Th. Phys.
49 (1973) 652.
2. PEP-II An Asymmetric B Factory, Conceptual Design Report, SLAC-418, LBL-5379
(1993).
3. The BABAR Collaboration, Letter of Intent for
the Study of CP Violation and Heavy Flavor
Physics at PEP-II, SLAC-443 (1994).
4. The BABAR Physics Book, Physics at
an Asymmetric B Factory, P.F. Harrison,
H.R. Quinn, Editors, SLAC-R-504, (1998).
5. The Physics Program of a High-Luminosity
Asymmetric B Factory at SLAC, SLAC-353
(1989); Proceedings of the Workshop on
Physics and Detector Issues for a High Luminosity Asymmetric B Factory at SLAC,
SLAC-373 (1991); Proceedings of B Factorys,
The State of the Art in Accelerators, Detectors and Physics, SLAC-400 (1992).
10
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
2. Detector Overview
The BABAR detector was designed and built by
a large, international team of scientists and engineers. Details of its original design are documented in the Technical Design Report [1], issued
in 1995.
Figure 1 shows a longitudinal section through
the detector center, and Figure 2 shows an endview with the principal dimensions. The detector is con gured around the PEP-II interaction
region. To maximize the acceptance, while taking into acount the kinematic boost chosen by
the asymmetric beam energies, the whole detector is o set by 0.37 m in the direction of the lower
energy beam. The inner detector consists of a
silicon vertex tracker, a drift chamber, a ringimaging Cherenkov detector, and a CsI calorimeter. These detector components are surrounded
by a superconducting solenoid that is designed
for a eld of 1.5 T. The iron ux return is instrumented for muon and neutral hadron detection.
All these components operate with good performance down to forward angles of 350 mrad and
backward angles of 400 mr. For general reference, forward and backward are de ned relative
to the higher energy beam. As indicated in the
two drawings, the right handed coordinate system is anchored on the main tracking system, the
drift chamber, with the z -axis coinciding with its
principal axis. This axis is o set relative to the
beam axis by about 20 mrad. The y-axis points
upward and the x-axis points away from the center of the PEP-II storage ring.
The detector is of relatively compact design,
its transverse dimension being constrained by the
elevation of the beam above the oor, 3.50 m.
The solenoid radius was chosen by balancing the
physics requirements and capabilities of the drift
chamber and calorimeter against the overall cost.
As in many similar systems, the calorimeter was
the most expensive single system and thus considerable e ort was made to minimize its total
volume without undue reduction in performance
of either the tracking system or the calorimeter. The geometric layout of the vertex detector is constrained by the space available between
the components of the PEP-II magnet system, a
February 6, 2001 { 10 : 40
Material / X0
337
DRAFT
1
10
EMC
-1
DRC
DCH
10
-2
SVT
-1 -0.8 -0.6-0.4-0.2 0 0.2 0.4 0.6 0.8 1
Cos(θ)
Figure 3. Amount of material surrounding the
interaction point.
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
pair of dipole magnets (B1) followed by a pair
of quadrupole magnets (Q1). These magnets are
made of SmCo; they extend to 21 cm on either side of the interaction point and the dipoles
limit the forward and backward acceptance of the
tracking system. The SVT and these two sets
of magnets are placed inside a long tube (4.5 m
long and 0.217 m inner diameter) that supports
the weight of these magnets. The central section
of the support tube is fabricated of carbon- bers
composite (CFC) with a thickness to 1.08 % of a
radiation length.
Since the average momentum of charged particles produced in B meson decay is less than
1 GeV=c, the errors on the measured track parameters are dominated by multiple Coulomb scattering, rather than the intrinsic spatial resolution of the detectors. Similarly, the detection efciency and energy resolution of low energy photons are severely impacted by material in front
of the calorimeter. Thus, special care has been
given to keep the material in the active volume
of the detector to a minimum. Figure 3 shows
the distribution of material in the various detector systems in units of radiation lengths. Speci -
Figure 1. BABAR detector elevation view.
6
K
,2558
'
6
6
(-*7*304;
)*9*(947
).7(
':(0.3,(4.1
2&,3*9.(8-.*1)
+47).7(
`
9IGRK
('('8)UUXJOTGZK9_YZKS
_
^
(7>4,*3.(
(-.23*>
S
/6
JKZKIZUX)2
.3897:2*39*)
+1:=7*9:73 .+7
'&77*1
.+7
*3)(&5
+47<&7) *3)51:,
K
8:5*7(43):(9.3,
(4.1
*1*(9742&,3*9.(
(&147.2*9*7
*2(
)7.+9(-&2'*7
)(-
8.1.(43;*79*=
97&(0*7
8;9
February 6, 2001 { 10 : 40
DRAFT
11
Figure 2. BABAR detector end view.
HZYF\F^XJHYNTS
`
).7(
^
)(-
S
8;9
('('8)UUXJOTGZK9_YZKS
_
9IGRK
DRAFT
,2558
*&79-6:&0*
.841&947
*&79-6:&0*
9.*)4<3
.+7(>1.3)7.(&175(X
*2(
8:5*7(43):(9.3,
(4.1
.+7'&77*1
12
February 6, 2001 { 10 : 40
February 6, 2001 { 10 : 40
DRAFT
13
14
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
DRAFT
cally, each curve indicates the material a particle
traverses before it reaches the rst active element
of a speci c detector system.
2.1. Detector Components
The charged particle tracking system is made
of two components, the Silicon Vertex Tracker
(SVT) and the larger cylindrical Drift Chamber
(DCH).
The SVT is composed of ve layers of doublesided silicon strip detectors that are assembled
from modules with read-out at each end, thus
avoiding inactive material in the acceptance volume. The inner three layers primarily provide
position and angle information for the measurement of the vertex position. They are mounted
as close to the water-cooled beryllium beam pipe
as practical, thus minimizing the impact of scattering in the material of the beam pipe on the
extrapolation to the vertex. The outer two layers layers are placed at much larger radii, thus
providing the coordinate and angle measurement
needed for linking SVT tracks to DCH tracks.
The principal purpose of the DCH is the momentum and angle measurement for charged particles. It also supplies information for the charged
particle trigger and a measurement of dE=dx for
particle identi cation. The drift chamber is of
compact design, with 40 layers of small, approximately hexagonal cells. Longitudinal information
is derived from wires placed at small angles to the
principal axis. By chosing low mass wires, and a
helium based gas mixture the multiple scattering
inside the drift chamber is kept to a minimum.
All of the read-out electronics is mounted on the
backward endplate of the chamber, minimizing
the amount of material in front of the calorimeter endcap.
The DIRC, the Detector of Internally Re ected
Cherenkov light, is a novel concept and technology employed primarily for the separation of pions and kaons from about 500 MeV=c to the kinematic limit of 4.5 GeV=c. Cherenkov light is produced in 4.9 m long bars of synthetic fused silica
of rectangular cross section, 1.7 cm 3.5 cm, and
transmitted by total internal re ection, while preserving the angle of emission, to an array of photomultipliers. This array forms the backward wall
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
February 6, 2001 { 10 : 40
of a donut shaped water tank that is located outside of the backward end of the magnet. Images
of the Cherenkov rings are reconstructed from the
location and time of arrival of the signals in the
photomultipliers.
The Electro-Magnetic Calorimeter (EMC) is
designed to detect electromagnetic showers with
excellent energy and angular resolution over the
full energy range from 20 MeV to 4 GeV. This
coverage will allow the detection of low energy
0 from B decays and higher energy photons and
electrons from electromagnetic, weak, or radiative
processes. The EMC is a nely segmented array
of projective geometry made of thallium doped
cesium iodide (CsI(Tl)) crystals. The crystals are
arranged in modules that are supported individually from an external support structure. This
structure is build from two units, a barrel and a
forward endcap. To obtain the desired resolution
the amount of material in front and in-between
the crystals is held to a minimum. The individual crystals are read-out by pairs of silicon PIN
diodes. Low noise analog circuits and frequent,
precise calibration of the electronics and energy
response over the full dynamic range are crucial
for the desired performance.
The Instrumented Flux Return (IFR) is designed to identify muons and to detect neutral
hadrons. For this purpose, the magnet ux return
steel in the barrel and the two endcaps is segmented into layers, increasing in thickness from
2 cm on the inside to 10 cm on the outside.
Between these steel absorbers, single gap Resistive Plate Chambers (RPC) are inserted which
detect streamers from ionizing particles via external capacitive read-out strips. There are 19
layers of RPCs in the barrel sectors and 18 layers
in the endcaps. Two additional cylindrical layers
of RPCs with four read-out planes are placed just
in front of the magnet cryostat to detect particles
exiting the EMC.
2.2. Electronics, Trigger, Data Acquisition
and On-line Computing
The electronics, trigger, data acquisition and
on-line computing represent a collection of tightly
coupled hardware and software systems. These
systems were designed to maximize the physics
February 6, 2001 { 10 : 40
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
DRAFT
data acceptance, maintainability and reliability
while minimizing complexity, deadtime, downtime and cost.
Front-End Electronics (FEE) assemblies are
physically located on the detector and consist of signal processing and digitization
electronics along with a standard digital
transport mechanism into the data acquisition system.
A robust and exible two-level trigger han-
dles the full accelerator collision rate, one
in hardware, Level 1 (L1), the other in software, Level 3 (L3). A provision is made for
an intermediate trigger (Level 2) should unexpected conditions require its implementation.
A data reduction, transport and event
building system, Online Data ow (ODF),
handles digitized data from the FEE
through the event building. This system
also includes hardware elements providing
fast control and timing signals.
A farm of commercial Unix processors and
associated software, Online Event Processing (OEP), provides the real-time environment within which complete events are processed by the L3 trigger algorithms, partial
event reconstruction is performed for monitoring, and event data is logged to an intermediate storage.
In a second farm of processors, Online
Prompt Reconstruction (OPR), completely
reconstructs on all physics events, performs monitoring and constants generation
in near real-time. Physics event data are
transferred to an object database and are
thus available for further analyses.
An Online Run Control (ORC) system han-
dles the logic for managing the state of
the detector systems, starting and stopping
runs, and performing calibrations. A user
interface provides operator access to detector operation.
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
15
A system to control and monitor the detec-
tor and its support systems, Online Detector Control (ODC), is based upon the Experimental Physics Industrial Control System (EPICS) toolbox. This system includes
communication links with the PEP-II accelerator complex. User interfaces provide
operator access and displays.
2.2.1. Electronics
A common electronics architecture is shared
by all BABAR detector subsystems; common vendors were chosen for many of its components such
as VME crates and power supplies. Event data
from the detector ows through the FEE circuitry, while monitoring and control signals are
handled by a separate, parallel system. All FEE
systems, listed in Table 1, are mounted directly
on the detector for performance reasons. This solution also minimizes the cable plant and avoids
noise pickup and grounding loops generated in
long analog signal cables. Electronics for all detector subsystems utilize standard BABAR interfaces to the global electronics and software.
Each FEE chain consists of an ampli er, digitizer, a trigger latency bu er for storing data
during the L1 trigger processing, and a derandomizing event bu er for storing data between
the L1 accept and and subsequent transfer to the
data acquisition system (Figure 4). Custom ICs
have been developed to perform the signal processing (Table 1). The digital L1 latency bu ers
function as xed length data pipelines and are
managed by common protocol signals generated
by the FCTS. All de-randomizing event bu ers
function as FIFOs capable of storing a xed number of events. During normal operation, analog
signal processing, digitization, and data readout
occur continuously and simultaneously.
Since many of the front-end circuits are totally
inaccessible or access requires signi cant downtime, stringent requirements were placed on reliability. Most components underwent comprehensive MTBF (mean time between failure) studies.
A burn-in procedure was followed for all circuits
prior to installation with the goal of minimizing
initial failure rates.
DRAFT
16
February 6, 2001 { 10 : 40
Table 1
Overview of the Front-End Electronics (PH=pulseheight, ASD=ampli er-shaper-disciminator)
System
Elements
PH Time
No.
Custom
(bits) (ns) Channels
ICs
SVT
Si Strips
4
150,000 ASD/storage
DCH
Wires
7,104 ASD
6
2
ADC/TDC
DIRC
PMTs
10,752 ASD
0.5
TDC/FIFO
EMC
PIN diodes
13,160 ampli er/shaper
17-18
6,580 ADC ranging chip
IFR
RPC Strips
1
51,584
AMP
Digitizer
L1 Latency
Buffer
Event
Buffer
612
613
614
615
to L1 trigger
L1 trigger accept
616
617
618
Figure 4. Front-End Electronics. Analog signals arrive from the left, proceed conditionally
through the indicated steps and are injected into
the remainder of the data acquisition system.
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
2.2.2. Trigger
The trigger system operates as a sequence of
two independent stages, the second conditional
upon the rst. The Level 1 trigger is performed
rst at the machine crossing rate. Its goal is to
suciently reduce that rate to a level acceptable
for the Level 3 software trigger which runs on a
farm of commercial processors.
The Level 1 trigger is optimized for simplicity
and speed. It consists of a pipelined hardware
processor. It is designed to provide an output
< 2 kHz. The L1 trigger selection
trigger rate of is based on a reduced data set from the DCH,
EMC and IFR. The maximum L1 response latency for a given collision is 12 s. Based on both
the complete event and L1 trigger information,
the Level 3 software algorithms select events of
interest stores them for processing. The L3 output rate is administratively limited to 120 Hz so
as to not overload the downstream storage and
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
processing capacity. The trigger architecture is
designed to accommodate a Level 2 should unexpected background conditions demand higher
performance in the future.
BABAR has no fast counters for triggering purposes, and bunch crossings are nearly continuous
at 4 ns. Dedicated Level 1 trigger processors receive data which is continuously clocked in from
the DCH, EMC and IFR detector subsystems.
These processors produce clocked outputs to the
fast control system at 30 MHz, the time granularity of resultant L1 Accept signals. The arrival
of an L1 Accept signal by the data acquisition
system causes a window of each subsystem's L1
Latency Bu er to be read out. This window size
varies from about 500 ns for the silicon vertex
tracker to only 4 ; 16 s wide for the calorimeter. Absolute timing information for the event,
i.e., associating an event with a particular beam
crossing, which is determined oine, is calculated
using timing distributions for drift chamber hits
within a track segment, waveforms from the electromagnetic calorimeter and accelerator timing
ducials.
2.2.3. Data Acquisition and Online Computing
The data acquisition and computing systems
are responsible for the transport of event data
from the detector FEE circuits all the way to
mass storage with a minimum of deadtime and is
shown schematically in Figure 5. These systems
interface with the trigger and enable specialized
February 6, 2001 { 10 : 40
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
DRAFT
modes of operation, such as calibration and testing. Other parts of these systems provide for the
control and monitoring of the detector and supporting facilities.
Hardware
The data acquisition system hardware consists
of VME crates, specialized VME-based processors called Readout Modules (ROMs) [reference],
the Fast Control and Timing System, a UNIX
processor farm, various server machines and an
Ethernet [3] network. A ROM consists of a commercial single board computer [4], event bu ers,
an interface to the FCTS, and a custom Personality Card which connects with the FEE circuits
via 1.2 Gbps ber optic cables. The ROM provides the standard interface between the detector speci c front-end electronics, the FCTS, and
the event builder. There are about 157 ROMs in
the system which are located in 19 physical VME
crates that represent 24 logical crates by virtue
of segmented backplanes. The FCTS system consists of a VME crate plus individual Fast Control
Distribution Modules in each of the data acquisition VME crates.
Detector monitoring and control is accomplished via a standard set of components, including a VME crate, a Motorola MVME177 singleboard computer, and various other VME modules [this needs to be expanded to a small list
of VME modules]. With the exception of the
solenoid magnet and gas systems, which have
their own specialized control and monitoring, all
other BABAR elements plug into this system.
The online computing system relies on a complex of workstation consoles and servers with
0.8 TBytes of attached storage, all interconnected
with switched 100 Mbps and 1 Gbps Ethernet
networks. Multiple Gbps Ethernet links connect
the experimental hall with the SLAC computing
center.
Online Data ow
The Online Data ow software connects, controls, and manages the ow of data in the data
acquisition hardware while incurring almost no
dead time. This code is divided between embed-
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
17
ded processors in the ROMs which run the VxWorks [5] real-time operating system and UNIX
processors running the Solaris operating system.
Data ow provides: a) con guration and read-out
of the FEE over ber links to the ROMs; b) data
transport, bu ering and event building from the
ROMs to the OEP farm; c) masking and prescaling of L1 triggers; and d) partitioning into multiple, independent data acquisition systems for
parallel calibrations and diagnostics. Additional
feature extraction (or FEX) code in the ROMs,
supplied by the detector system groups, extracts
physical signals from the raw data, performs gain
and pedestal correction, data sparsi cation, and
data formatting. Data from electronics calibrations are accumulated in the ROMs, the channel response functions evaluated, the results compared to reference data and subsequently applied
in feature extraction. Calibration data is stored
in a Conditions Database.
Online Event Processing
The On-line Event Processing receives and processes data from the DataFlow event builder on
each of the 32 UNIX processors. In particular,
OEP orchestrates the following tasks: a) L3 trigger algorithms; b) \fast monitoring" to assure
data quality; and c) logging the selected events
to disk. Logging involves a process to merge the
multiple data output streams to a single le.
Online Prompt Reconstruction
Bridging the online and oine worlds is
OPR [6]. This system reads raw data recorded to
disk by OEP and, operating on farm of 150 Unix
processors, selects physics events, performs complete reconstruction, performs a \rolling" calibration, collects extensive monitoring data for quality assurance and writes the result into an object
database. A \rolling" calibration is the generation of reconstruction constants during normal
event processing which are then applied to the
processing of subsequent data.
Online Detector and Run Control
The ODC system controls and extensively
monitors the electronics and environment of the
DRAFT
18
raw
detector
signals
BABAR
detector
processed
detector
signals
Front−End
Electronics
digital
event
data
VME Dataflow
Crates
Event Bldg
L3 Trigger
Monitoring
Intermediate
Event Store
L1 Accept, clocks
and trigger data
trigger
data
L1 Trigger
Processor
February 6, 2001 { 10 : 40
24
Fast Control
and Timing
trigger
lines
Figure 5. Data acquisition schematic.
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
detector and assures the safety of the detector.
Its implementation is based on EPICS [2], providing detector-wide standardization for the control
and monitoring, diagnostics and alarm handling.
ODC also provides communication with PEP-II
and the magnet control system. Monitoring data
are archived in the Ambient Database.
The ORC system ties together the various components of the online system and provides the operator with a single graphical interface to control
the detector operation. Complex con gurations
are stored in a Con guration Database ; keys to
the con guration used for any run is stored along
with the data. The Event, Ambient, Conditions
and Con guration Databases are implemented in
an object database [7].
REFERENCES
1. BABAR Technical Design Report, SLAC-R457 (1995)
2. EPICS
3. Any of the IEEE 802 family of standards for
computer networking, running at rates of 10
Mbps, 100 Mbps or 1000 Mbps.
4. PowerPC
5. VxWorks Real-time operating system (Tornado??), Wind River Systems, Inc., 500 Wind
River Way, Alameda, CA 94501 USA.
6. Francesco Safai Tehrani, "The Babar Prompt
Reconstruction Manager: A Real Life Ex-
762
763
764
765
766
767
ample of a Constructive Approach to Software Development.", submitted to Computer
Physics Communications
7. Objectivity/DB, Objectivity, Inc., 301B East
Evelyn Avenue, Mountain View, California,
94041, USA.
February 6, 2001 { 10 : 40
768
769
770
771
772
773
774
775
776
777
778
779
780
781
DRAFT
3. PEP II Storage Rings and Their Impact
on the BABAR Detector
3.1. PEP-II Storage Rings
PEP-II is an e+e; storage ring system designed
to operate at a center of mass (c.m.) energy
of 10.58 GeV, corresponding to the mass of the
(4S ) resonance. The parameters of these energy asymmetric storage rings are presented in
Table 2. PEP-II has surpassed its design luminosity, both in terms of the instantaneous and
the integrated daily luminosity, with signi cantly
fewer bunches than anticipated. A full description of the design and operational experience of
PEP-II has recently been published [1] [2].
Table 2
PEP-II beam parameters. Values are given both
for design and typical colliding beam operation
in the rst year. HER and LER refer to the high
energy e; and low energy e+ ring, respectively.
Lx, Ly , and Lz refer to the horizontal, vertical,
and longitudinal rms size of the luminous region.
Parameters
Design Typical
Energy HER/LER (GeV)
9.0/3.1 9.0/3.1
Current HER/LER (A)
0.75/2.15 0.7/1.3
# of bunches
1658
553-829
Bunch spacing (ns)
4.2
6.3-10.5
( m)
110
120
( m)
3.3
5.6
(mm)
9
9
Luminosity (1033 cm;2 s;1 )
3
2.5
Luminosity ( pb;1 /d)
135
120
Lx
Ly
Lz
782
783
784
785
786
787
788
789
790
791
792
793
794
PEP-II typically operates in a series of 40
minute lls during which the colliding beams
coast. At the end of each ll, it takes about three
minutes to replenish the beams for the next ll.
After a loss of the stored beams, it takes approximately 10-15 minutes to re ll the two beams.
BABAR divides the data into runs, de ned as periods of three hour duration or less during which
beam and detector conditions are judged to be
stable. While most of the data are recorded at
the peak of the (4S ) resonance, some 12% are
taken at a c.m. energy 40 MeV below to allow
for studies of the non-resonant background.
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
19
3.2. Impact of PEP-II on BABAR Layout
The large beam currents inherent to the high
luminosity of PEP-II and the necessity to separate closely-spaced bunches as close as possible to
the interaction point (IP), tightly couple the issues of detector design, interaction region layout,
and remediation of machine-induced background.
The bunches collide head-on and are separated
magnetically in the horizontal plane by a pair of
dipole magnets (B1), followed by a series of o set
quadrupoles. The tapered B1 dipoles, located at
21 cm on either side of the IP, and the Q1
quadrupoles are made of samarium-cobalt and
operate inside the eld of the BABAR solenoid,
while Q2, Q4, and Q5, located outside or in the
fringe eld of the solenoid, are standard iron magnets. The collision axis is o -set from the z -axis
of the BABAR detector by about 20 mrad in the
horizontal plane [3].
The interaction region is enclosed by a watercooled beam pipe of 27.9 mmouter radius, composed of two thin layers of beryllium (0.83 mm
and 0.53 mm) with a 1.48- mm water channel inbetween. To attenuate synchrotron radiation, the
inner surface of the pipe is covered with a thin
layer of gold (approximately 4 m). In addition,
the beam pipe is wrapped with 150 m of tantalum foil on either side of the IP, i.e., beyond
z = +10:1 cm and z = ;7:9 cm. The total
thickness of the central beam pipe section at normal incidence corresponds to 1.06% of a radiation
length.
The beam pipe, the permanent magnets and
the SVT are assembled and aligned and then enclosed in a 4.5 m-long support tube. This rigid
structure is inserted into the BABAR detector,
spanning the IP. The central section of this tube
is fabricated from carbon- ber epoxy composite
thus reducing its thickness to 1.08% of a radiation
length.
3.3. Luminosity, Beam Energies and Position
The beam parameters that are most critical for
the BABAR data analysis are the luminosity, the
energies of the two beams and the position and
size of the luminous region.
20
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
DRAFT
3.3.1. Luminosity
While PEP-II uses a high-rate luminosity monitor sampling radiative Bhabha scattering to provide a fast relative measurement of the luminosity
for operations, BABAR derives the absolute luminosity o -line from QED processes. The best result is obtained from + ; pairs. For a sample
of 1 fb;1 , the statistical error is 1.3%, compared
to a systematic error of 0.5% on the relative, and
1.5% on the absolute value of the luminosity. This
error is currently dominated by uncertainties in
the Monte Carlo generator and the simulation of
the detector. It is expected that with a better understanding of the eciency, the overall systematic error on the absolute value of the luminosity
can be reduced by a factor of two.
3.3.2. Beam Energies
During operation, the mean energies of the
two beams are calculated from the total magnetic bending strength (including the e ects of
o -axis quadrupole elds, steering magnets, and
wigglers) and the average deviations of the accelerating frequencies from their central values.
While the systematic uncertainty in the PEP-II
calculation of the absolute beam energies is estimated to be 5-10 MeV, the relative energy setting for each beam is accurate and stable to about
1 MeV. The energy spread of the LER and HER
is 2.3 MeV and 5.5 MeV, respectively.
To ensure that data are recorded close to the
peak of the (4S ) resonance, the observed ratio of
BB enriched hadronic events to lepton pair production is monitored on-line. At the peak of the
resonance, a 2.5% change in the BB production
rate corresponds to a 2 MeV change in the c.m.
energy, a value that is close to the tolerance to
which the energy of PEP-II can be held. However, a drop in the BB rate does not distinguish
between energy settings below or above the (4S )
peak. The sign of the energy change must be determined from other indicators. The best monitor and absolute calibration of the c.m. energy
is derived from the measured c.m. momentum
of fully reconstructed B mesons combined with
the known B meson mass. An absolute error of
1.1 MeV can be obtained for an integrated luminosity of 1 fb;1 . This error is limited by both
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
February 6, 2001 { 10 : 40
the knowledge of the B mass [4] and the detector
resolution.
The beam energies are necessary input for the
calculation of two kinematic variables that are
commonly used to separate signal from background in the analysis of exclusive B meson decays. These variables make optimum use of the
measured quantities and are largely uncorrelated.
They are chosen to be invariant under Lorentz
transformations and thus can be evaluated both
in the laboratory and c.m. frame.
The rst variable, E , can be expressed in a
Lorentz invariant form as
p
(1)
E = (2qB qi ; s)=2 s;
where qB = (EB ; ~pB ) and qi = (Ei ; p~i ) are the
Lorentz vectors representing the momentum of
the B candidate and of the initial state, qi =
qe+ + qe; . In the c.m. frame, E takes the familiar form
;
E = EB ; Ebeam
(2)
where EB is the reconstructed energy of the
equals half the c.m. energy.
B meson and Ebeam
The spread in E receives a sizable contribution
from the beam energy spread, but it is generally
dominated by detector resolution.
The second variable is the energy-substituted
mass, mES , de ned as mES2 = qB2 with the constraint E = 0. In the laboratory frame, this
quantity can be determined from the measured
three-momentum ~pB of the B candidate without explicit knowledge of the masses of the decay
products:
r1
mES = ( 2 s + ~pB~pi )2 =Ei2 ; p2B :
(3)
In the c.m. frame (p~i = 0) this variable takes the
familiar form
q 2 2
mES = Ebeam
; pB ;
(4)
where pB is the c.m. momentum of the B meson,
derived from the momenta of its decay products.
.
The B meson energy is substituted by Ebeam
The spread of mES is dominated by the spread
, E = 2:6 MeV which can be meain Ebeam
beam
sured from the mES distribution for samples of
fully reconstructed B meson decays.
February 6, 2001 { 10 : 40
931
932
933
934
935
936
937
938
939
940
941
942
DRAFT
3.3.3. Beam Direction
The direction of the beams relative to BABAR
is measured iteratively run-by-run using e+e; !
e+e; and e+ e; ! + ; events. The resultant
uncertainty in the direction of the boost from the
laboratory to the c.m. frame, ~, is about 1 mrad,
dominated by alignment errors. This translates
into an uncertainty of about 0.3 MeV in mES . ~
is consistent to within 1 mrad with the orientation of the elongated beam spot (see below). It is
stable to better than 1 mrad from one run to the
next.
977
978
979
980
981
982
983
984
985
986
987
988
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
3.3.4. Beam Size and Position
The size and position of the luminous region are
critical parameters for the decay-time-dependent
analyses and their values are monitored continuously on-line and o -line. The design values for
the size of the luminous region are presented in
Table 2. The vertical size is too small to be measured directly. It is inferred from the measured
luminosity, the horizontal size, and the beam currents; it varies typically by 1-2 m.
The transverse position, size and angles of the
luminous region relative to the BABAR coordinate
system are determined by analyzing the distribution of the distance of closest approach to the
z -axis of the tracks in well measured two-track
events as a function of the azimuth . The longitudinal parameters are derived from the longitudinal vertex distribution of the two tracks. A
combined t to 9 parameters (3 average coordinates, 3 widths, and 3 small angles) converges
readily, even after signi cant changes in the beam
position. The uncertainties in the average beam
position are of the order of a few m in the transverse plane and 100 m along the collision axis.
Run-by-run variations in the beam position are
comparable to these measurement uncertainties,
indicating that the beams are stable over the period of a typical run. The t parameters are
stored run by run in the conditions database.
These measurements are also checked o -line by
measuring the primary vertices in multi-hadron
events. The measured horizontal and longitudinal beam sizes, corrected for tracking resolution,
are consistent with those measured by PEP-II.
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
21
3.4. Beam Background Sources
The primary sources of steady-state accelerator backgrounds are, in order of increasing importance: synchrotron radiation in the vicinity of
the interaction region; interactions between the
beam particles and the residual gas in either ring;
and electromagnetic showers generated by beambeam collisions [5] [6] [7]. In addition, there are
other background sources that uctuate widely
and can lead to very large instantaneous rates,
thereby disrupting stable operation.
3.4.1. Synchrotron Radiation
Synchrotron radiation in nearby dipoles, the
interaction-region quadrupole doublets and the
B1 separation dipoles generates many kW of
power and is potentially a severe background.
The beam orbits, vacuum-pipe apertures and
synchrotron-radiation masks have been designed
such that most of these photons are channeled to
a distant dump; the remainder are forced to undergo multiple scatters before they can enter the
BABAR acceptance. The remaining synchrotron
radiation background is dominated by x-rays
(scattered from the tungsten tip of a mask) generated by dipole elds in HER low- quadrupoles.
This residual background is very low and has not
presented a signi cant problem.
3.4.2. Beam-Gas Scattering
Beam-gas bremsstrahlung and Coulomb scattering o residual gas molecules cause beam particles to escape the acceptance of the ring if their
energy loss or scattering angle are suciently
large. The intrinsic rate of these processes is proportional to the product of the beam current and
the residual pressure (which itself increases with
current). Their relative importance, as well as the
resulting spatial distribution and absolute rate of
lost particles impinging the vacuum pipe in the
vicinity of the detector, depend on the beam optical functions, the limiting apertures, and the
residual-pressure pro le around the entire rings.
The separation dipoles bend the energy-degraded
particles from the two beams in opposite directions and consequently most BABAR detector systems exhibit occupancy peaks in the horizontal
plane, i.e., the LER background near = 0 and
DRAFT
22
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
HER background near = 180.
During the rst few months of operation and
during the rst month after a local venting of
the machine, the higher pressures lead to signi cantly enhanced background from beam-gas scattering. The situation has improved signi cantly
with time due to scrubbing of the vacuum pipe
by synchrotron radiation. Towards the end of the
rst year of data-taking, the dynamic pressure in
both rings had dropped below the design goal,
and the corresponding background contributions
are much reduced. Nevertheless, beam-gas scattering remains the primary source of radiation
damage in the SVT and the dominant source of
background in all detectors systems, except the
DIRC.
3.4.3. Luminosity Background
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
Radiative Bhabha scattering results in energydegraded electrons or positrons hitting aperture
limitations within a few meters of the IP and
spraying BABAR with electromagnetic shower debris. This background is directly proportional to
the instantaneous luminosity and thus is expected
to contribute an increasing fraction of the total
background in the future.
1082
3.4.4. Background Fluctuations
1091
In addition to these steady-state background
sources, there are instantaneous sources of radiation that uctuate on diverse time scales:
1083
1084
1085
1086
1087
1088
1089
1090
1092
1093
1094
1095
beam losses during injection,
1096
intense bursts of radiation, varying in du-
1098
1097
ration from a few ms to several minutes,
currently attributed to very small dust particles, and
1099
non-Gaussian tails from beam-beam inter-
1103
1100
1101
1102
actions (especially of the e+ beam) that are
highly sensitive to adjustments in collimator settings and ring tunes.
1104
These e ects typically lead to short periods of
high background and have resulted in a large
number of BABAR-initiated beam aborts (see below).
1108
1105
1106
1107
1109
1110
1111
1112
February 6, 2001 { 10 : 40
3.5. Radiation Protection and Monitoring
A system has been developed to monitor the instantaneous and integrated radiation doses, and
to either abort the beams or to halt or limit the
rate of injection, if conditions become critical. In
addition, DCH and IFR currents, as well as DIRC
and IFR counting rates, are monitored; abnormally high rates signal critical conditions.
Radiation monitoring and protection systems
are installed for the SVT, the DCH electronics,
and the EMC. The radiation doses are measured
with silicon photo-diodes. For the SVT, 12 diodes
are arranged in three horizontal planes, bottom,
middle and top, with four diodes in each plane,
placed at z = +12:1 cm and z = ;8:5 cm and at
a radial distance of 3 cm from the beam line [8].
The diode leakage current, after correction for
temperature and radiation damage e ects, is proportional to the dose rate. The four diodes in the
middle are exposed to about ten times more radiation than the others. These mid-plane diodes
are connected to the beam abort system, while
the remaining eight diodes at the top and bottom
are used to monitor the radiation dose delivered
to the SVT bulk. The accuracy of the measured
average dose rate is better than 0.5 mRad/s. The
integrated dose, as measured by the SVT diodes,
is presented in Figure 6.
The radiation level at the drift chamber and the
calorimeter is more than two orders of magnitude
lower than at the SVT. To amplify the signal, the
PIN diodes are mounted on small CsI(Tl) crystals
(about 10 cm3). These silicon diodes are installed
in sets of four. Three sets are mounted on the
front face of the endcap calorimeter and one set
on the backward endplate of the drift chamber,
close to the read-out electronics. The signals of
the four diodes in each set are summed, ampli ed
and fed into the radiation protection electronics.
Only one of the three diode sets of the EMC is
used at any given time. The DCH and the EMC
use identical hardware and decision algorithms.
They limit injection rates or abort beams, whenever an instantaneous dose equivalent to about
1 Rad/day is exceeded.
The SVT employs a di erent strategy and circuitry to assess whether the measured radiation
levels merit a beam abort or a reduction in single-
February 6, 2001 { 10 : 40
DRAFT
23
80
Dose Budget
HER (horiz)
LER (horiz)
HER+LER (non-horiz)
60
400
Trips / day
Dose Integral / kRad
600
40
200
20
0
0
-200
0
200
Day of Year 2000
0
Figure 6. The integrated radiation dose as measured by PIN diodes located at three di erent positions near the SVT. Also shown is the SVT radiation budget for the rst year of operation.
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
beam injection rate. It was realized that every
beam dump initiated by BABAR would be followed
by a 5-10 minute period of injection with significant radiation exposure. Thus to minimize the
ratio of the integrated radiation dose to the integrated luminosity, it has been bene cial to tolerate transient high-dose events during data-taking
as long as the integrated dose remains less than
the typical dose accumulated during injection. To
di erentiate between very high instantaneous radiation and sustained high dose rates, di erent
trip thresholds are enforced on two di erent time
scales: an instantaneous dose of 1 Rad accumulated over 1 ms, and an average of 50 mRad/s
measured over a 5-minute period. In addition,
higher thresholds are used during injection, since
an aborted injection will delay the return to datataking.
Figure 7 shows the daily rate of beam aborts
initiated by the SVT protection diodes since January 2000. Initially as many as 80 beam aborts
were triggered per day, while the average for sta-
100
200
Day of Year 2000
300
Figure 7. Daily rates of beam aborts initiated
by the SVT radiation protection diodes, summed
over regular data-taking and PEP-II injection.
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
ble operation is now signi cantly less than ten.
The measures described above, combined with a
signi cant reduction in large background uctuations, have been very e ective in protecting the
detector against radiation damage, as well as very
important to the e ort of increasing the combined
live time of the machine and detector to greater
than 75%.
3.6. Impact of Beam-Generated Background on BABAR
Beam-generated backgrounds a ect the detector in multiple ways. They cause radiation damage to the detector components and the electronics and thus may limit the lifetime of the experiment. They may also cause electrical breakdown
and damage or generate large numbers of extraneous signals leading to problems with bandwidth
limitations of the data acquisition system and
with event reconstruction. The event reconstruction is a ected both by degraded resolution and
by losses in eciency.
The impact of the beam-generated background
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
DRAFT
on the lifetime and on the operation of the different detector systems varies signi cantly. Table 3 lists the limits on the instantaneous and
integrated background levels in terms of the total
dose and instantaneous observables. These limits
are estimates derived from tests and/or experience in earlier experiments. For each detector system, an annual radiation allowance has been established taking into account the total estimated
lifetime of the components and the expected annual operating conditions. The typical values accumulated for the rst year of operation are also
stated in the table.
Systematic studies of background rates were
performed with stable stored beams. Measurements of the current-dependence of the backgrounds were carried out for single beams, two
beams not colliding, and colliding beams with the
goal to identify the current background sources,
to develop schemes of reducing these sources and
to extrapolate to operation at higher luminosity [7]. These experimental studies were complemented by Monte Carlo simulations of beam-gas
scattering and of the propagation of showers in
the detector. The studies show that the relative importance of the single-beam and luminosity background contributions varies, as illustrated
in Figure 8. Data for the IFR are not shown because this system is largely insensitive to beamgenerated backgrounds, except for the outer layer
of the forward endcap due to insucient shielding
of the external beam line components.
The experience of the rst year of operation
and the concern for future operation for each of
the detectors are summarized as follows.
SVT: The most signi cant concern for the
SVT with regard to machine background is the
integrated radiation dose. The instantaneous and
integrated dose rates in the radiation protection
diodes are representative, to within about a factor of two, of the radiation doses absorbed by
the SVT modules. The exposure in the horizontal planes is an order of magnitude larger than
elsewhere, averaging 15-25 mRad/s during stable beam operation. The worst integrated dose
is 450 kRad, 30% below the allowance, giving
con dence that the SVT can be operated several
more years (see Figure 6).
February 6, 2001 { 10 : 40
LER Only
Background Fraction
24
HER Only
Collisions
>1MeV >10MeV
1.00
0.75
0.50
0.25
1-2001 0
8583A31
SVT
DCH
DIRC
EMC
EMC TRG
Figure 8. Fractional steady-state background
contributions in BABAR detector systems, measured for single beams and colliding beams under
stable conditions (I + = 1:25 A, I ; = 0:75 A,
L = 2:3 1033 cm;2 s;1 ). The contributions are
derived from the measured doses in the horizontal plane for the SVT, the total currents in the
DCH, the rates in the DIRC photo-tubes, the occupacany and number of photons above 10 MeV
in the EMC, and the L1 triiger rates.
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
DCH: For the DCH, the currents on the wires
are the main concern, both because of the limited capacity of the HV power supplies and the
e ect of wire aging. The currents drawn are approximately uniformly distributed among the 44
HV supplies, one for each quadrant of superlayers 2-10, and two per quadrant for superlayer 1.
Consequently, the total current limit is close to
the sum of the limits of the individual supplies.
During stable operation the total chamber current is 200-300 A. However, radiation spikes can
lead to currents that occasionally exceed the limit
of 1000 A, causing HV supplies to trip. Other
background e ects are measured to be well below the estimated lifetime limits and thus not a
serious issue at this time. The average wire occupancy has not exceeded 1-2% during stable operation, but the extrapolation to future operation at
higher luminosity and currents remains a major
concern.
February 6, 2001 { 10 : 40
DRAFT
25
Table 3
BABAR background tolerance. Operational limits are expressed either as lifetime limits (radiation-damage
and ageing-related quantities), or in terms of instantaneous observables (DCH current, DIRC and L1trigger rates).
Detector system
Limiting factor
Operational
First-year
and impact
limit
typical
SVT sensors
Integrated dose:
2 MRad
0.33 MRad
& electronics
radiation damage
(hor.-plane modules)
0.06 MRad
(other modules)
SVT sensors
Instantaneous dose:
1 Rad/ms
N/A
diode shorts
DCH: electronics
Integrated dose:
20 kRad
100 Rad
radiation damage
DCH: wire current Accumulated charge: 100 mC/cm
8 mC/cm
wire aging
DCH: total current HV system limitations 1000 A
250 A
(steady-state)
DIRC photo tubes
Counting rate:
200 kHz 110 kHz (steady-state,
TDC dead time
well-shielded sector)
EMC crystals
Integrated dose:
10 kRad
0.25 kRad
radiation damage
(worst case)
L1 trigger
Counting rate:
2 kHz
0.7 kHz
DAQ dead time
(steady-state)
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
DIRC: The DIRC quartz bars were tested up
to doses of 100 kRad without showing any measurable e ects and thus radiation damage is not
a concern. The present operational limit of the
DIRC is set by the TDC electronics which induces
signi cant dead time at rates above 200-300 kHz,
well above the stable beam rate of 110 kHz in
well shielded areas. Roughly half the present rate
originates from radiative Bhabha scattering. The
counting rate is due to debris from electromagnetic showers entering the water- lled stand-o
box. E orts are underway to improve the shielding of the beam pipe nearby.
EMC: The lifetime of the EMC is set by
the reduction in light collection in the CsI crystals due to radiation damage. The cumulative
dose absorbed by the EMC is measured by a
large set of RADFETs [23] distributed over the
barrel and endcap. The absorbed dose increases
approximately linearly with the integrated luminosity. The highest dose to date was observed
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
in the innermost ring of the endcap, close to
250 Rad, while the barrel crystals accumulated
about 80 Rad. The observed reduction in light
collection of 10-15% in the worst place, and 4-7%
in the barrel, is consistent with expectation (see
Chapter 11).
The energy resolution is dependent on the single crystal readout threshold, currently set at
1 MeV. During stable beam conditions the average crystal occupancy for random triggers is 16%,
with 10% originating from electronics noise in the
absence of any energy deposition. The spectrum
of photons observed in the EMC from the LER
and HER is presented in Figure 9. The HER produces a somewhat harder spectrum. The average
occupancy for a threshold of 1 MeV and the average number of crystals with a deposited energy
of more than 10 MeV are shown in Figure 10 as
a function of beam currents for both single and
colliding beams. The occupancy increases signi cantly at smaller polar angles, in the forward
DRAFT
February 6, 2001 { 10 : 40
10 2
10
HER only
Beams in Collision
LER only
1
10
10
10
Crystal Occupancy (%)
Crystals / Event / 2.5 MeV
26
-1
-2
-3
0
0.02 0.04 0.06 0.08
0.1
0.12 0.14
18
Beams Separated
14
(a)
10
Figure 9. The energy spectrum of photons
recorded in the EMC by random triggers with
single beams at typical operating currents, LER
at 1.1 A and HER at 0.7 A. The electronic noise
has been subtracted.
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
endcap and the backward barrel sections, and in
the horizontal plane. The rate increase is approximately linear with the single beam currents.
The data recorded with separated beams produce
backgrounds that are consistent with those produced by single beams. For colliding beams, there
is an additional ux of photons originating from
small angle radiative Bhabha scattering. This effect is larger for low energy photons and thus it
is expected that at higher luminosities the low
energy background will raise the occupancy and
thereby limit the EMC energy resolution.
L1 Trigger: During stable beam operation
the typical L1 trigger rate is below 1 kHz, about a
factor of of two below the data acquisition bandwidth limit of about 2-2.5 kHz. Experience shows
that background bursts and other rate spikes can
raise the data volume by as much as a factor of
two and thus it is necessary to aim for steady state
rates signi cant below the stated limit. For the
L1 trigger, the dominant sources of DCH triggers
Number of Crystals with E>10 MeV
Single Crystal Energy (GeV)
(a)
Beams in Collision
8
Beams Separated
6
4
2
(b)
0
0
200
400
HER Current (mA)
600
1-2001
8583A8
Figure 10. Average rates in the EMC for random
triggers as a function of the HER current for a
xed LER current of 1.1 A, both for separated
and colliding beams; a) the single crystal occupancy for thresholds of 1 MeV and b) the number
of crystals with a deposited energy greater than
10 MeV. The solid curves represent a t to the
colliding beam data, the dashed curves indicate
the sum of data recorded for single beams.
February 6, 2001 { 10 : 40
HER + LER in Collision
HER + LER
HER
600
L1 Rate (Hz)
DRAFT
1311
1312
1313
1314
1315
1316
400
1317
1318
1319
200
1320
1321
1322
1323
0
1-2001
8583A7
0
400
HER Current (mA)
1324
1325
1326
1327
1328
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
Figure 11. The L1 trigger rate as a function of
the HER current for single beam only, for both
beams, separated and colliding (with a LER current of 1.1 A).
1329
are particles generated by interactions in vacuum
anges and the B1 magnets (see Figure XX in
Chapter ??). This e ect is most pronounced in
the horizontal plane. At present, the HER background is twice as high as that of the LER, and
the luminosity trigger contributes less than half
of the combined LER and HER single beam triggers.
1334
3.7. Summary and Outlook
Towards the end of the rst year of data-taking,
PEP-II routinely delivered beams close to design luminosity. Due to the very close cooperation with the PEP-II operations team, the
machine-induced backgrounds have not been a
major problem once stable conditions were established. The background monitoring and protection system has become a reliable and useful tool
to safeguard the detector operation.
Currently planned upgrades are expected to
raise the luminosity to 1:5 1034 cm;2 s;1 within
a few years. The single beam backgrounds will
increase with beam currents and the luminosity
background is projected to exceed, or at best
1330
1331
1332
1333
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
27
remain comparable to, the beam-gas contribution. Measures are being prepared to reduce
the sources and the impact of machine-related
background on BABAR, among them upgrades
to the DCH power supply system and to the
DIRC TDC electronics, the addition of localized
shielding against shower debris (especially for the
DIRC stando -o box), new vacuum chambers,
adjustable collimators, and additional pumping
capacity in critical regions upstream of the interaction point.
With the expected increase in LER current
and luminosity both the single-beam and the
luminosity-generated L1 trigger rates will increase and are projected to exceed 2 kHz (see
Figure 11). Therefore, the DCH trigger is being
upgraded to improve the rejection of background
tracks originating from outside the luminous region. In addition, the data acquisition and data
processing capacity will need to be expanded to
meet the demands of higher luminosity.
Overall, the occupancy in all systems, except
the IFR, will probably reach levels that are likely
to impact the resolution and reconstruction eciency. For instance, the occupancy in the EMC
is expected to more than double. Thus, beyond the relatively straight forward measures currently planned for BABAR system upgrades, detailed studies of the impact of higher occupancy
will be necessary for all systems.
REFERENCES
1. F. Author et al., The PEP-II Storage
Rings submitted to Nucl. Instr. and Methods (2001), SLAC-PUB8xxx.
2. J. Seeman et al., Status Report on PEP-II
Performance, in Proceeedings of the 7th European Particle Accelerator Conference (EPAC
2000), Vienna, Austria (2000).
3. M. Sullivan, B -Factory Interaction Region
Designs, in Proceedings of the IEEE Particle
Accelerator Conference (PAC97), Vancouver,
B.C., Canada (1997), SLAC-PUB-7563.
4. S.E. Csorna et al., (CLEO Collaboration),
Phys. Rev. 61 (2000) 111101.
5. T. Mattison et al., Background Measurements
during PEP-II Commissioning in Proceedings
28
1357
1358
1359
1360
1361
6.
7.
1362
1363
1364
8.
1365
1366
1367
1368
9.
DRAFT
of the IEEE Particle Accelerator Conference
(PAC99), New York, NY, USA (1999).
W. Kozanecki, Nucl. Instr. and Methods A
446 (2000) 59.
C. Hast et al., Report of the High-Luminosity
Background Task Force, BABAR Note-522
(2000).
T.I. Meyer et al., contribution to DPF 2000,
Meeting of the Division of Particles and Fields
of the American Physical Society, Columbus,
OH, USA (2000).
RADFET
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
February 6, 2001 { 10 : 40
4. The Solenoid Magnet and Flux Return
4.1. Overview
The BABAR magnet system consists of a superconducting solenoid [1], a segmented ux return
and a eld compensating coil. This system provides the magnetic eld which enables charged
particle momentum measurement, serves as the
hadron absorber for hadron/muon separation,
and provides the overall structure and support for
the detector components. Figures 1 and 2 show
key components of the BABAR magnet system and
some of the nearby PEP-II magnets.
The magnet coil cryostat is mounted inside the
hexagonal barrel ux return by four brackets on
each end. The end-doors are each split vertically
and mounted on skids that rest on the oor. To
permit access to the inner detector, the doors can
be raised and moved on rollers. At the interface
between the barrel and the end-doors, approximately 60% of the area is occupied by structural
steel and ller plates; the remaining space is reserved for cables and utilities from the inner detectors. A vertical triangular chase cut into the
backward end-doors contains the cryostat chimney. Table 4 lists the principal parameters of the
magnet system. The total weight of the ux return is approximately 870 metric tons.
To optimize the detector acceptance for unequal beam energies, the center of the BABAR detector is o set by 370 mm in the electron beam
direction. The beam collision axis is rotated in
the horizontal plane by 20 mrad relative to axis
of the solenoid to minimize the perturbation of
the beams by the solenoid eld. The principal
component of the magnetic eld, Bz , lies along
the +z axis; this is also the approximate direction
of the electron beam. The backward end-door is
tailored to accommodate the DIRC bar boxes and
to allow access to the drift chamber electronics.
Both ends allow space and adequate shielding for
the PEP-II quadrupoles.
4.2. Magnetic Field Requirements and Design
4.2.1. Field Requirements
A solenoid magnetic eld of 1.5 T was speci ed
in order to permit the desired momentum resolu-
February 6, 2001 { 10 : 40
DRAFT
Table 4
Magnet Parameters
Field Parameters
Central Field
1.5 T
Max. Radial Field
<0.25 T
at Q1 and r = 200 mm
Leakage into PEP-II
<0.01 T
Stored Energy
27 MJ
Steel Parameters
Overall Barrel Length
4050 mm
Overall Door Thickness
1149 mm
(incl. gaps for RPCs)
Overall Height
6545 mm
Plates in Barrel
18
9
20 mm
4
30 mm
3
50 mm
2
100 mm
Plates in Each Door
18
9
20 mm
4
30 mm
4
50 mm
1
100 mm
Main Coil Parameters
Mean Diameter of
3060 mm
Current Sheet
Current Sheet Length
3512 mm
Number of layers
2
Operating Current
4596 A
Conductor Current
1.2 kA/ mm2
Density
Inductance
2.57 H
Bucking Coil Parameters
Inner Diameter
1906 mm
Operating Current
200 A
Number of Turns
140
Cryostat Parameters
Inner Diameter
???
Radial Thickness
???
Total Length
???
Total Material
???
1415
1416
1417
1418
1419
1420
1421
1422
tion for charged particles. To simplify track nding and fast and accurate track tting, the magnitude of the magnetic eld within the tracking
volume was required to be constant within a few
percent.
The magnet was designed to minimize disturbance of operation of the PEP-II beam elements. The samarium-cobalt B1 dipole and
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
29
Q1 quadrupole magnets are located inside the
solenoid as shown in Figure 1. Although these
magnets can sustain the high longitudinal eld of
1.5 T, they cannot tolerate a large radial component. Speci cally, the eld cannot exceed 0.25 T
at a radius r = 200 mm (assuming a linear dependence of Br on r) without degrading their eld
properties due to partial demagnetization. The
conventional iron quadrupoles Q2, Q4, and Q5
are exposed to the solenoid stray elds. To avoid
excessive induced skew octupole components, the
stray eld leaking into these beam elements is required to be less than 0.01 T, averaged over their
apertures.
4.2.2. Field Design Considerations
Due to unavoidable saturation of the steel near
the coil and the gap between the coil and the steel,
eld non-uniformities arise. To control these nonuniformities the current density of the coil is increased at the ends relative to the center by reducing the thickness of the aluminum stabilizer.
While the requirements on the radial eld component at Q1 inside the solenoid can be satised easily at the forward end, the shape of the
backward plug is speci cally designed to simultaneously control eld uniformity and unwanted
radial components.
Sizable leakage of the magnetic ux is a problem, in particular at the backward end. A compensating coil, mounted at the face of the backward door and surrounding the DIRC strong support tube, is designed to reduce the stray eld to
an acceptable level for the DIRC photomultipliers
and the PEP-II quadrupoles.
4.2.3. Magnetic Modeling
Extensive calculations of the magnetic eld
were performed to develop the detailed design of
the ux return, the solenoid coil, and the compensating coil. To cross-check the results of these
calculations the elds were modeled in detail in
two and three dimensions using commercial software [2]. The shape of the hole in each end-door
was designed by optimizing various parameters,
such as the minimum steel thickness in areas of
saturation. The design of the hole in the forward
door was particularly delicate because the highly
30
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
DRAFT
saturated steel is very close to the Q2 quadrupole.
The multiple \ nger" design of the hole was chosen to control the saturation of the steel.
Most of the design work was performed in two
dimensions, but some three dimensional calculations were necessary to assure the accuracy of
modeling the transitions between the end-doors
and the barrel, the leakage of eld into the PEPII magnets, and the impact of that leakage on the
multipole purity [3,4]. The computations of the
leakage eld were done for central eld of 1.7 T
instead of 1.5 T to provide some insurance against
uncertainties in the modeling of complex steel
shapes and the possible variations of the magnetic properties of the steel.
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
4.3. Steel Flux Return
4.3.1. Mechanical and Magnetic Forces
The magnet ux return supports the detector
components on the inside, but this load is not
a major issue. Far greater demands are placed
on the structural design by the magnetic forces
and the mechanical forces from a potential earthquake.
Magnetic forces are of three kinds. First, there
is a symmetric magnetic force on the end-doors
which was taken into consideration in their design and construction method. Second, there is
an axial force on the solenoid due to the forwardbackward asymmetry of the steel. Because the
steel is highly saturated in places, the magnitude
of the eld asymmetry changes when the current
is raised from zero and there is no position of the
solenoid at which the force remains zero at all
currents. Because it is important that this axial
force should not change sign, which could cause
a quench, the superconducting solenoid was deliberately o set by 30 mm towards the forward
door. This o set was chosen to accommodate a
worst case scenario, including uncertainties in the
calculation. Third, during a quench of the superconducting coil, eddy currents in conducting components inside the magnetic volume could generate sizable forces. These forces were analyzed
for components such as the end-plates of the drift
chamber and the electromagnetic calorimeter and
were found not to be a problem.
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
February 6, 2001 { 10 : 40
4.3.2. Earthquake Considerations
Because SLAC is located in an earthquake
zone, considerable attention has been given to
protecting the detector against severe damage
from such an event. The entire detector is supported on four earthquake isolators, one at each
corner, which limit the component acceleration in
the horizontal plane to 0.4 g. However, these isolators o er no protection in the vertical direction.
Vertical ground accelerations of 0.6 g are considered credible and actual component accelerations
may be considerably larger due to resonances. By
taking into account resonant frequencies and the
expected frequency spectra of earthquakes, the
magnet and all detector components have been
designed to survive these accelerations without
serious damage. Because the magnet is isolated
from the ground moving beneath it, worst case
clearances to external components, e.g. PEP-II
components, are provided. It is expected that
even during a major earthquake, damage would
be modest.
4.3.3. Fabrication
The ux return was fabricated [5] from drawings prepared by the BABAR engineering team. A
primary concern was the magnetic properties of
the steel. The need for a high saturation eld
dictated the choice of a low carbon steel, specied by its chemical content (close to AISI 1006).
The manufacturer supplied sample steel for critical magnetic measurements and approval. The
availability of very large tools at the factory made
it possible to machine the entire face of each end
of the assembled barrel, thus assuring a good t
of the end-doors. The entire ux return was assembled at the factory, measured mechanically,
and inspected before disassembly for shipment.
4.4. Magnet Coils
The design of the superconducting solenoid
is conservative and follows previous experience.
The superconducting material is composed of
niobium-titanium (46.5% by weight Nb) laments, each less than 40 m in diameter. These
laments are then wound into 0.8 mm strands,
16 of which are then formed into Rutherford
cable measuring 1.4 x 6.4 mm. The nal
February 6, 2001 { 10 : 40
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
DRAFT
conductor [6] consists of Rutherford cable coextruded with pure aluminum stabilizer measuring 4.93 x 20.0 mm for use on the outer,
high current density portion of the solenoid, and
8.49 x 20.0 mm for the central, lower current
density portion. The conductor is covered in an
insulating dry wrap berglass cloth vacuum impregnated with epoxy. The conductor has a total
length of 10.3 km.
The solenoid is indirectly cooled to an operating point of 4.5 K using a thermo-syphon technique. Liquid helium [7] is circulated in channels
welded to the solenoid support cylinder. Liquid helium and cold gas circulate between the
solenoid, its shields, the lique er/refrigerator and
a 4000 ` storage dewar via 60 m of coaxial, gasscreened, exible transfer line. The solenoid coil
and its cryostat were fabricated [8] to drawings
prepared by the BABAR engineering team. Before shipment [9], the fully assembled solenoid was
cooled to operating temperature and tested with
currents of up to 1000 A, limited by coil forces in
the absence of the iron ux return.
To reduce the leakage elds into the PEP-II
components and the DIRC photomultipliers, an
additional external compensating coil is installed.
This is a conventional water cooled copper coil
consisting of 10 layers. Although the nominal operating current is 200 A, a current of up to 575 A
may be needed to demagnetize the DIRC shield.
To optimally control the stray elds and avoid
a magnetization of the DIRC magnetic shield, the
currents in the solenoid and the compensating
coil are ramped together under computer control.
High precision transducers are used to measure
the currents and provide the feedback signals to
the power supplies. The values of the currents are
recorded in the BABAR database.
4.5. Magnetic Field Map
The goal of the magnetic eld mapping and
subsequent corrections was to determine the magnetic eld in the tracking volume to a precision
of 0.2 mT.
4.5.1. Mapping Procedure
A eld mapping device was built speci cally for
the BABAR magnet based on a design concept de-
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
31
veloped at Fermilab [10]. The magnetic eld sensors were mounted on a \propeller" at the end of
a long cantilevered spindle which reached through
the hole in the forward end-door. The spindle in
turn rode on a carriage which moved on precisionaligned rails. The propeller rotated to sample the
magnetic volume in , and the carriage moved
along its axis to cover z . Measurements were obtained from ve sets of Br and Bz and two B
Hall probes, all of which were mounted on a plate
at di erent radial positions. This plate was attached to the propeller and its position could be
changed to cover the desired range in the radial
distance r from the axis. Precision optical alignment tools were used to determine the position of
the sensors transverse to the z axis.
The Br and Bz probes were two-element devices with a short term precision of 0.01%, the B
probes were single element devices with a precision of 0.1% [11]. In addition to the Hall probes,
an NMR probe [12] was mounted at a radius of
89 mm on the propeller to provide a very precise
eld reference near the z axis as a function of z
for jz j < 1000 mm, where z = 0 at the magnet
center. The NMR measurements set the absolute
scale of the magnetic eld.
The magnetic eld was mapped at the nominal
central operating eld of 1.5 T, as well as at 1.0 T.
Measurements were recorded in 100 mm intervals
from ;1800 to +1800 mm in z , and in 24 azimuthal positions spaced by 15 for each of three
di erent positions of the Hall probe plate. Thus
for each z and position, the components Br
and Bz were measured at 13 radii from 130 mm
to 1255 mm and B at six radii between 505 mm
and 1180 mm.
The eld map was parameterized in terms of a
polynomial of degree up to 40 in r and z plus additional terms to account for expected perturbations [13]. The t reproduced the measurements
to within an average deviation of 0.2 mT throughout the tracking volume. The tting procedure
also served as a means of detecting and removing
questionable measurements.
4.5.2. Perturbations to the Field Map
During the mapping process, the permanent
magnet dipoles (B1) and quadrupoles (Q1) were
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
not yet installed. Their presence inside the
solenoid results in eld perturbations of two
kinds. The rst is due to the fringe elds of the
B1 and Q1 permanent magnets, and of the dipole
and quadrupole trim coils mounted on Q1. The
B1 eld strength reaches a maximum of 20 mT
close to the surface of the B1 casing and decreases
rapidly with increasing radius. The elds associated with the trim coils were measured and parameterized prior to installation; they are essentially dipole in character.
The second eld perturbation is due to the permeability of the permanent magnet material. Sintered samarium-cobalt has a relative permeability of 1.11 to 1.13 in the z direction, and as a
result the solenoid eld is modi ed signi cantly.
Probes between the B1 and Q1 magnets at a radius of about 190 mm measure the e ect of the
permeability. The eld perturbation is obtained
from a two-dimensional, nite element analysis
which reproduces the r and z dependence of Br
and Bz . The induced magnetization increases Bz
by about 9 mT at the interaction point, the e ect
decreases slowly with increasing radius.
4.5.3. Field Quality
To illustrate the quality of magnetic eld, Figure 12 shows the eld components Bz and Br as
a function of z for various radial distances r. In
the tracking volume the eld is very uniform, the
B component does not exceed 1 mT. The variation of the bend eld, i.e. the eld transverse
to the trajectory, along the path of a high momentum track is at most 2.5% from maximum to
minimum within the tracking region, as shown in
Figure 13.
4.5.4. Field Computation
In order to reduce the computation of the magnetic eld for track reconstruction and momentum determination, the eld values averaged over
azimuth are stored in a grid of r - z space points
spanning the volume interior to the cryostat. Local values are obtained by interpolation. Within
the volume of the SVT, a linear interpolation is
performed in a 20 mm grid; elsewhere the interpolation is quadratic in a 50 mm grid. Azimuthal dependence is parameterized by means
February 6, 2001 { 10 : 40
1.55
1.45
Bz (T)
1654
DRAFT
r
0.80
0.54
0.27
0.00
IP
1.35
Drift Chamber
0.05
Br (T)
32
–0.05
r
0.27
0.54
0.80
IP
Drift Chamber
–0.15
-1.0
1-2001
8583A2
0.0
1.0
Z (m)
Figure 12. The magnetic eld components Bz
and Br as a function of z for various radial distances r (in m). The extent of the drift chamber
and the location of the interaction point (IP) are
indicated.
DRAFT
February 6, 2001 { 10 : 40
1.02
1718
1719
1720
1721
1722
1723
Bt(L)/Bt(0)
1724
˚
25
1.00
1725
1726
1727
˚ 140
˚
1728
55
0.98
0.0
1-2001
8583A1
1729
˚
18
1730
1731
1732
1.0
2.0
Track Length (m)
1733
1734
1735
1736
1737
Figure 13. Relative magnitude of magnetic eld
transverse to a high momentum track as a function of track length from the IP for various polar
angles (in degrees). The data are normalized to
the eld at the origin.
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
of a Fourier expansion at each r ; z point. The
Fourier coecients at the point of interest are obtained by interpolation on the r ; z grid, and the
average eld value is corrected using the resulting
Fourier series.
4.6. Summary
Since its successful commissioning, the magnet
system has performed without problems. There
have been no spontaneous quenches of the superconducting solenoid. In the tracking region
the magnetic eld meets speci cations, both in
magnitude and uniformity. The eld compensation and magnetic shielding work well for the
DIRC photomultiplier array and the external
quadrupoles.
REFERENCES
1. IEEE Transactions on Applied Superconductivity, 9 #2 (1999) 847.
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
33
2. Opera-2D, Vector Fields, Inc., Aurora, Illinois 60505 (USA).
ANSYS by ANSYS Inc., Canonsburg, PA
15317 (USA).
Mermaid ( c 1994) by Sim Limited, Novosibirsk (Russia).
3. A. Onuchin, et al., Magnetic Field Calculation in the BABAR Detector, BABAR Note 344
(1996).
4. L. Keller, et al., Magnetic Field Calculation in
the BABAR Detector, BABAR Note 370 (1997).
5. Kawasaki Heavy Industries (KHI), Kobe
(Japan).
6. Europa Metali & Cortaillod (Switzerland).
7. Solenoid cool-down and cryogenic Helium is
supplied by a modi ed Linde TCF-200 liqueer/refrigerator.
8. Ansaldo Energia, Genova (Italy).
9. The solenoid was shipped from Genova on a
C5-B transport plane of the US Air Force.
10. C. Newman-Holmes, E.E. Schmidt, and R.
Yamada, Nucl. Instr. and Meth. A 274 (1989)
443.
11. Sentron model 2MR-4A/3B-14B25-20 (2element) and, AMR-3B-14B25-20 (1-element)
Hall Probes, GMW Associates, San Carlos,
CA (USA).
12. Metrolab model PT 2025 Telsameter with
model 1060 NMR probe, Metrolab Instruments, SA, CH-1228, Geneva (Switzerland).
13. A. Boyaraki, et al., \Field Measurements in
the BABAR Solenoid" BABAR Note 514 (2000).
34
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
DRAFT
5. Silicon Vertex Tracker
5.1. Goals and Design Requirements
1796
1797
The silicon vertex tracker (SVT) has been
designed to provide precise reconstruction of
charged particle trajectories and decay vertices
near the interaction region. It forms one of
the two detectors responsible for charged particle tracking in BABAR, the other being the drift
chamber (DCH). The SVT is the critical detector
for the measurement of time-dependent CP asymmetries via the measurement of B 0 meson decay
vertices. The design choices were driven primarily by direct requirements from physics measurements and constraints imposed by the PEP-II interaction region and BABAR experiment. In this
chapter we will discuss the mechanical and electronic design of the SVT, with some discussion of
the point resolution per layer and dE=dx performance. We discuss the tracking performance and
eciency of the SVT alone and in combination
with the DCH, in Section 9.
1798
5.1.1. Requirements and Constraints
1818
Physics requirements are derived from the decay topologies of B mesons in the laboratory
frame. To avoid signi cant impact on the CP
asymmetry measurement (< 10% with respect to
perfect vertex reconstruction) the mean spatial
resolution on each B decay vertex along the z axis must be better than 80 m [1]. The required
resolution in the x ; y plane arises from the need
to reconstruct nal states in B decays as well as
in and charm decays. For example, in decays of
the type B 0 ! D+ D; separating the two D vertices is important. The distance between the two
D's in the x ; y plane for this decay is typically
275 m. Hence the SVT needs to provide x ; y
resolution of order 100 m.
Many of the decay products of B mesons are
at low pt . The SVT must provide stand-alone
tracking for particles with transverse momentum
less than 120 MeV=c, the minimum that can be
reliably measured in the DCH alone. This feature
is fundamental for the identi cation of slow pions
from D meson decays: a tracking eciency of
70% or more is desirable for tracks with a transverse momentum in the range 50{120 MeV=c.
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
February 6, 2001 { 10 : 40
This also means that the material traversed by
particles must be minimized.
The stand-alone tracking capability and the
need to link SVT tracks to the DCH were crucial in choosing the number of layers. Beyond
the stand-alone tracking capability, the SVT provides the best measurement of track angles at the
IP, which is required to achieve design resolution
for the Cherenkov angle.
Additional constraints are imposed by the storage rings and the environment of the BABAR detector. The SVT is located inside the 4.5 m long
support tube, that extends all the way through
the detector. To maximize the angular coverage,
the SVT must extend down to 350 mrad (17.2)
in polar angle from the beamline in the forward
direction. The region at smaller polar angles is
occupied by the B1 permanent magnets. In the
backward direction, it is sucient to extend the
SVT sensitive area down to 30.
The SVT must withstand 2 MRad of ionizing
radiation. A radiation monitoring system capable of aborting the beams is required. The expected radiation dose is 240 kRad/yr in the horizontal plane immediately outside the beam pipe
(where the highest radiation is concentrated), and
33 kRad/yr on average otherwise.
The SVT is inaccessible during normal detector operations. Hence, reliability and robustness
are essential: all components of the SVT inside
the support tube should have long mean-time-tofailure, because the time needed for any replacement is estimated to be 4-5 months. Redundancies are built in whenever possible and practical.
Because the SVT is physically well insulated
from the external environment, the SVT is cooled
to remove the heat generated by the electronics
or radiated by the water cooled beam pipe. In
addition, it operates in the 1.5T magnetic eld.
To achieve the position resolution necessary to
carry out physics analyses, the relative position
of the individual silicon sensors should be stable
over long time periods. The assembly allows for
relative motion of the support structures with respect to the B1 magnets. This feature is also
necessary to sustain an earthquake of moderate
intensity.
These requirements and constraints have led to
February 6, 2001 { 10 : 40
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
DRAFT
the choice of a SVT made of ve layers of doublesided silicon strip sensors. To ful ll our physics
requirements, the spatial resolution, for perpendicular tracks, must be 10-15 m in the three
inner layers and about 40 m in the two outer
layers. The inner three layers perform the impact
parameter measurements, while the outer layers
are necessary for pattern recognition and low pt
tracking.
5.2. SVT Layout
The ve layers of double-sided silicon strip sensors, which form the SVT detector, are organized
in 6, 6, 6, 16, and 18 modules, respectively: a photograph is shown in Figure 14. The strips on the
two sides are oriented orthogonally to each other:
the measuring strips (\ strips") run parallel to
the beam and the z measuring strips (\z strips")
are oriented transversely to the beam axis. The
modules of the inner 3 layers are straight, while
the modules of layers 4 and 5 are arch-shaped
(Figures 15 and 16). This arch design was cho-
Figure 14. Fully assembled Silicon Vertex
Tracker. The silicon sensors of the outer layer are
visible, as is the carbon- ber space frame (black
structure) that surrounds the silicon.
1864
1865
1866
1867
1868
35
sen to minimize the amount of silicon required to
cover the solid angle, while increasing the crossing
angle for particles near the edges of acceptance.
A photograph of an outer layer arch module is
Figure 16. Schematic view of SVT: tranverse section.
1869
1870
shown in Figure 17. The modules are divided
electrically into two half-modules, which are read
out at the ends.
36
DRAFT
February 6, 2001 { 10 : 40
Figure 15. Schematic view of SVT: longitudinal section. The roman numerials label the six di erent
types of sensors.
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
Figure 17. Photograph of an SVT arch module
in an assembly jig.
1871
1897
1898
1899
1900
To satisfy the di erent geometrical requirements of the ve SVT layers, ve di erent sensor shapes are required to assemble the planar
sections of the layers. The smallest detectors
are 48x40 mm2, and the largest are 55x52 mm2.
Two identical trapezoidal sensors are added (one
each at the forward and backward ends) to form
the arch modules. The half-modules are given
mechanical sti ness by means of two carbon
ber/kevlar ribs, which are visible in Figure 17.
The strips of sensors in the same half-module
are electrically connected with wire bonds to form
a single readout strip. This results in a total strip
length up to 14 cm (24 cm) in the inner (outer)
layers.
The signals from the z strips are brought to
the readout electronics using fanout circuits consisting of conducting traces on a thin (50 m)
insulating Upilex [2] substrate. For the innermost three layers, each z strip is connected to
its own preampli er channel, while in layers 4
and 5 the number of z strips on a half module
exceeds the number of electronics channels available, requiring that two z strips on di erent detectors are electrically connected (ganged) to a
single electronics channel. The length of a z strip
is about 5 cm (no ganging) or 10 cm (two strips
connected). The ganging introduces an ambiguity on the z coordinate measurement, which must
February 6, 2001 { 10 : 40
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
DRAFT
be resolved in the pattern recognition procedure.
The total number of readout channels is approximately 150,000.
The inner modules are tilted in by 5, allowing an overlap region between adjacent modules,
a feature that is very useful for the alignment.
The outer modules cannot be tilted, because of
the arch geometry. To avoid gaps and to have
a suitable overlap in , layers 4 and 5 are divided into two sub-layers (4a, 4b, 5a, 5b) placed
at slightly di erent radii. The relevant geometrical parameters of each layer are summarized in
Table 5.
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
Table 5
Geometric parameters for each layer and readout
plan of the SVT. \Floating strips" refers to the
number of strips between readout strips. Note:
parts of the sides of layers 1 and 2 are bonded at
100 m and 110 m pitch, respectively, with one
oating strip. Strip length of z -strips for layers
4 and 5 takes includes ganging. The radial range
for layers 4 and 5 includes the radial extent of the
arched sections.
Layer/ Radius Readout Floating Strip
view
(cm)
pitch
strips length
( m)
(mm)
1z
3.2
100
1
40
1
3.2
50-100
0-1
82
2z
4.0
100
1
48
2
4.0
55-110
0-1
88
3z
5.4
100
1
70
3
5.4
110
1
128
4z
9.1-12.7
210
1
104
4
9.1-12.7
100
1
224
5z
11.4-14.4
210
1
104
5
11.4-14.4
100
1
265
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
In order to minimize the material in the detector acceptance region, the readout electronics are
mounted entirely outside the active detector volume. The forward electronics must be mounted
in the 1 cm space between the 350 mrad stayclear space and B1 magnet. This implies that the
hybrids carrying the front-end chip must be positioned at an angle of 350 mrad relative to the
sensor for the inner layers, and at an even larger
1960
1961
1962
1963
1964
1965
1966
1967
1968
37
angle for the outer ones (Figure 15). In the backward direction the available space is larger and
the inner layer electronics can be placed in the
sensor plane, allowing a simpli ed assembly procedure.
The module assembly and the mechanics are
quite complicated, especially for the arch modules, and are described in detail elsewhere [3].
The SVT support structure (Figure 14) is a rigid
body made from two carbon- ber cones, connected by a \space-frame", also made of carbonber epoxy laminate.
An optical survey of the SVT on its assembly jig indicated that the global error in placement of the sensors with respect to design was
200m, FWHM. Subsequently the detector was
disassembled and shipped to SLAC, where it was
re-assembled on the IR magnets. The SVT is attached to the B1 magnets by a set of gimbal rings,
in such a way as to allow for relative motion of
the two B1 magnets while xing the position of
the SVT relative to the forward B1 and the orientation relative to the axis of both B1 dipoles. The
support tube structure is mounted on the PEP-II
accelerator supports, independently BABAR, leading to the possible movements between the SVT
and the rest of BABAR. Precise position monitoring of the beam interaction point is necessary,
this is described in section 5.4.
The total active silicon area is 0.96 m2 and the
material traversed by particles is 4% of a radiation length (see Section 2). The geometrical
acceptance of SVT is 92% of the solid angle in
the center-of-mass system.
5.3. SVT Components
A block diagram of SVT components is shown
in Figure 18. The basic components of the detector are the silicon sensors, the fanout circuits, the
front end electronics and the data transmission
system. Each of these components is discussed
below.
5.3.1. Silicon sensors
The SVT sensors [4] are made of 300 m thick
double-sided silicon strip devices. They were designed at INFN Pisa and Trieste (Italy) and fabricated commercially [5]. They are built on high-
DRAFT
38
Sensors
Inside
Support
Tube
Atom
chips
HDI
Fiber Optics
to/from DAQ
CAN
Bus
Kapton
Tail
DAQ
link
Card
Table 6
Electrical parameters of the SVT, shown for the
di erent layers and views. Cinput refers to the
total input capacitance, Rseries is the series resistance. The ampli er peaking time is 200 ns for
layers 1{3 and 400 ns for layers 4{5.
Layer/ Cinput Rseries
Noise,
view
(pF)
( ) calc. meas.
(elec) (elec)
1z
6.0
40. 550
880
1
17.2
164. 990 1200
2z
7.2
48. 600
970
2
18.4
158. 1030 1240
3z
10.5
70. 700 1180
3
26.8
230. 1470 1440
4z
16.6
104. 870 1210
4
33.6
224. 1380 1350
5z
16.6
104. 870 1200
5
39.7
265. 1580 1600
Matching
card
HDI
link
Card
Front
cables
MUX Power
Power Supplies
MUX
module
Figure 18. Schematic block diagram showing the
di erent components of the SVT.
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
resistivity (6-15 k -cm) n-type substrates with
p+ strips and n+ strips on the two opposite sides.
The insulation of the n+ strips is provided by individual p-stops, so as to achieve an inter-strip
resistance greater than 100 M at operating bias
voltage, normally about 10 V above the depletion voltage. Typical depletion voltages are in
the range 25-35 V. On both sides the strips are
biased with polysilicon resistors (4-20 M ) to
ensure the required radiation hardness, keeping
the voltage drop across resistors and the parallel
noise as low as possible. Strips are AC-coupled to
electronics via integrated decoupling capacitors,
whose capacitance depends on the sensor shape,
but is always greater than 14 pF/cm. The sensors were designed to maximize the active area,
which extends to within 0.7 mm of the physical edges. Another design goal was to control
the inter-strip capacitance: values between 0.7
pF/cm and 1.1 pF/cm were obtained for the various sensor shapes. To achieve the required spatial
resolution, while keeping the number of readout
channels as low as possible, most of the modules
have a oating strip between two readout strips.
The leakage currents, because of the excellent
performance of the manufacturing process, were
as low as 50 nA/cm2 on average, measured at
February 6, 2001 { 10 : 40
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
10 V above depletion voltage. The silicon sensor parameters have been measured after irradiation with 60 Co sources. Apart from an increase in the inter-strip capacitance of about
12% during the rst 100 kRad, the main e ect
was an increase of the leakage current by 0.7
A/cm2/MRad. However, in a radiation test performed in a 1 GeV electron beam, an increase in
leakage current of about 2 A/cm2/MRad, and a
signi cant shift in the depletion voltage, dependent on the initial dopant concentration, were observed. A shift of about 8-10 V was seen for irradiation corresponding to a dose of approximately
1 MRad. These observations indicate signi cant
bulk damage caused by energetic electrons. Due
to the change in depletion voltage, the SVT sensors could undergo type inversion after about 13 MRad. Preliminary tests show that the sensors continue to function after type inversion [6].
Studies of the behavior of SVT modules as a function of radiation dose continue.
5.3.2. Fanout circuits
The fanout circuits, which route the signals
from the strips to the electronics, have been designed to minimize the series resistance and the
inter-strip capacitance. As described in [7], a
February 6, 2001 { 10 : 40
2023
2024
2025
2026
2027
2028
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
trace on the fanout has a series resistance about
1.6 /cm, an inter-strip resistance > 20 M , and
an inter-strip capacitance < 0:5 pF/cm. The electrical parameters of the nal assembly of sensors
and fanouts (referred to as Detector Fanout Assemblies or DFAs) are summarized in Table 6.
Due to the di erent strip lengths, there are large
di erences between the inner and the outer layers. Smaller di erences are also present between
the forward and backward halves of the module,
that are of di erent lengths.
2064
2065
PRE
AMP
signals from all hit strips must be retained,
in order to improve the spatial resolution
through interpolation, while keeping the
number of transmitted hits as low as possible. A `hit" means a deposited charge
greater than 0:95 fC, corresponding to 0.25
MIP (a MIP is the average charge deposited
by a minimum ionizing particle) ;
the ampli er must be sensitive to both negative and positive charge;
the peaking time must be programmable,
with a minimum of 100 ns (in layers 1 and
2, because of the high occupancy), up to
400 ns (outer layers, with high capacitance)
capability to accept random triggers with a
latency up to 11.5 s and a programmable
jitter up to 1 s, without dead time;
radiation hardness greater than 2.5 MRad;
small dimensions: 128 channels in a 6.2 mm
wide chip.
Shaper
CAC
Comp
ToT Counter
Time Stamp
Buffer
Chan #
15 MHz
CINJ
The electrical parameters of a DFA and the
general BABAR requirements are the basic inputs
that drove the design of the SVT front-end IC:
the ATOM (A Time-Over-Threshold Machine).
In particular, the front-end IC had to cope with
the following requirements:
imum ionizing particle (MIP) signals for all
modules;
These requirements are fully satis ed by the
ATOM chip [8], which is depicted schematically
in Figure 19. The linear analog section consists
Thresh
DAC
5.3.3. Front end electronics
signal to noise ratio greater than 15 for min-
39
Sparsification
Readout Buffer
2029
DRAFT
CAL
DAC
From Silicon
Circular
Buffer
193 Bins
Buffer
Event Time
and Number
Serial
Data Out
Figure 19. Schematic diagram of the ATOM front
end IC.
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
of a charge-sensitive preampli er followed by a
shaper. Gains of 200 mV/fC (low) or 300 mV/fC
(high) may be selected. The channel gains on a IC
are uniform to 5 mV/fC. Signals are presented to
a programmable-threshold comparator, designed
so that the output width of the pulse (time over
threshold or ToT) quasi-logarithmic function of
collected charge. This output is sampled at 30
MHz and stored in a 193 location bu er. Upon
receipt of a rst level trigger, the time and ToT is
retrieved from this latency bu er, sparci ed, and
stored in a four event bu er. Upon the receipt of
a L1 accept command from the data acquisition
system, the output data (the 4 bits for the ToT,
5 bits for the time stamp, and 7 bits for the strip
address) are formatted, serialized, and delivered.
The IC also contains a test charge injection circuit. The typical noise behavior of the ATOM, as
described by the equivalent noise charge (ENC)
of the linear analog section is given in Table 7.
The average noise for the various module types
is shown in Table 6. Given that shot noise due to
sensor leakage current is negligible, the expected
noise may be calculated from the parameters of
Tables 6 and 7. The results of such a calculation
are also shown in Table 6. The maximum average
noise is 1,600 electrons, leading to a signal to noise
ratio greater than 15.
40
DRAFT
Table 7
ATOM chip Equivalent Noise Charge (ENC) parameters at di erent peaking times
ATOM
Noise Charge at
Noise
peaking time zero Capacitance
slope
100 ns
380 e;
40.9 e;/pF
200 ns
280 e;
33.9 e;/pF
;
400 ns
220 e
25.4 e;/pF
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
The power consumption of the IC is about
4.5 mW/channel. Radiation hardness was
checked up to 2.4 MRad of 60 Co radiation. At
that dose, the gain decreased 20%, and the noise
increased less than 15%.
The ATOM IC's are mounted on thick- lm
double-sided hybrid circuits (known as High
Density Interconnects or HDIs) based on an
aluminum-nitride substrate with high thermal
conductivity. The electronics are powered
through a oating power supply system, in such
a way as to guarantee a small voltage drop (< 1
V) across the detector decoupling capacitors.
5.3.4. Data transmission
The digitized signals are transmitted from the
ATOM chips through a thin kapton cable to the
matching cards, from where they are routed to
more conventional cables. Just outside the detector signals are multiplexed by the MUX modules,
converted into optical signals and transmitted to
the ROMs. The MUX modules also receive digital
signals from the DAQ via a ber optical connection. The SVT is connected to the BABAR online detector control and monitoring system via
the industry standard CAN bus. Details on SVT
data transmission system and DAQ can be found
in [10,11]. Power to SVT modules (silicon sensor
bias voltage and ATOM low voltages) is provided
by a CAEN A522 power supply system [12].
5.4. Monitoring and calibration
To identify immediately any operational problems, the SVT is integrated in the control and
monitoring system (see Section 14). Major concerns for SVT monitoring are temperature and
humidity, mechanical position and radiation dose.
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
February 6, 2001 { 10 : 40
5.4.1. Temperature and humidity monitors
The total power dissipation of the SVT modules is about 350 W, mainly dissipated in the
ATOM chip. External cooling is provided by
chilled water at 8 C. In addition, humidity is reduced by a stream of dry nitrogen in the support
tube.
Since excessive temperature can permanently
damage the front-end electronics, temperature
monitoring is very important to the safe operation of the SVT. Thermistors are located on the
HDIs (for the measurements of front-end electronics temperature), around the SVT, along the
cooling systems and in the electronics (MUX)
crates. The absolute temperatures are monitored
to 0.2 C and relative changes of 0.1C. Additionally, a series of humidity sensors are employed
to monitor the performance of the dry nitrogen
system. The temperature and humidity monitors
also serve as an interlock to the HDI power supplies.
5.4.2. Position monitoring sensors
The relative alignment of the two tracking systems (DCH and SVT) is monitored with a precision equal to the spatial resolution of the outermost SVT layer (40 m). This is done on an
hourly basis with charged particles from Bhabha
and muon pairs from e+ e; ! + ; events with
a precision of approximately 10 m. To detect
sudden unexpected movements, it is important
to monitor the SVT position on a much faster
time scale. This is achieved by a system of capacitive sensors that measure the position of the
SVT with respect to the PEP-II B1 magnets and
the position of the support tube with respect to
the drift chamber.
An example of the understanding that can be
achieved by the monitoring systems is given in
Figure 20. The relative DCH-SVT horizontal position during a period of 6 days, as measured by
the capacitive sensors, is shown. This curve is
compared with the temperature, measured by the
thermistors around the SVT: a correlation is evident, thus explaining the diurnal movement. The
displacement measurement performed by the capacitive sensors is also con rmed by the measurements obtained with Bhabha events and + ;
February 6, 2001 { 10 : 40
DRAFT
41
35
(a)
(b)
30
–0.4
Lower Curve T (C)
Upper Curve X (mm)
0
25
–0.8
1-2001
8583A24
4
6
6
Day of July
9
12
Figure 20. (a) Horizontal motion measured with the capacitive sensors (curve) between the DCH and the
support tube and mean x coordinate of the collision point (circles) measured with e+ e; and + ; events
for a three day period in July 1999. An arbitrary o set and scale has been applied to the beamspot data.
(b) Diurnal horizontal motion of the drift chamber relative to the SVT as measured with the capacitive
sensors (upper curve) and temperature near the detector (lower curve) as a function of time, for six days
in July 1999.
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
pairs.
5.4.3. Radiation monitors
Radiation monitoring is extremely important
to ensure the SVT does not exceed its radiation
budget, which could cause permanent damage to
the device. To date, the measured radiation absorbed by the SVT is well within the allowed budget.
The monitoring of radiation dose to the SVT
is discussed in detail in Section 3.
5.4.4. Calibrations
Once a day, and each time the SVT con guration has changed, calibrations are performed in
absence of circulating beams. All electronic channels are tested with pulses through test capacitors, for di erent values of the injected charge.
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
Gains, thresholds and electronic noise are measured, and defective channels are identi ed. The
calibration results have proved very stable and repeatable. The main variation in time is the occasional discovery of a new defective channel. The
calibration procedures have also been very useful
for monitoring noise sources external to the SVT.
5.4.5. Defects
Due to a series of minor mishaps incurred during the installation of the SVT, 9 out of 208
readout sections (each corresponding to one side
of a half-module) were damaged and are currently not functioning. There is no single failure mode, but several causes: defective connectors, mis-handling during installation and notfully-understood problems on the front-end electronics hybrid. There has been no module failure
42
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
DRAFT
February 6, 2001 { 10 : 40
due to radiation damage. It should be noted that
due to the redundancy a orded by the ve layers
of the SVT, the presence of the defective modules
has minimal impact on physics analyses.
In addition, there are individual channel defects, of various types, at a level of about 1%. Calibrations have revealed an increase in the number of defective channels at a rate of less than
0.2%/year.
5.5. Data analysis and performance
This section describes the clustering of the raw
data into hits, the SVT internal and global alignment, single hit eciency, and resolution and
dE=dx performance of the SVT.
5.5.1. Cluster and hit reconstruction
Under normal running conditions the average
occupancy of the SVT in a time window of 1 s is
about 3% for the inner layers, with a signi cant
azimuthal variation due to beam-induced backgrounds, and < 1% for the outer layers, where
noise hits dominate. Figure 21 shows the typical
occupancy as a function of IC number (equivalent
to azimuthal angle, in this case) for layer 1, side.
In the inner layers, the occupancy is dominated
by machine backgrounds, which are signi cantly
higher in the horizontal plane, seen in the plot
as the peaks near IC numbers 3 and 25, approximately.
The rst step of the reconstruction program
consists in discarding out-of-time channels. A
time correction, the time between the passage
of the particle and the time the shaper exceeds
threshold, is performed, after which hits with
times more than 200 ns from the event time (determined by the drift chamber) are discarded.
The loss of real hits from this procedure is negligible. The resulting in-time hits are then passed to
the cluster nding algorithm. First, the charge
pulse height (Q) of a single pulse is calculated
from the ToT value. In a rst pass, clusters are
formed grouping adjacent strips with consistent
times. In a second pass clusters separated by
just one strip are merged into one cluster. The
two original clusters plus the merged cluster are
made available to the pattern recognition algorithm, which chooses among the three.
Figure 21. Typical occupancy in percent as a
function of IC number in layer 1, side for
a) forward half-modules and b) backward halfmodules. IC number corresponds to the azimuthal angle and the increased occupancy in the
horizontal plane is visible near ICs 3 and 25.
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
The position x of a cluster formed by n strips is
determined, using the \head-to-tail" algorithm:
Qn ; Q1 ) ;
x = (x1 +2 xn ) + p2 ((Q
n + Q1 )
where xi and Qi are the position and collected
charge of i-th strip, respectively, and p is the readout pitch. This formula results in a cluster position that is always within p=2 of the geometrical
center of the cluster. The cluster pulse height
is simply the sum of the strip charges, while the
cluster time is the average of the signal times.
5.5.2. Alignment
The alignment of the SVT is performed in two
steps. The rst step consists of determining the
internal (or local) relative positions of the 340
silicon sensors. Once this is accomplished, the
next step is to align the SVT as a whole within
the global coordinate system de ned by the drift
chamber. The primary reason for breaking the
February 6, 2001 { 10 : 40
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
DRAFT
alignment process into these two steps is that
the local positions are relatively stable in time
compared to the global position. Also, the local
alignment procedure is considerably more complex than the global alignment procedure. Thus,
the global alignment can be updated on a runby-run basis, while the local alignment constants
are changed as needed, typically after magnet
quenches or detector access.
The local alignment procedure is performed using tracks from e+ e; ! + ; events and cosmic
rays. Well-isolated high momentum tracks from
hadronic events are also used to supplement sensors that are not suciently illuminated by dimuon and cosmic events. Data samples sucient
to perform the local alignment are collected in 1
to 2 days of typical running conditions.
In + ; events the two tracks are simultaneously t using a Kalman ltering technique that
utilizes the known beam momentum. The use
of tracks from cosmic rays reduces any systematic distortion that may be introduced in the local alignment due to imprecise knowledge of the
beam momentum. For all tracks, no information
from the DCH is used, e ectively decoupling the
SVT and DCH.
In addition to the information from tracks,
data from an optical survey performed during the
assembly of the SVT is included in the alignment
procedure. The typical precision of these optical
measurements is 4 m. This survey information
is only used to constrain sensors relative to other
sensors in the same module, but not one module
to another or one layer to another. Furthermore,
only degrees of freedom in the plane of the sensor are constrained as they are expected to be the
most stable given the assembly procedure.
Using the hit residuals from the aforementioned
set of tracks and the optical survey information, a
2 is formed for each sensor and minimized with
respect to the sensor's 6 local parameters. Because each sensor's 2 is minimized separately,
the process must be iterated. The combination of
constraints coming from the overlapping regions
of the silicon sensors, the di-muon t, the cosmic
rays and the optical survey result in an internally
consistent local alignment.
After the internal alignment, the SVT is con-
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
43
sidered as a rigid body. The second alignment
step consists in determining the position of the
SVT with respect to the drift chamber. Tracks
with sucient numbers of SVT and DCH hits are
t two times: once using only the DCH information and again using only the SVT hits. The six
global alignment parameters, three translations
and three rotations, are determined by minimizing the di erence between track parameters obtained with the SVT-only and the DCH-only ts.
As reported above, because of the diurnal movement of the SVT with respect to the drift chamber, this global alignment needs to be performed
once per run ( every 2-3 hours). The alignment constants obtained in a given run are then
used to reconstruct the data in the subsequent
run. This procedure, known as \rolling calibrations", ensures that track reconstruction is always
performed with up-to-date global alignment constants.
Figure 22 shows a comparision between the optical alignment made during the SVT assembly in
February 1999 and a local alignment using data
taken during January, 2000. The alignment from
tracking data was made without using cosmics or
constraints from the optical survey. The width
of the distributions in these plots has four contributions: 1) The SVT was disassembled from
its assembly jig and re-assembled on the IR magnets, 2) time dependent motion of the SVT after
mounting, 3) statistical errors, and 4) systematic
errors. The second set of plots shows the difference in two alignment sets for data taken in
January 2000 as compared to March 2000. In
general, the stability of the inner three layers is
excellent, with slightly larger tails in the outer
two layers. The radial coordinate is less tighly
constrained in all measurements because the radial location of the charge deposition is not well
known, and most of the information about the radial locations comes only from constraints in the
overlap region of the sensors.
5.5.3. Performance
The SVT hit eciency can be calculated for
each half-module by comparing the number of associated hits to the number of tracks crossing the
active area of the module. As can be seen in
44
DRAFT
Figure 22. Comparison of a local alignment of all
the sensors in the SVT using data from January
2000 with the optical survey of the SVT made
during assembly in February 1999 in the (a) R,
(b) Z and R coordinates. Plots (d), (e), and
(f) show the di erence between two local alignments using data from January 15-19 and March
6-7, 2000 for the R, Z , and R coordinates,
respectively. In all the plots, the shaded regions
correspond to the sensors in the rst 3 layers.
In comparing the di erent alignments and optical
survey, a six parameter t (three global translations and three global rotations) has been applied
between the data sets.
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
Figure 23, a combined hardware and software efciencies of 97% is measured, excluding defective
readout sections (9 out of 208), but employing no
special treatment for other defects, such as broken
AC coupling capacitors or dead channels on frontend chips. Actually, since most of the defects
a ect a single channel, they do not contribute
heavily to ineciency, because most charge depositions involve two or more strips, due to track
crossing angles and, to some degree, charge di usion.
The spatial resolution of SVT hits is determined by measuring the distance (in the plane of
the sensor) between the track trajectory and the
hit, using high-momentum tracks in two prong
events. The uncertainty due to the track trajectory is subtracted from the width of the residual distribution to obtain the hit resolution. Fig-
February 6, 2001 { 10 : 40
Figure 23. SVT hit reconstruction eciency, as
measured on + ; events for a) forward halfmodules and b) backward half-modules. The
plots show the probability of associating both
a and z hit to a track passing through the
active part of the detector. The horizontal
axis corresponds to the di erent modules, with
the vertical lines separate the di erent layers as
numbered. Missing values correspond to nonfunctioning half-modules.
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
ure 24 shows the SVT hit resolution for z and side hits as a function of track incident angle, for
each of the ve layers. The measured resolutions
are in excellent agreement with expectations from
Monte Carlo simulations.
Initial studies have shown that hit reconstruction eciency and spatial resolution are e ectively independent of occupancy for the occupancy levels observed so far.
Measurement of the ToT value by the ATOM
chips enables one to obtain the pulse height, and
hence the ionization dE=dx in the SVT sensor.
The values of ToT are converted to pulse height
using a lookup table computed from the pulse
shapes obtained in the bench measurements. The
pulse height is corrected for track length vari-
DRAFT
February 6, 2001 { 10 : 40
2400
2401
Layer 1
Layer 2
(a)
2402
2403
z Resolution (µm)
2404
2405
2406
Layer 3
Layer 4
2407
2408
2409
2410
Layer 5
angle (degrees)
2411
2412
2413
2414
angle (degrees)
2415
2416
2417
(b)
Layer 1
Layer 2
2418
φ Resolutiion (µm)
2419
2420
2421
2422
Layer 3
Layer 4
2423
2424
2425
2426
Layer 5
angle (degrees)
2427
2428
2429
2430
angle (degrees)
2431
2432
2433
Figure 24. SVT hit resolution in the a) z and b)
coordinate in microns, plotted as a function of
track incident angle in degrees. Each plot shows
a di erent layer of the SVT. The plots in the coordinate for layers 1-3 are asymmetric around
= 0 because of the \pin-wheel" design of the
inner layers. There are fewer points in the resolution plots for the outer layers as they subtend
smaller angles than the inner layers.
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
45
ation. The double sided sensors provide up to
10 measurements of dE=dx per track. For every
track with signals from at least 4 sensor in the
SVT, a 60% truncated mean dE=dx is calculated.
The cluster with the smallest dE=dx energy is
also removed to reduce sensitivity to electronics
noise. For minimum-ionizing particles, the resolution on the truncated mean dE=dx is approximately 14%. We can achieve a 2 separation
between the kaons and pions up to momentum
of 500 MeV/c, and between kaons and protons
beyond 1 GeV/c.
5.6. Summary and Outlook
The Silicon Vertex tracker has been operating
eciently since its installation in the BABAR experiment in May 1999. The ve layer device,
based on double-sided silicon sensors, has satised the original design goals, in particular the
targets speci ed for eciency, hit resolution and
low transverse momentum track reconstruction.
The radiation dose during the rst 25 fb;1 of integrated luminosity is within the planned budget, and no modules have failed due to radiation
damage. The performance of the SVT modules
at high radiations dose is currently being studied. Early results indicate that the sensors will
continue to function after type inversion (at 13 MRad), but further tests with irradiated sensors and ATOM ICs need to be performed. A
program of spare module production has commenced, with the goal of replacing modules that
are expected to fail due to radiation damage.
Beam generated backgrounds are expected to rise
with increasing luminosity. Physics studies at ve
times the current backgrounds levels indicate no
change in mass or vertex resolution for the mode
B 0 ! J= KS and a 20% loss of resolution
in the D+ ; D0 mass di erence. In this study
the detector eciency for both decay modes was
lower by 15{20%.
REFERENCES
1. D. Boutigny et al., SLAC-R-0443 (1994) 69.
2. UBE Industries, Japan. see also [3]
3. C. Bozzi et al., Nucl. Instr. and Methods A447 (2000) 20.
46
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
DRAFT
4. D. Barbieri et al., Nuo. Cim. A112 (1999)
113.
5. MICRON Semiconductor Ltd., Lancing, U.K.
6. L. Bosisio, private communication of preliminary results.
7. G. Della Ricca et al., Nucl. Instr. and Methods A409 (1998) 258.
8. V. Re at al., Nucl. Instr. and Methods A409
(1998) 354.
9. F. Lanni and F. Palombo, Nucl. Instr. and
Methods A379 (1996) 399. The HDIs are
manufactured by AUREL, Milano, Italy (?).
10. J. Beringer et al., \The Data Transmission System for the BABAR Silicon Vertex
Tracker", BABAR Note # 518, May 2000.
11. R. Claus et al., SLAC-PUB-8134 (1999).
12. CAEN { Costruzioni Apparecchiature Elettroniche Nucleari { Via Vetraia, 11 { 55049
Viareggio, Italy.
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
February 6, 2001 { 10 : 40
6. Drift Chamber
6.1. Purpose and Design Requirements
The principal purpose of the drift chamber
(DCH) is the ecient detection of charged particles and the measurement of their momenta and
angles with high precision. These high precision measurements enable the reconstruction of
exclusive B and D meson decays with minimal
background. The DCH complements the measurements of the impact parameter and the directions of charged tracks provided by the SVT
near the IP. At lower momenta, the DCH measurements dominate the errors on the extrapolation of charged tracks to the DIRC, EMC, and
IFR. Most critical are the angles at the DIRC,
because the uncertainties in the charged particle
track parameters add to the uncertainty in the
measurement of the Cherenkov angle.
The reconstruction of decay and interaction
vertices outside of the SVT volume, for instance
the KS0 decays, relies solely on the DCH. For this
purpose, the chamber should be able to measure
not only the transverse momenta and positions,
but also the longitudinal position of tracks, with
a resolution of 1 mm.
The DCH also needs to supply information for
the charged particle trigger with a maximum time
jitter of 0.5 s (Section ??).
For low momentum particles, the DCH is required to provide particle identi cation by measurement of ionization loss (dE=dx). A resolution of about 7% will allow =K separation up
to 700 MeV=c. This capability is complementary
to that of the DIRC in the barrel region, while
in the extreme backward and forward directions,
the DCH is the only device providing some discrimination of particles of di erent mass.
Since the average momentum of charged particles produced in B and D meson decays is less
than 1 GeV=c, multiple scattering is a signi cant,
if not the dominant limitation on the track parameter resolution. In order to reduce this contribution, material in front of and inside the chamber volume has to be minimized.
Finally, the DCH must be operational in
the presence of large beam-generated backgrounds, which were predicted to generate rates
February 6, 2001 { 10 : 40
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
DRAFT
of 5 kHz/cell in the innermost layers.
6.2. Mechanical Design and Assembly
6.2.1. Overview
The DCH is relatively small in diameter, but
almost 3 m long, with 40 layers of small hexagonal cells providing up to 40 spatial and ionization loss measurements for charged particles with
transverse momentum greater than 180 MeV=c.
Longitudinal position information is obtained by
placing the wires in 24 of the 40 layers at small
angles with respect to the z -axis. By choosing
low-mass aluminium eld wires and a heliumbased gas mixture the multiple scattering inside
the drift chamber is held to a minimum, less than
0.2% X0 of material. The properties of the chosen
gas, a 80:20 mixture of helium:isobutane, are presented in Table 8. This mixture has a radiation
length that is ve times larger than commonly
used argon-based gases. The smaller Lorentz angle results in a rather uniform time-distance relationship and thereby improved spatial resolution.
Table 8
Properties of helium-isobutane gas mixture at atmospheric pressure and 20C. The drift velocity
and Lorentz angle are given for an electric eld of
600 V/cm with no magenetic eld and with 1.5 T,
respectively.
Parameter
Values
Mixture He : C4 H10 80:20
Radiation Length
807 m
Primary Ions
21.2 /cm
Drift Velocity
22 m/ ns
Lorentz Angle
32
dE=dx Resolution 6.9%
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2532
2533
2534
2535
2536
2537
2538
2539
2540
The inner cylindrical wall of the DCH is kept
thin to facilitate the matching of the SVT and
DCH tracks, to improve the track resolution for
high momentum tracks, and to minimize the
background from photon conversions and interactions. Material in the outer wall and in the
forward direction is also minimized so as not to
degrade the performance of the DIRC and the
EMC. For this reason, the HV distribution and
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
47
all of the readout electronics are mounted on the
backward endplate of the chamber. This choice
also eliminates the need for a massive, heavily
shielded cable plant.
A longitudinal cross section and dimensions of
the DCH are shown in Figure 25. The DCH is
bounded radially by the support tube at its inner
radius and the DIRC at its outer radius. The device is asymmetrically located with respect to the
interaction point. The forward length of 1749 mm
is chosen so that particles emitted at polar angles
of 300 mrad traverse at least half of the layers
of the chamber before exiting through the front
endplate. In the backward direction, the length
of 1015 mm means that particles with polar angles down to ;475 mrad traverse at least half of
the layers. This choice ensures sucient coverage
for forward-going tracks, and thus avoids signi cant degradation of the invariant mass resolution,
while at the same time maintaining a good safety
margin on the electrical stability of the chamber. The DCH extends beyond the endplate by
485 mm at the backward end to accommodate the
readout electronics, cables, and an rf shield. It
extends beyond the forward endplate by 68 mm
to provide space for wire feed-throughs and an rf
shield.
6.2.2. Structural Components
Details of the DCH mechanical design are presented in Figure 26. The endplates, which carry
an axial load of 31,800 kN, are made from aluminium plates of 24 mm thickness. At the forward end, this thickness is reduced to 12 mm beyond a radius of 46.9 cm to minimize the material
in front of the calorimeter endcap. For this thickness, the estimated safety margin on the plastic
yield point for endplate material (6061T651 aluminium) is not more than a factor of two. The
maximum total de ection of the endplates under
loading is small, about 2 mm or 28% of the 7 mm
wire elongation under tension. During installation of the wires, this small de ection was taken
into account by over-tensioning the wires.
The inner and outer cylinders are load bearing
to reduce the maximum stress and de ections of
the endplates. The stepped forward endplate created a complication during the assembly, because
DRAFT
48
630
1015
February 6, 2001 { 10 : 40
1749
68
Elec–
tronics
809
27.4°
485
1358 Be
e–
17.2°
236
e+
IP
469
1-2001
8583A13
Figure 25. Longitudinal section of the drift chamber with principal dimensions; the chamber center is
o set by 370 mm in z from the interaction point.
4
9
Honeycomb
Carbon Fiber
3
12.5
R809
Outer Wall
R809
24
R808.5
1
12
24
R469
Back
Inner Wall
5
R236
3.5
1
Beryllium
Front
3.5
1-2001
8583A30
Figure 26. Details of the structural elements of the DCH. All components are made of aluminium, except
for the 1 mm-thick inner beryllium wall and the 9 mmthick outer composite wall.
February 6, 2001 { 10 : 40
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
DRAFT
the thinner forward endplate would de ect more
than the thicker backward endplate. The outside
rim of the forward endplate had to be pre-loaded,
i.e., displaced by 2.17 mm in the forward direction, to maintain the inside and outside rims of
the rear endplate at the same longitudinal position after the load transfer to the outer cylinder
was complete.
Prior to installation on the inner cylinder, the
two endplates were inspected on a coordinatemeasuring machine. All sense wire holes, as well
as 5% of the eld and clearing eld wire holes,
were measured to determine their absolute locations. The achieved concentricity was 38 m
for both sense and eld wires, better than the
speci cation by more than a factor of two. In
addition, the diameters of the same sample of
endplate holes were checked with precision gauge
pins. All holes passed the diameter speci cation
(4:50000::025
000 for sense wires and 2:50000::025
00 for
the eld and guard wires.
The inner cylindrical wall of the DCH, which
carries 40% of the wire load, was made from
ve sections, a central 1 mm-thick beryllium
tube with two aluminium extensions which were
electron-beam welded to aluminium end anges
to form a 3 m-long cylindrical part. The central section was made from three 120 segments
of rolled and brazed beryllium. The end anges
have precision surfaces onto which the endplates
were mounted. These surfaces set the angles of
the two endplates with respect to the axis and signi cantly constrain the concentricity of the tube.
The inner cylinder also provides a substantial rf
shield down to the PEP-II bunch-gap frequency
of 136 kHz.
The outer wall bears 60% of axial wire load
between the endplates. To simplify its installation, this external wall was constructed from two
half-cylinders with longitudinal and circumferential joints. The gas and electrical seals for these
joints were made up in situ. The main structural
element consists of two 1.6 mm-thick (0.006X0)
carbon ber skins laminated to a 6 mm-thick honeycomb core. The outer shell is capable of withstanding a di erential pressure of 30 mbar and
temperature variations as large as 20C, conditions that could be encountered during ship-
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
49
ping or installation. Aluminium foil, 25 m-thick
on the inside surface and 100 m on the outside,
are in good electrical contact with the endplates,
thereby completing the rf shield for the chamber.
The total thickness of the drift chamber at normal incidence is 1.08% X0 , of which the wires and
gas mixture contribute 0.2% X0 , and the inner
wall 0.28% X0 .
6.2.3. Wire Feed-Throughs
A total of ve di erent types of feed-throughs
were required for the chamber to accommodate
the sense, eld, and clearing eld wires, as well as
two di erent endplate thicknesses. The ve types
are illustrated in Figure 27. They incorporate
crimp pins of a simple design which fasten and
precisely locate the wires. The choice of pin material (gold-plated copper for the signal wires and
gold-plated aluminium for the eld wires) and
wall thickness in the crimp region was optimized
to provide an allowable range of almost 150 m in
crimp size, as a primary means for avoiding wire
breakage.
Crimp pins were either press- t into an insulator made from a single piece of injectionmolded thermoplastic reinforced with 30% silica
glass ber [1], or swaged into a copper jacket for
the eld wires. The plastic insulates the sense,
guard, and clearing eld wires from the electrically grounded endplates, while the metal jackets
provide good ground contact for eld wires (<
0:1 ) on the backward endplate. The outer diameter of the eld and clearing eld feed-throughs
:000
was maintained at 2:475+0
sense
;0:025 mm while+0the
wire feed-through had a larger (4:5;0::000
025 mm)
outer diameter and a longer body (41.7 mm).
This choice provided both thicker insulating walls
and a longer projection into the gas volume to
better shield the HV from the grounded endplate.
6.2.4. Assembly and Stringing
Assembly of the chamber components and installation of the wires was carried out in a large
clean room (Class 10,000) at TRIUMF in Vancouver. The wires were strung horizontally without
the outer cylindrical shell in place. The endplates
were mounted and aligned onto the inner cylinder
which in turn was supported by a central shaft in
DRAFT
50
2697
2698
Copper
Field
2699
2700
Sense
Guard
AL
2701
2702
Cu
Celenex
2703
2704
2705
AL
2706
24 mm
2707
2708
2709
Sense
Cu
2710
2711
Celenex
2712
2713
Field/
Guard
AL
2714
12 mm
1-2001
8583A11
2715
2716
2717
Figure 27. Design of the ve wire feed-throughs
for the 24 mm-thick endplates and the 12 mmthick endplate. The copper jacketed feed-through
is for grounded eld wires, the other four are for
sense wires (4.5 mm diameter), and guard and
clearing eld wires (2.5 mm diameter), all made
from a Celenex insulator surrounding the crimp
pins.
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
a mobile xture. The endplates were mounted
on the inner cylinder at the inside rim and and
attached to support rings at the outside. These
rings were connected by radial spiders to the central shaft of the stringing frame.
Two teams of two operators each worked in parallel as the wires were strung from the inner radius outward. The two teams were each assisted
by an automated wire transporter [2]. A wire was
attached to a needle and inserted through an endplate hole where it was captured magnetically by
one of the transporters, and then transported and
inserted though the appropriate hole in the other
endplate. Once a wire was threaded through the
feed-throughs, the feed-throughs were glued into
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
February 6, 2001 { 10 : 40
the endplates, and the wire was tensioned and
crimped. The automated wire transporters were
largely built from industrial components, employing commercial software and hardware. The semiautomatic stringing procedure ensured the correct hole selection, accelerated the stringing rate
and greatly improved the cleanliness and quality
of the stringing process. The installation of a total of 30,000 wires was completed in less than 15
weeks.
6.3. Drift Cells
6.3.1. Layer Arrangement
The DCH consists of a total of 7,104 small drift
cells, arranged in 40 cylindrical layers. The layers
are grouped by four into ten superlayers, with the
same wire orientation and equal numbers of cells
in each layer of a superlayer. Sequential layers
are staggered by half a cell. This arrangement
enables local segment nding and left-right ambiguity resolution within a superlayer, even if one
out of four signals is missing. The stereo angles of
the superlayers alternate between axial (A) and
stereo (U,V) pairs, in the order AUVAUVAUVA,
as shown in Figure 28. The stereo angles vary between 45 mrad and 76 mrad; they have been
chosen such that the drilling patterns are identical for the two endplates. The hole pattern has a
16-fold azimuthal symmetry which is well suited
to the modularity of the electronic readout and
trigger system. Table 9 summarizes parameters
for all superlayers.
6.3.2. Cell Design and Wires
The drift cells are hexagonal in shape, 11.9 mm
by approximately 19.0 mm along the radial and
azimuthal directions, respectively. The hexagonal cell con guration is desirable because approximate circular symmetry can be achieved over a
large portion of the cell. The choice of aspect ratio has the bene t of decreasing the number of
wires and electronic channels, while allowing a 40
layer chamber in a con ned radial space. Each
cell consists of one sense wire surrounded by six
eld wires, as shown in Figure 28. The properties of the di erent types of gold-coated wires
that make up the drift cells are given in Table 10.
The sense wires are made of tungsten-rhenium [3],
DRAFT
February 6, 2001 { 10 : 40
16
0
15
0
14
0
13
0
12
-57
11
-55
10
-54
9
-52
8
50
7
48
6
47
5
45
Table 9
The DCH superlayer (SL) structure, specifying
the number of cells per layer, radius of the innermost sense wire layer, the cell widths, and wire
stereo angles, which vary over the four layers in
a superlayer as indicated. The radii and widths
are speci ed at the mid-length of the chamber.
SL # of Radius Width
Angle
Cells (mm)
(mm) (mrad)
1 96
260.4 17.0-19.4
0
2 112 312.4 17.5-19.5 45-50
3 128 363.4 17.8-19.6 -(52-57)
4 144 422.7 18.4-20.0
0
5 176 476.6 16.9-18.2 56-60
6 192 526.1 17.2-18.3 -(63-57)
7 208 585.4 17.7-18.8
0
8 224 636.7 17.8-18.8 65-69
9 240 688.0 18.0-18.9 -(72-76)
10 256 747.2 18.3-19.2
0
2743
4
0
3
0
2
0
1
Layer
0
Stereo
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
4 cm
Sense
Field
Guard
20 m in diameter and tensioned with a weight
of 30 g. The de ection due to gravity is 200 m
at mid-length. Tungsten-rhenium has a substantially higher linear resistivity (290 /m), compared to pure tungsten (160 /m), but it is considerably stronger and has better surface quality.
While the eld wires are at ground potential, a
positive high voltage is applied to the sense wires.
An avalanche gain of approximately 5 104 is
obtained at a typical operating voltage of 1960 V
and a 80:20 helium:isobutane gas mixture.
Table 10
DCH wire speci cations (all wires are gold
plated).
Type Material Diameter Voltage Tension
(m)
(V)
(g)
Sense
W-Re
20
1960
30
Field
Al
120
0
155
Guard
Al
80
340
74
Clearing
Al
120
825
155
Clearing
1-2001
8583A14
Figure 28. Schematic layout of drift cells for
the four innermost superlayers. Lines have been
added between eld wires to aid in visualization
of the cell boundaries. The numbers on the right
side give the stereo angles (mrad) of sense wires
in each layer. The 1 mm-thick beryllium inner
wall is shown inside of the rst layer.
51
2754
2755
The relatively low tension on the sense wires
was chosen so that the aluminium eld wires have
52
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
DRAFT
matching gravitational sag and are tensioned well
below the elastic limit. A simulation of the electrostatic forces shows that the cell con guration
has no instability problems. At the nominal operating voltage of 1960 V, the wires de ect by less
then 60 m.
The eld wires [4] are tensioned with 155 g to
match the gravitational sag of the sense wires to
within 20 m. This tension is less than one-half
the tensile yield strength of the aluminium wire.
For cells at the inner or outer boundary of a superlayer, two guard wires are added to improve
the electrostatic performance of the cell and to
match the gain of the boundary cells to those of
the cells in the inner layers. At the innermost
boundary of layer 1 and the outermost boundary
of layer 40, two clearing wires have been added
per cell to collect charges created through photon conversions in the material of the walls.
Sense
Guard
Field
6.3.3. Drift Isochrones
The calculated isochrones and drift paths for
ions in adjacent cells of layer 3 and 4 of an axial superlayer are presented in Figure 29. The
isochrones are circular near the sense wires, but
deviate greatly from circles near the eld wires.
Ions originating in the gap between superlayers
are collected by cells in the edge layers after a delay of several s. These lagging ions do not a ect
the drift times measurements, but they contribute
to the dE=dx measurement.
6.3.4. Cross Talk
A signal on one sense wire produces oppositelycharged signals on neighboring wires due to capacitive coupling. The cross talk is largest between adjacent cells of adjacent layers, ranging
from ;0:5% at a superlayer boundary to ;2:7%
for internal layers within superlayers. For adjacent cells in the same layer, the cross talk ranges
from ;0:8 to ;1:8%, while for cells separated by
two layers it is less than 0.5%.
6.4. Electronics
6.4.1. Design Requirements and Overview
The DCH electronic system is designed to provide a measurement of the drift time and the integrated charge, as well as a bit for every hit wire
to the trigger system [5]. In the 80:20 helium-
February 6, 2001 { 10 : 40
1-2001
8583A16
Figure 29. Drift cell isochrones, i.e., contours of
equal drift times of ions in cells of layers 3 and 4 of
an axial superlayer. The isochrones are spaced by
100 ns. They are circular near the sense wires, but
become irregular near the eld wires, and extend
into the gap between superlayers.
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
isobutane gas mixture, there are on average some
22 (44) primary (total) ionization clusters produced per cm. The position of the primary ionization clusters is derived from timing of the leading
edge of the ampli ed signal. The design goal was
to achieve a position resolution of 140 m, averaged over the cells. To reduce the time jitter in
the signal arrival and at the same time maintain
a good signal-to-noise ratio, the signal threshold
was set at about 2.5 primary electrons. For the
dE=dx measurement, a resolution of 7% was projected for a 40-layer chamber.
The small cell size and the dicult access
through the DIRC strong support tube require a
very high density of electronics components. As a
consequence, a compact and highly modular design was chosen. The readout is installed in well
shielded assemblies that are plugged into the end-
February 6, 2001 { 10 : 40
2820
2821
2822
2823
2824
2825
2826
2827
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
plate and are easily removable for maintenance.
A schematic overview of the DCH electronics
is presented in Figure 30 [6]. The 16-fold azimuthal symmetry of the cell pattern is re ected
in the readout segmentation. The DCH ampli er
and digitizer electronics are installed in 3 16
electronics front-end assemblies (FEAs) that are
mounted directly onto the rear endplate. There
are 16 radial bars that extend from the inner
to the outer chamber walls. These bars provide mechanical support and water cooling for the
FEAs. The assemblies connect to the sense wires
through service boards, which route the signals
and HV distribution. A readout interface board
in each FEA organizes readout of the digitized
data. Data I/O and trigger I/O modules multiplex serial data from the FEAs to high-speed
optical bers for transfer to the readout modules
that are located in the electronics building.
Ground
Post
Signal
Board
Hypertronics
Connectors
6.4.3. Front-End Assemblies
The front-end assemblies (FEAs) plug into
connectors on the back side of the service
boards. These custom wedge-shaped crates are
aluminium boxes that contain a single readout
interface board (ROIB) and 2, 3, or 4 amplier/digitizer boards (ADB) for superlayers 1{4,
5{7, and 8{10, respectively, as shown in Figure 32. The crates are mounted with good thermal contact to the water cooled radial support
bars. The total heat load generated by the FEAs
HV
Board
Feedthroughs
Al
Endplate
1-2001
8583A22
6.4.2. Service Boards
Service boards provide the electrostatic potentials for signal, guard, and clearing wires, and
pass signals and ground to the front-end readout
electronics. A side view of a service board is
shown in Figure 31. The lower board contains
the high voltage buses and ltering, current limiting resistors, and blocking capacitors. Jumpers
connect adjacent boards. The stored energy is
minimized by 220 pF HV blocking capacitors.
The series resistors for the protection circuits
carry the signals to the upper signal board which
contains the protection diodes and standard output connectors. Mounting posts, anchored into
the rear endplate, also serve as ground connections.
53
24 mm
2828
DRAFT
Sense Wires
Field Wires
Guard Wires
Figure 31. Side view of service boards showing
two-tiered structure for HV distribution and signal collection.
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
is 1.3 kW.
The ADBs are built from basic building blocks
consisting of two 4-channel ampli er IC [7] feeding a single 8-channel digitizer custom ASICs [8].
The number of channels serviced by an ADB is
60, 48, or 45, for the inner, middle, and outer
FEA modules, respectively.
The ampli er IC receives the input signal from
the sense wire and produces a discriminator output signal for the drift time measurement and a
shaped analog signal for the dE=dx measurement.
Both outputs are fully di erential. The discriminator has gain and bandwidth control, and a voltage controlled threshold. The analog circuit has
integrator and gain control.
The digitizer IC incorporates a 4-bit TDC for
time measurement and a 6-bit 15 MHz FADC to
measure the total deposited charge. The TDC is
a phase-locked digital delay linear vernier on the
sample clock of 15 MHz, which achieves a 1 ns
precision for leading edge timing. The FADC design is based on a resistor-divider comparator ladder that operates in bi-linear mode to cover the
DRAFT
54
February 6, 2001 { 10 : 40
Signals
SL 1
SL 2
SL 3
SL 4
SL 5
SL 6
FEA 1
SL 8
SL 9
FEA 2
ADB
ADB
ROIB
Trigger I/0
Module
SL10
FEA 3
ADB
ROIB
FEA = Front End Assembly
ADB = Amplifier/Digitizer Board
ROIB = Readout Interface Board
SL = Superlayer
SL 7
ROIB
Data I/0
Module
G-Link
to Trigger
G-Link
to ROM
1-2001
8583A12
Figure 30. Block diagram for a 1/16th wedge of the readout system, showing logical organization of the
three front-end assemblies and their connections to the trigger and data I/O modules
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
full dynamic range. The digitized output signals
are stored in a trigger latency bu er for 12.9 s,
after which a L1 Accept initiates the transfer of
a 2.2 s block of data to the readout bu er. In
addition, trigger information is supplied for every
channel, based either on the presence of a TDC
hit during the sample period or FADC di erential
pulse height information, should a higher discriminator level be desirable.
The ROIB interprets FCTS commands to control the ow of data and trigger information.
Data are moved to FIFOs on the ROIBs, and then
to data and trigger I/O modules via 59.5 MHz serial links. A total of four such links are required
per 1/16th wedge, one for each of the outer two
FEAs and two for the innermost of the FEA. Each
data I/O module services all FEAs one quadrant and transmits the data to a single ROM
via one optical ber link. The trigger stream is
rst multiplexed onto a total of 30 serial lines per
wedge for transmission to the trigger I/O module. Trigger data from two wedges of FEAs are
then transmitted to the trigger system via three
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
optical links. Thus, a total of 28 optical bers,
four for the data and 24 for the charged particle
trigger, are required to transfer the DCH data to
the electronics building.
6.4.4. Data Acquisition
The data stream is received and controlled by
four BABAR standard readout modules. Drift
chamber-speci c feature extraction algorithms
convert the raw FADC and TDC information into
drift times, total charge, and a status word. The
time and charge are corrected channel-by-channel
for time o sets, pedestals, and gain constants.
These algorithms take about 1 s per channel,
and reduce the data volume by roughly a factor
of four.
6.4.5. High Voltage System
The HV bias lines on the chamber are daisychained together so that each superlayer requires
only four power supplies, except for superlayer
1 which has eight. The high voltages are supplied to the sense, guard and clearing wires by a
DRAFT
February 6, 2001 { 10 : 40
2949
2950
2951
SL 8-10
2952
2953
2954
2955
2956
2957
2958
SL 5-7
2959
Cooling
Channel
2960
2961
2962
2963
2964
2965
SL 1-4
10 cm
2966
2967
2968
Amplifier
Digitizer
Board
2969
2970
1-2001
8583A17
2971
2972
2973
Figure 32. Layout of 1=16th of the rear endplate,
showing 3 FEA boxes between water cooled channels.
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
CAEN SY527 HV mainframe [9], equipped with
24-channel plug-in modules. The sense wires are
supplied by 44 HV channels providing up to 40 A
of current each that can be monitored with a resolution of 0.1%.
6.5. Gas System
The gas system has been designed to provide a stable 80:20 helium-isobutane mixture at
a constant over pressure of 4 mbar [10]. Gas
mixing and recirculation is controlled by precise
mass ow controllers; the total ow is tuned to
15 `/min, of which 2.5 `/min are fresh gas. During normal operation, the complete DCH gas volume is re-circulated in six hours, and one full volume of fresh gas is added every 36 hours. The
pressure in the DCH is measured by two indepen-
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
55
dent pressure gauges, one of which is connected
to a regulator controlling the speed of the compressor. The relative pressure in the chamber is
controlled to better than 0:05 mbar.
Oxygen is removed from the gas mixture using a palladium catalytic lter. The water content is maintained at 3500 200 ppm by passing
an adjustable fraction of the gas through a water
bubbler. This relatively high level of water vapor
is maintained to prevent electrical discharge. In
addition to various sensors to monitor pressure,
temperature, and ow at several points of the system, a small wire chamber with an 55 Fe source
continuously monitors gain of the gas mixture.
6.6. Calibrations and Monitoring
6.7. Electronics Calibration
The front-end electronics are calibrated daily to
determine the channel-by-channel correction constants and thresholds. Calibration pulses are produced internally and input to the preampli er at
a rate of about 160 Hz. The calibration signals
are processed in the ROM to minimize the data
transfer and fully exploit the available processing power. The results are stored for subsequent
feature extraction. The entire online calibration
procedure takes less than two minutes.
6.7.1. Environmental Monitoring
The operating conditions of the DCH are monitored in real time by a variety of sensors and
read out by the detector-wide CAN bus system.
These sensors monitor the ow rate, pressure,
and gas mixture; the voltages and currents applied to the wires in the chamber; the voltages
and currents distributed to the electronics from
power supplies and regulators; instantaneous and
cumulative radiation doses; temperature and humidity around the chamber electronics and in the
equipment racks. Additional sensors monitor the
atmosphere in and around the detector for excess
isobutane, which could pose a ammability or explosive hazard in the event of a leak.
Many of the sensors are connected to hardware
interlocks, which ensure that the chamber is automatically put into a safe state in response to an
unsafe condition. All of these systems have performed reliably. In addition, automated software
56
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
monitors raw data quality, chamber occupancies
and eciencies to sense variations in electronics
performance that might indicate more subtle operational problems.
Drift Distance (mm)
2995
2996
DRAFT
6.7.2. Operational Stability
The design of the DCH speci es a voltage of
1960 V on the sense wires to achieve the desired
gain and resolution. The chamber voltage was
lowered for part of the run to 1900 V out of concern for a small region of the chamber that was
damaged during the commissioning phase by inadvertently applying 2 kV to the guard wires.
Wires in this region were disconnected when continuous discharge was observed over extended periods of time.
6.8.1. Time-to-Distance Relation
The precise relation between the measured drift
time and drift distance is determined from samples of e+ e; and + ; events. For each signal,
the drift distance is estimated by computing the
distance of closest approach between the track
and the wire. To avoid bias, the t does not use
the hit on the wire under consideration. The estimated drift distances and measured drift times
are averaged over all wires in a layer, but the data
are accumulated separately for tracks passing on
the left of a sense wire and on the right. The timedistance relation is t to a sixth-order Chebychev
polynomial. An example of such a t is shown in
Figure 33.
8
4
Left
Right
0
0
6.8. Performance
The DCH was rst operated with full magnetic eld immediately after the installation into
BABAR. Cosmic ray data were recorded and extensive studies of the basic cell performance were
performed to develop calibration algorithms for
the time-to-distance and dE=dx measurements.
These algorithms were then implemented as described below for colliding beam data. Calibrations are monitored continuously to provide feedback to the operation; some time varying parameters are updated continuously as part of OPR.
For charge particle tracking the DCH and SVT
information is combined; the performance of the
combined tracking system is described in Chapter 9.
February 6, 2001 { 10 : 40
1-2001
8583A18
200
400
Drift Time (ns)
600
Figure 33. The drift time versus distance relation
for left and right half of a cell, obtained from a t
to data averaged over all cells in layer 18 of the
DCH.
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
An additional correction is made for tracks
with varying entrance angle into the drift cell.
This angle is de ned relative to the radial vector
from the IP to the sense wire. The correction is
applied as a scale factor to the drift distance and
was determined layer-by-layer from a Gar eld [11]
simulation. The entrance angle correction is implemented as a tenth-order Chebychev polynomial of the drift distance, with coecients which
are functions of the entrance angle.
Figure 34 shows the position resolution as a
function of the drift distance, separately for the
left and the right side of the sense wire. The
resolution is taken from Gaussian ts to the distributions of residuals obtained from unbiased
track ts. The results are based on multi-hadron
events, for data averaged over all cells in layer 18.
6.8.2. Charge Measurement
The speci c energy loss, dE=dx, for charged
particles traversing the DCH is derived from measurement of total charge deposited in each drift
February 6, 2001 { 10 : 40
DRAFT
57
dE/dx vs momentum
80% truncated mean (arbitrary units)
Resolution (mm)
0.4
0.3
0.2
0.1
0
–10
1-2001
8583A19
–5
0
5
Distance from Wire (mm)
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
p
d
BABAR
K
π
10 3
e
µ
10
10
cell. The charge collected per signal cell is measured as part of the feature extraction algorithm
in the ROM. Individual measurements are corrected for gain variations, pedestal-subtracted
and integrated over a period of approximately
1.8 s.
The truncated mean of the speci c energy loss
per track is computed from the lowest 80% of the
individual measurements. Various corrections are
applied to remove sources of bias that degrade the
accuracy of the primary ionization measurement.
These corrections include the following:
changes in gas pressure and/or temperature, leading to 9% variation in dE=dx,
corrected by a single overall multiplicative
constant;
di erences in cell geometry and charge collection (8% variation), corrected by a set
of multiplicative constants for each wire;
signal saturation due to space charge buildup (11% variation), corrected by a second-
-1
1
10
Track momentum (GeV/c)
Figure 34. Position resolution as a function of
the drift distance, separately for tracks on the
left and right of the sense wire. The data are
averaged over all cells in layer 18 of the DCH.
3062
10 4
Figure 35. Measurement of dE=dx in the DCH
as a function of track momenta. The data include large samples of beam background triggers,
as evident from the high rate of protons.
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
order polynomial
in the dip angle, , of the
p
form 1= sin2 + const;
non-linearities in the most probable energy
loss at large dip angles (2:5% variation),
corrected with a fourth-order Chebychev
polynomial as a function of ; and
variation of cell charge collection as a function entrance angle (2:5% variation), cor-
rected using a sixth-order Chebychev polynomial in the entrance angle.
The overall gas gain is updated continuously
based on calibrations derived as part of prompt
reconstruction of the colliding beam data; the
remaining corrections are determined once for a
given HV voltage setting and gas mixture.
Corrections applied at the single cell level can
be large compared to the single-cell dE=dx resolution, but have only a modest impact on the average resolution of the ensemble of hits. Global
DRAFT
58
3124
3125
3126
300
3127
Tracks
3128
February 6, 2001 { 10 : 40
the intrinsic position and dE=dx resolution have
been met. Backgrounds are acceptable at present
beam currents, but there is concern for rising occupancies and DAQ capacity at the high end of
the planned luminosity upgrades.
200
100
3129
0
1-2001
8583A21
-0.4
0
0.4
(dE/dxmeas.– dE/dxexp.) / dE/dxexp.
3130
3131
3132
3133
3134
Figure 36. Di erence between the measured and
expected energy loss dE=dx for e from Bhabha
scattering, measured in the DCH at an operating
voltage of 1900 V.
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
corrections applied to all hits on a track are therefore the most important for the resolution.
Figure 35 shows the distribution of the corrected dE=dx measurements as a function of track
momenta. The superimposed Bethe-Bloch predictions for particles of di erent masses have been
determined from selected control samples.
The measured dE=dx resolution for Bhabha
events is shown in Figure 36. The performance
achieved to date is typically 7.5%, limited by the
number of samples and Landau uctuations. This
value is close to the expected resolution of 7%.
Further re nements and additional corrections
are being considered to improve performance.
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
REFERENCES
1. Celenex 3300-2 polyester thermoplastic with
30% silica glass ber
2. E. Borsato et al., Nucl. Instr. and Methods A451 (2000) 414.
3. Luma Metall AB, Kalmar, Sweden.
4. California Fine Wire, Grover Beach, CA,
USA.
5. A. Berenyi et al., IEEE Trans. Nucl. Sci. 46
(1999), 348; ibid. 46 (1999) 928.
6. J. Albert et al., IEEE Trans. Nucl. Sci. 46
(1999) 2027; A. Bouchan et al., Nucl. Instr.
and Methods A409 (1998) 46; G. Sciolla et
al., Nucl. Instr. and Methods A419 (1998)
310.
7. D. Dorfan et al., Nucl. Instr. and Methods A409 (1998) 310.
8. S.F. Dow et al., IEEE Trans. Nucl. Sci. 46
(1999) 785.
9. CAEN SY527 HV mainframe by CAEN,
Viareggio, Italy.
10. Y. Karyotakis, D. Boutigny, LAPP Annecy,
private communication.
11. Gar eld: Simulation of Gaseous Detectors,
CERN Program Library (1992).
6.9. Conclusions
The drift chamber has been performing close
to design expectations from the start of operations. With the exception of a small number of
wires that were damaged by an unfortunate HV
incident during the commissioning phase, all cells
are fully operational. The DCH performance has
proven very stable over time. The design goal for
3154
7. Drift Chamber
February 6, 2001 { 10 : 40
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
DRAFT
8. Charged Particle Tracking
3201
The principal purpose of the BABAR charged
particle tracking systems, the SVT and the DCH,
is the ecient detection of charged particles and
the measurement of their momentum and angles
with high precision. Among many applications,
these high precision measurements allow for the
reconstruction of exclusive B and D meson decays with high resolution and thus minimal background.
The reconstruction of multiple decay vertices
of weakly decaying B and D mesons, is of prime
importance to the physics goals of BABAR. The
design goal was to achieve a mean spatial resolution for each B decay vertex along the z -axis of
better than 80 m.
Track measurements are also important for the
extrapolation to the DIRC, EMC, and IFR. At
lower momenta the DCH measurements are more
important, while at higher momenta the SVT
measurements dominate. Most critical are the
angles at the DIRC, because the uncertainties in
the charged particle track parameters add to the
uncertainty in the measurement of the Cherenkov
angle. Thus the track errors from the combined
SVT and DCH measurements should be small
compared to the average DIRC angle measurements, i.e., of order of 1 mrad, particularly at the
highest momenta.
Charged particle tracking has been studied
with large samples of cosmic ray muons, e+e; ,
+ ; , and + ; events, as well as multi-hadrons.
At this time, these studies are far from complete
and the results are still preliminary. In particular, many issues related to the intrinsic alignment of the SVT, the variation with time of the
relative alignment of the SVT and the DCH, and
movement of the beam position relative to BABAR
remain under study.
3202
8.1. Track Reconstruction
3241
The reconstruction of charged particle tracks
relies on data from both tracking system, the SVT
and the DCH. Charged tracks are de ned by ve
parameters (d0 ; 0 ; !; z0; tan ) and their associated error matrix. These parameters are measured at the point of closest approach to the z -
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3242
3243
3244
3245
3246
3247
3248
59
axis; d0 and z0 are the distances of this point from
the origin of the coordinate system in the x ; y
plane and along the z -axis, respectively. The angle 0 is the azimuth of the track, the dip angle
relative to the transverse plane, and ! = 1=pt is
its curvature. d0 and ! are signed variables; their
sign depends on the charge of the track. The
track nding and the tting procedures make use
of Kalman lter algorithm [1] that takes into account the detailed distribution of material in the
detector and the full map of the magnetic eld.
The o -line charged particle track reconstruction builds on information available from the L3
trigger and tracking algorithm. It begins with an
improvement of the event start time t0 , obtained
from a t to the parameters d0 ; 0 ; and t0 based
on the four-hit track segments in the DCH superlayers. The next step is to select tracks in the
DCH by performing helix ts to the hits found
by the L3 track nding algorithm. A search for
additional hits in the DCH that may belong on
these tracks is performed, while t0 is further improved by using only hits associated with tracks.
Two more sophisticated tracking procedures are
applied which are designed to nd tracks that either pass through fewer than the full ten DCH
superlayers or do not originate from the interaction point. These algorithms use primarily track
segments that have not already been assigned to
other tracks, and thus bene t from a progressively
cleaner tracking environment with a constantly
improving t0 . At the end of this process, tracks
are t again using a Kalman lter t.
The resulting tracks are then extrapolated into
the SVT, and SVT track segments are added provided they are consistent with the expected error
in the extrapolation through the intervening material and inhomogeneous magnetic eld. Among
the possible SVT segments, those with the smallest residuals and the largest number of SVT layers
are retained and a Kalman t is performed to the
full set of DCH and SVT hits.
Any remaining SVT hits are then passed to
two complementary stand-alone track nding algorithms. The rst reconstructs tracks starting
with triplets of space points (matched and z
hits) in layers 1, 3 and 5 of the SVT, and adding
consistent space points from the other layers. A
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
minimum of four space points are required to form
a good track. This algorithm is ecient over a
wide range of impact parameters and z0 values.
The second algorithm starts with circle trajectories from hits and then adds z hits to form
helices. This algorithm is less sensitive to large
combinatorics and to missing z information for
some of the SVT modules.
Finally, an attempt is made to combine tracks
that are only found by one of the two tracking systems and thus recover pattern recognition problems caused by multiple scattering at the support
tube. The relative alignment of the two tracking
devices and the internal alignment of the SVT
modules are discussed in Chapter 5.
February 6, 2001 { 10 : 40
1
0.9
0.7
0.6
8.2. Tracking Eciency
The eciency for reconstructing tracks in the
DCH has been measured as a function of transverse momentum, polar and azimuthal angles in
multi-track events, as well as the charged track
multiplicity. These measurements rely on speci c
nal states and exploit the fact that the track reconstruction can be performed independently in
the two tracking devices, the SVT and the DCH.
A comparison of the tracking eciency in data
and Monte Carlo simulation has been performed
for a sample of + ; pairs with 1-prong plus 3prong decay topology. The events are selected
without any requirements on the third track of
the 3-prong decay, and the fraction of events
for which all tracks are successfully reconstructed
provides a measurement of the overall reconstruction eciency for tracks in the range 0:2 < pt <
3:0 GeV=c. Initial results indicated that the measured tracking eciency was lower than expected
from the simulations. Since then, a number of
improvements in the track nding software have
been made, and the DCH operating voltage was
raised by 60 V to the design value of 1960 V.
These measures have restored the eciency for
data close to the expected level.
The absolute DCH tracking eciency is determined as the ratio of reconstructed DCH tracks
to the tracks detected in the SVT with the requirement that they fall within the acceptance of
the DCH. Corrections are made for fake tracks in
the SVT and DCH as well as for a bias in the
0.8
0.5
0
0.5
1
1.5
2
Transverse Momentum (GeV/c)
1
0.9
Efficiency
3249
DRAFT
Efficiency
60
0.8
0.7
0.6
0.5
0
0.5
1
1.5
Polar angle
2
2.5
Figure 37. DCH track reconstruction eciency
at operating voltages of 1900 V and 1960 V, as a
function of a) transverse momentum, and b) polar
angle. The eciency is measured in multi-hadron
events as a fraction all tracks detected in the SVT
for which the DCH portion is also reconstructed.
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
track fnding algoriths. Such studies have been
performed for di erent samples of multi-hadron
events. Figure 37 shows the result of one such
study for the two voltage settings. The results of
the di erent samples agree well within the estimated error. For 1900 V, the data show a reduction in eciency by about 5% at close to normal
incidence, indicating that the cells are not fully
ecient at this voltage. At the design voltage of
1960 V, the eciency averages (981)% per track
of more than 200 MeV=c and polar angle > 0:5.
The errors of the measurements are dominated
by the uncertainty in the contamination of SVT
tracks.
As a cross-check and measure of systematic uncertainties in the above method, an analysis is
performed using a sample of D0 ! K ; + + ;
decays, where the D0 is reconstructed from the
decay D+ ! D0 + . All of the tracks from the
D0 decay are required to have SVT hits and the
fraction of D0 for which all four tracks have DCH
tracks is taken as the relative DCH tracking eciency. The results agree with the more inclusive
method to within the estimated error of 1%.
The stand-alone SVT tracking algorithms have
a high eciency for tracks with low transverse
momentum, which do not reach the drift chamber, as illustrated in Figure 38. This feature is
very important for the detection of D decays
which is used for a variety of inclusive studies
of B and D mesons. To study the eciency,
the decays D+ ! D0 + are selected by reconstructing events of the type B ! D+ X followed by D+ ! D0 + ! K ;+ + , where the
pion from the D decay has a soft momentum
spectrum. The majority of these low momentum
tracks do not reach the inner layers of the DCH,
their momentum resolution is in most cases limited by multiple scattering, but the production
angle can be determined from the hits in innermost layers of the SVT. Figure 38 shows the momentum of the slow pions and the mass di erence
M = M (K ; + + ) ; M (K ;+ ), for the total
sample and the subsample of events in which the
slow pion has been reconstructed in both the SVT
and the DCH. To derive an estimate of the tracking eciency for these low momentum tracks, a
detailed Monte Carlo simulation was performed.
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
61
In particular, the spectrum of the pions was derived from simulation of the inclusive D production in BB events, and the events were selected in
the same way as for the data. Contributions for
non-BB events were subtracted. A comparison of
the detected slow pions spectra with the Monte
Carlo prediction is presented in Figure 39. Also
shown is the detection eciency derived from the
Monte Carlo simulation, which is taken as an estimate, based on the good agreement between the
spectra. While there is some remaining concern
about adequate details in the simulation of the
SVT, one can conclude that the SVT signi cantly
extends the capability of the charged particle detection of tracks down to transverse momenta of
50 MeV=c.
b)
Efficiency
3296
DRAFT
Tracks/10 MeV/c
February 6, 2001 { 10 : 40
0.8
0.4
0.0
8000
a)
Data
Simulation
4000
1-2001
8583A27
0
0.0
0.1
0.2
0.3
0.4
Transverse Momentum (GeV/c)
Figure 39. Monte Carlo studies of low momentum
tracking in the SVT a) comparison of data (nonresonant data have been subtracted) and simulation of the transverse momentum spectrum of
pions from D+ ! D0 + in BB events, and b) efciency for pion detection derived from simulated
events.
DRAFT
62
10000
10000
Tracks /10 MeV/c
a)
b)
8000
8000
6000
6000
4000
4000
2000
2000
0
1-2001
8583A26
February 6, 2001 { 10 : 40
0
0.2
0.4
Transverse Momentum (GeV/c)
0
0.140
0.150
M(Kππ) – M(Kπ) (GeV/c2)
Figure 38. Reconstruction of low momentum tracks in the SVT: a) Transverse momentum spectrum of
pions from D+ ! D0 + in BB events, and b) the mass di erence, M = M (K ; + + ) ; M (K ;+ ),
both for all detected events and for events in which the slow pion track is reconstructed both in the SVT
and DCH. Non-resonant data have been subtracted.
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
8.3. Track Parameter Resolutions
The resolution in the ve track parameters is
monitored in OPR using e+ e; and + ; pair
events. It is further investigated o -line for tracks
in multi-hadron events and cosmic ray muons.
Cosmic rays that are recorded during normal
data-taking o er a simple way of measuring the
resolution of track resolution. The upper and
lower halves of the muon tracks traversing the
DCH are t as two separate tracks, and the resolution measurements are taken from the di erence of the measured parameters for the two track
halves. To assure that the tracks pass close to
the beam interaction point, cuts are applied on
the d0 , z0 , and tan . The results of this comparison for the impact parameters and the angles
are shown in Figure 40 for tracks with momenta
above pt of 3 GeV=c. The distributions for the
impact parameters and the angles are symmetric; the non-Gaussian tails are small. The z0 and
tan distributions show a clear o -set, attributed
to residual problems with the internal alignment
of the SVT. Based on the full width at half maxi-
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
mum of these distributions the resolutions for single tracks are
d0 = 23m
z0 = 29m
0 = 0:43 mrad
tan = 0:53 10;3:
The dependence of the resolution in the impact
parameter, d0 and z0 , on the transverse momentum pt is presented in Figure 41. The measurement is based on tracks in multi-hadron events.
The resolution is determined as the width of the
distribution of the di erence between the measured impact parameters, d0 and z0 , and the coordinates of the vertex reconstructed from the remaining tracks in the event. These distributions
peak at zero, but have a tail for positive values
due to the e ect of weak decays. Consequently,
only the negative part of the distributions re ects
the measurement error and is used in the Gaussian t. Event shape cuts and a cut on the 2 of
the vertex t are applied to further reduce the effect of weak decay on this measurement. The contribution from the vertex errors are removed from
the measured resolutions in quadrature. The d0
DRAFT
February 6, 2001 { 10 : 40
63
800
Tracks
a)
b)
c)
d)
400
0
1-2001
8583A29
–0.2
0
4
0
0
0.2 –0.2
0.2 –4
0
4 –4
∆d0 (mm)
∆z0 (mm)
∆Φ0 (mrad)
∆tanλ (10-3)
Figure 40. Resolution for track parameters of cosmic ray muons reconstructed in the SVT and DCH. The
resolution is measured as the di erence between the tted track parameters of the two halves of cosmic
ray tracks; a) d0 , b) z0 , c) 0 , and d) tan .
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
and z0 resolutions so measured are about 25 m
and 40 m respectively at pt = 3 GeV=c and
above. These values agree well with expectations
and Monte Carlo simulation, and are also in reasonable agreement with the results obtained from
cosmic rays.
While the impact parameter and angle measurements near the IP are dominated by the SVT
measurements, the DCH contributes primarily to
the pt measurement. Figure 42 shows the resolution in the transverse momentum derived from
cosmic muons. The data are well represented by
a linear, rather than the usual quadratic function,
pt = (0:13 0:01)% p + (0:45 0:03)%,
t
pt
where the transverse momentum pt is measured
in GeV=c. These values for the resolution parameters are very close to the initial estimates and
are well reproduced by Monte Carlo simulations.
More sophisticated treatment of the DCH timeto-distance relations and overall resolution function are presently under study.
0.3
0.2
0.1
0
1-2001
8583A28
0
1
2
3
Transverse Momentum (GeV/c)
Figure 41. Impact parameter resolution for tracks
in multi-hadron events as a function of the transverse momentum, in the x ; y plane and for the
projection on the z -axis. The data are corrected
for the impact of particle decays and vertexing
errors.
8.4. Conclusions
The two tracking devices, the SVT and DCH,
have been performing close to design expectations
from the start of operations. Studies of track resolution at lower momenta and as a function of
polar and azimuthal angles are still under way.
Likewise, the position and angular resolution at
σz0
σd0
0.4
σ (mm)
3405
3433
3434
3435
the entrance to the DIRC or EMC are still being
studied. Such measurements are very sensitive to
internal alignment of the SVT and relative place-
DRAFT
64
3444
3445
2.0
σ(pt)/pt(%)
3446
3447
3448
3449
3450
1.0
3451
3452
3453
3454
3455
3456
0
1-2001
8583A23
0
4
8
Transverse momentum (GeV/c)
3457
3458
3459
3460
Figure 42. Resolution in the transverse momentum pt determined from cosmic ray muons
traversing the DCH and SVT.
3436
3437
3438
3439
3440
ment of the SVT and the DCH. A better understanding will not only reduce the mass resolution
for the reconstruction of exclusive states, it will
also be particularly important for improvement
of the performance of the DIRC.
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3441
3442
REFERENCES
3478
1. Kalman Filter reference needed here
3480
3479
3481
3482
3483
3484
3485
3486
3487
3443
9. Tracking
3488
3489
3490
February 6, 2001 { 10 : 40
10. DIRC
10.1. Purpose and Design Requirements
The study of CP-violation using hadronic nal
states of the BB meson system requires the ability to tag the avor of one of the B mesons via the
cascade decay b ! c ! s, while fully reconstructing the nal state of the other over a large region
of solid angle and momentum. The momenta of
the kaons used for avor tagging extend up to
about 2 GeV/c, with most below 1 GeV/c. On
the other hand, pions from the rare two-body decays B 0 ! + ; (K ;+ ) must be well-separated
from kaons, and have momenta between 1.5 and
4.5 GeV/c with a strong momentum-polar angle
correlation between the tracks (higher momenta
occur at the more forward angles because of the
c.m. system boost) [1].
The particle identi cation (PID) system inside
the calorimeter volume should be thin and uniform in radiation lengths (to minimize degradation of the calorimeter energy resolution) and thin
in the radial dimension to reduce the volume,
hence, the cost of the calorimeter. Finally, for
high-luminosity running conditions, the PID system must have fast signal response, and be able
to tolerate high backgrounds.
The PID system being used in BABAR is a new
kind of ring imaging detector called the DIRC [2]
(the acronym DIRC stands for detection of internally re ected Cherenkov light). It is expected
to be able to provide =K separation of 4
or greater, for all tracks from B meson decays
with momentum greater than 600 MeV/c. Particle identi cation below 700 MeV/c is provided
by dE/dx measurements in the DCH and SVT.
The DIRC is a ring imaging Cherenkov detector, based on the principle that the magnitudes of
angles are maintained upon re ection from a at
surface. Figure 43 shows a schematic of the DIRC
geometry that illustrates the principles of light
production, transport, and imaging. The radiator
material of the DIRC is synthetic fused silica in
the form of long, thin bars with rectangular cross
section. These bars serve both as radiators and
as light pipes for the portion of the light trapped
in the radiator by total internal re ection. Fused
DRAFT
February 6, 2001 { 10 : 40
PMT + Base
10,752 PMT's
3517
3518
Light Catcher
Purified Water
Standoff
Box
3520
3521
17.25 mm Thickness
(35.00 mm Width)
3522
Bar Box
Track
Trajectory
PMT Surface
Wedge
3523
3524
Mirror
3525
Bar
Window
4.9 m
{
3519
4 x 1.225m Bars
glued end-to-end
1.17 m
{
8-2000
8524A6
3526
3527
3528
3529
3530
3531
Figure 43. Schematics of the DIRC fused silica
radiator bar and imaging region. Not shown is a
6 mrad angle on the bottom surface of the wedge
(see text).
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
silica (Spectrosil [3]) is chosen because of its resistance to ionizing radiation, its large index of
refraction, low chromatic dispersion, long attenuation length, and because it allows an excellent
optical nish on the surfaces of the bars [4].
The variable c is used to designate the
Cherenkov angle, c denotes the azimuthal angle
of the Cherenkov photon around the track direction, and n represents the mean index of refraction of fused silica (n = 1:473), with the familiar
relation cos c = 1=n .
For particles with 1, some photons will always lie within the total internal re ection limit,
and will be transported to either one or both ends
of the bar, depending on the particle incident angle. To avoid having to instrument both bar ends
with photon detectors, a mirror is placed at the
forward end, perpendicular to the bar axis, to
re ect incident photons to the backward (instrumented) bar end.
Once photons arrive at the instrumented end,
most of them emerge into a water- lled expansion region, called the stando box. A fused silica wedge at the exit of the bar re ects photons
at large angles and thereby reduces the size of
the required detection surface. The photons are
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
65
detected by an array of densely packed photomultiplier tubes (PMTs), each surrounded by reecting \light catcher" cones [5] to capture light
which would otherwise miss the PMT active area.
The PMTs are placed about 1.2 m from the bar
end. The expected Cherenkov light pattern at
this surface is essentially a conic section, whose
cone opening-angle is the Cherenkov production
angle modi ed by refraction at the exit from the
fused silica window.
The DIRC is intrinsically a three-dimensional
imaging device. Photons are focused onto the
phototube detection surface via a \pin-hole" dened by the exit aperture of the bar, so that the
photon propagation angles can be measured in
two-dimensional space ( x ; y ). The time taken
for the photon to travel down the bar is also related to the photon propagation angle ( z ) with
respect to the bar axis. As the track position and
angles are known from the tracking system, these
three angles can be used to (over)-determine
the two Cherenkov angles (c ; c ). Imaging in the
DIRC occurs in all three of these dimensions, by
recording the time at which a given PMT is hit.
This over-constraint on the angles is particularly
useful in dealing with ambiguities (see below) and
high background rates.
10.2. Mechanical Design/Physical Description
The DIRC bars are arranged in a 12-sided
polygonal barrel. Because of the beam energy
asymmetry, particles are produced preferentially
forward in the laboratory. To minimize interference with other detector subsystems in the forward region, the DIRC photon detector is placed
at the backward end.
The principal components of the DIRC are
shown schematically in Figs. 44,45. The bars
are placed into 12 hermetically sealed containers,
called bar boxes, made of very thin aluminumhexcel panels. Dry nitrogen gas ows through
each box, and is monitored for humidity to ensure
that the bar box/water interface remains tightly
sealed. Each bar box, shown in Figure 45, in turn
contains 12 long bars, for a total of 144 long bars.
Within a bar box the twelve bars are optically isolated by a 150 m air gap between neighboring
66
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
DRAFT
bars, enforced by custom shims made from aluminum foil.
The bars are 17 mm thick, 35 mm wide, and
4.9 m long. Each long bar is assembled from
four 1.225 m \short" bars that are glued end-toend; that length being the longest high-quality
bar currently obtainable from industry [4].
The bars are supported at 600 mm intervals on
small nylon buttons for optical isolation from the
bar box. Each long bar has a fused silica wedge
glued to it at the readout end. The wedge is made
of the same material as the bar, 91 mm long with
very nearly the same width as the bars (33 mm)
and a trapezoidal pro le (27 mm high at bar end,
and 79 mm at the light exit end). The bottom of
the wedge has a slight ( 6 mrad) upward slope
to minimize the displacement of the downward
re ected image due to the nite bar thickness.
The wedges are glued to a 10 mm thick fused
silica window, which provides the interface and
seal to the puri ed water in the stando box.
The mechanical support of the DIRC, shown
in Figure 44, is cantilevered from the iron of the
instrumented ux return (IFR). The strong support tube (SST) is a steel cylinder located inside
the end-doors of the IFR and provides the basic
support for the entire DIRC. It, in turn, is supported by an iron support gusset that xes the
SST to the Barrel magnet iron. It also minimizes
the magnetic ux gap caused by the DIRC bars
extending through the IFR, and supports the axial load of the inner magnetic plug surrounding
the beam in this region.
The bar boxes are supported in the active region by an aluminum tube, the central support
tube (CST), attached to the SST via an aluminum transition ange. The CST is a thin,
double-walled, cylindrical shell, using aircrafttype construction with stressed aluminum skins
and bulkheads having riveted or glued joints. The
CST also provides the support for the drift chamber.
The stando box is made of stainless steel,
consisting of a cone, cylinder, and 12 sectors of
PMTs. It contains about 6,000 liters of puried water. Water is used to ll this region because it is inexpensive, has an index of refraction
(n 1:346) reasonably close to that of fused sil-
February 6, 2001 { 10 : 40
Figure 44. Exploded view of the DIRC mechanical support structure.
Figure 45. Schematics of the bar box assembly.
3612
3613
3614
3615
3616
3617
ica, thus minimizing the total internal re ection
at their interface, and its chromaticity index is
a close match to that of fused silica, e ectively
eliminating dispersion at the silica-water interface. The iron gusset supports the stando box.
An iron shield, supplemented by a bucking coil,
February 6, 2001 { 10 : 40
DRAFT
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
Figure 47. 1/6 cross-section view of the nominal
DIRC system geometry. All dimensions are given
in mm.
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
surrounds the stando box to reduce the eld in
the PMT region to below 1 Gauss [7].
The PMTs at the rear of the stando box lie
on a surface that is approximately toroidal. Each
of the 12 PMT sectors contains 896 PMTs (ETL
model 9125 [8,9]) with 29-mm diameter, closely
packed, inside the water volume. Each PMT
is mounted from the inside of the stando box
and is connected via a feed-through to a base
mounted outside. A hexagonal \light catcher"
cone is mounted in front of the photocathode of
each PMT, which results in an e ective active surface area light collection fraction of about 90%.
The geometry of the DIRC is shown in an elevation view and a cross-section in Figures 46 and
47.
The DIRC occupies 80 mm of radial space in
the central detector volume including supports
and construction tolerances, with a total radiation length thickness of about 19% at normal incidence. The radiator bars subtend about 94%
of the azimuthal angle and 87% of the center-ofmass polar angle cosine.
The distance from the end of the bar to the
PMTs is 1.174 m, which together with the size of
the bars and PMTs, gives a geometric contribution to the single photon Cherenkov angle resolution of 7 mrad. This is a bit larger than the
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
67
resolution contribution from the photon production (dominated by a 5:4 mrad chromatic term)
and transmission dispersions. The overall single
photon resolution expected is about 9 mrad.
10.2.1. Cherenkov Photon Detection Eciency
Figure 48 shows the contribution of various optical and electronic components of the DIRC to
the Cherenkov photon detection eciency (number of detected photoelectrons per Cherenkov
photon) as a function of wavelength. The data
pertain to a particle entering the center of the
bar at 90 . A typical design goal for the photon
transport in the bar was that no single component
speci cation should be responsible for more than
10{20% loss of detection eciency. To satisfy this
requirement implied an extremely high internal
re ection coecient of the bar surfaces (greater
than 0.9992 per bounce), so that about 80% of the
light is maintained after multiple bounces down
the bars: 365 bounces in the example of Figure 48. The ultraviolet cut-o is 300 nm, determined by the epoxy (Epotek 301-2 [10]) used
to glue the fused silica bars together. The dominant contributor to the overall detection eciency is the quantum eciency of the photomultiplier tube, taken from the manufacturer's
data. On the basis of these data, and taking into
account wavelength independent factors such as
the PMT packing fraction and the geometrical
eciency for trapping Cherenkov photons in the
fused silica bars via total internal re ection, the
number of expected photoelectrons (Npe ) is 28
for a = 1 particle entering normal to the surface
at the center of a bar.
10.2.2. DIRC Water System
The DIRC water system is designed to maintain good transparency at wavelengths as small
as 300 nm. The only sure way to do this is to use
ultra-pure, de-ionized water, close to the theoretical limit of 18 M /cm resistivity. In addition, the
water must be de-gassed and the entire system
kept free of bacteria. To maintain the necessary
level of water quality, all components are made of
stainless steel or of polyvinylidene uoride.
The system contains an input line with six me-
68
DRAFT
February 6, 2001 { 10 : 40
Figure 46. Elevation view of the nominal DIRC system geometry. For clarity, the end plug is not shown.
All dimensions are given in mm.
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
chanical lters (three 10 m, two 5 m, and one
1 m), a reverse osmosis unit, de-ionization beds,
a Te on microtube de-gasser and various pumps
and valves. To prevent bacteria growth, it is
equipped with a UV lamp (254 nm wavelength)
and lters (two 1 m, two 0.2 m, and charcoal
lters). Sampling ports are provided to check the
water quality and to monitor resistivity, temperature, and ow. A gravity feed return system
prevents overpressure. The water volume can be
recirculated up to four times a day.
The operating experience with the water system so far has been very good. The water transparency is routinely measured using lasers of
three di erent wavelengths. The transmission is
better than 92% per meter at 266 nm and exceeds
98% per meter at 325 nm and 442 nm.
Potential leaks from the water seals between
the bar boxes and the stando box are detected
by a water leak detection system of 20 custom
water sensors in and about the bar box slots, two
commercial ultrasonic ow sensors to monitor wa-
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
ter ow in two (normally dry) drain lines in addition to the twelve humidity sensors on a nitrogen
gas output line from each bar box (see below).
Should water be detected, a valve in a 100 mm
diameter drain line is opened, and all the water
is drained in about 12 minutes.
10.2.3. DIRC Gas System
Nitrogen gas from boil-o is used to prevent humidity from condensing on the bars and to detect
water leaks. The gas ows through each bar box
at the rate of 100 ; 200 cm3 /min, and is monitored for humidity to ensure that the seal around
the bar box/water interface remains tight. The
gas is ltered through a molecular sieve and three
mechanical lters to remove particulates (7 m,
0.5 m, and 0.01 m). Dew points of the gas
returned from the bar boxes are typically -40C.
About one third of the input nitrogen gas leaks
from the bar boxes and keeps the bar box slots in
the mechanical support structure free of condensation.
DRAFT
February 6, 2001 { 10 : 40
Transmission or Reflecticity or Q.E.
Water transmission (1.2m)
Mirror reflectivity
Internal reflection coeff. (365 bounces)
Epotek 301-2 transmission (25µm)
EMI PMT 9215B quantum efficiency (Q.E.)
PMT Q.E. ⊗ PMT window transmission
Final Cherenkov photon detection efficiency
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3755
3756
3757
3758
3761
3762
0.8
3763
3764
0.6
3765
3766
0.4
3767
3768
3769
0.2
3770
3771
0
300
400
500
600
700
Figure 48. Transmission, re ectivity and quantum eciency for various components of the
DIRC as a function of wavelength for a = 1
particle at normal incidence to the center of a
bar [11].
3737
3754
3760
Wavelength [nm]
3736
3753
3759
1
200
3735
3752
10.3. Electronics
10.3.1. DIRC PMT Electronics
The DIRC PMT base system contains a single printed circuit board, equipped with surface
mounted components. The operating high voltage (HV) of the PMTs is typically 1.1 kV, with a
range between 900 and 1.3 kV.
Groups of 16 tubes are selected for uniformity
of gain to allow their operation at a common HV
provided from a single distribution board.
The HV is provided by a CAEN SY-527 high
voltage distribution system. Each of the 12 sectors receives HV through 56 high voltage channels, distributed through a single cable bundle.
Each voltage can be set between 0 and 1.6 kV. A
total of 672 high voltage channels are needed for
the entire DIRC system.
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
10.3.2. DIRC Front-End Electronics
69
The DIRC front-end electronics (FEE) is designed to measure the arrival time of each
Cherenkov photon detected by the phototube array [12] to an accuracy that is limited by the intrinsic 1.5 ns transit time spread of the PMTs.
In addition, the electronics also allow the photoelectron charge spectra to be measured in calibration mode to ensure that the PMTs operate
on the HV plateau. However, because the ADC
information is not needed to reconstruct events,
64 PMT are multiplexed onto a single ADC for
monitoring and calibration. The design contains
a pipeline to deal with the level one trigger latency of 12 s, and can handle random background rates of up to 200 kHz/PMT with zero
deadtime.
The DIRC FEE is mounted on the outside
of the stando box and highly integrated in order to minimize cable lengths and to retain the
required single photo-electron sensitivity. Each
of the 168 DIRC front-end boards (DFB) processes 64 PMT inputs, housing eight custom analog chips along with their associated level translators, four custom TDC chips, one 8-bit ash ADC
(FADC), two digitally controlled calibration generators, multi-event bu ers and test hardware.
The PMT signals are ampli ed, and pulse
shaped by an eight channel analog chip. A digital pulse timed with the peak of the input pulse
is output by a zero-crossing discriminator, as well
as a pulse shaped by a CR-RC lter with 80 ns
peaking time. The 80 ns shaping time was chosen
to allow the ADC multiplexing mechanism time
to take place. The multiplexer selects the analog
channel to be digitized by the FADC for calibration.
The TDC chip [13] is a 16-channel TDC with
0.5 ns binning, input bu ering and selective readout of the data in time with the trigger. To cope
with the L1 maximum trigger latency of 12 s
and jitter of 1 s, the selective readout process extracts data in time with the trigger within a programmable time window. The acceptance window width (latency) is programmable between 64
ns and 2 s (16 s) and is typically set at 600 ns.
The six read-out modules (ROMs) are connected
by 1.2 Gbit/s optical bers to twelve DIRC crate
70
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
DRAFT
controllers (DCCs) that form the interface to the
DIRC VME front-end crates.
10.3.3. DAQ Feature Extraction
Raw data from the DFBs are processed in
the ROMs by a feature extraction algorithm before being transmitted to the segment and event
builder. This software algorithm reduces the data
volume by roughly 50% under typical background
conditions. DFB data that contain errors are
agged and discarded. The only data errors seen
to date have been traced to damaged DFBs. Because the data ow system can reliably transmit
at most 32 kBytes/crate, the feature extraction
must sometimes truncate data to limit the event
size. Hit data are replaced with a per-DFB occupancy summary when a ROM's hit occupancy
exceeds 56%, which occurs in roughly one in 104
events. An appropriate ag is inserted into the
feature extraction output whenever truncation or
deletion occurs. Errors, truncation, and feature
extraction performance are continuously monitored online.
10.3.4. DIRC Calibration
The DIRC uses two independent approaches for
a calibration of the a priori unknown PMT time
response and the delays introduced by the frontend electronics chain and the fast control system.
The rst is a conventional pulser calibration, and
the second uses reconstructed tracks from collision data.
The pulser calibration is performed online using a light pulser system, which generates precisely timed 1 ns duration pulses of blue LED
light. This light is transmitted through approximately 47 m long optical bers to di users
mounted on the stando box wall opposite the
PMT detector surface of each of the 12 DIRC
sectors. The pulser produces roughly 10% photoelectron occupancy nearly uniformly throughout the stando box. The LEDs are triggered
by the global fast control calibration strobe command sent to the DCCs. The DCC triggers an
individual LED for each sector upon receipt of
calibration strobe. Pulses in adjacent sectors are
staggered by 50 ns to prevent light crosstalk between sectors. The pulser is run at roughly 2
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
3867
3868
3869
3870
3871
3872
3873
3874
3875
3876
3877
3878
3879
3880
3881
3882
3883
3884
3885
3886
3887
3888
3889
3890
3891
3892
February 6, 2001 { 10 : 40
kHz for the time delay calibration. Histograms of
TDC times for each PMT are accumulated in parallel in the ROMs, and then t to an asymmetric
peaked function. About 65,000 light pulses are
used to determine the mean time delay of each
of the PMTs in the stando box to a statistical
accuracy of better than 0.1 ns. The LED pulser
is also used to monitor the phototube gains using
the ADC readout. As with the TDC calibration,
histograms and ts of the ADC spectrum are accumulated and t in the ROM. A calibration run
including both TDC and ADC information for
all PMTs completes in a few minutes, and is run
once per day. Daily calibrations not only verify
the time delays, but allow defective hardware to
be xed quickly.
The data stream calibration uses reconstructed
tracks from the collision data. For calibration of
the global time delay, the values of uncalibrated
di erences of observed and expected arrival times,
t , are collected during the online prompt reconstruction processing using all DIRC channels. To
calculate the individual channel calibrations, t
values for each DIRC channel are accumulated
until sucient statistics of about 100,000 tracks
are achieved. The collected distribution for each
channel is tted to extract the global time o set
calibration.
The data stream and online pulser calibrations
of the electronic delays and the PMT time response and gain yield fully consistent results, although the data stream results in 15% better timing resolution than the pulser calibration. The
time delay values per channel are typically stable
to an rms of less than 0.1 ns over more than one
year of daily calibrations.
10.3.5. DIRC Environmental Monitoring
System
The DIRC environmental monitoring system is
divided into three parts, corresponding to three
separate tasks. The rst deals with the control
and monitoring of the high voltage system for
the photomultipliers. The second is devoted to
monitoring low voltages related to the front-end
electronics. The third controls a variety of other
detector parameter settings. An interlock system,
based on a standard VME module (SIAM), is pro-
February 6, 2001 { 10 : 40
3893
3894
3895
3896
3897
3898
3899
3900
3901
3902
3903
3904
3905
3906
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
DRAFT
vided. For the purposes of the DIRC, three dedicated VME CPUs are running the application
code. The communication between the HV mainframes and the monitoring crate is achieved by a
CAENET controller (V288). The high voltage
monitor task controls the step sizes for ramping
the HV up or down as well as the communication
of alarm conditions, and the values and limits for
the HV and current of each channel.
The purpose of FEE monitoring is to control
and monitor parameters related to the front-end
electronics. For each DIRC sector, a custom
multi-purpose board, the DCC, equipped with
a micro-controller [14] incorporating the appropriate communication protocol (CANbus) is situated in the same crate as the DFB. All monitoring
and control tasks are implemented on this card.
The parameters monitored are the low voltages
for the DFBs and DCCs, the status of the optical
link (Finisar), the temperature on supply boards
and the VME crate status.
The third part of the monitoring system is
based on a custom ADC VME board (VSAM)
used to monitor various type of sensors: magnetic
eld mappers, an ensemble of 12 beam monitoring scalers, 16 CsI radiation monitors, the level of
the water in the stando box as well as the pHvalue, resistivity and temperature of the water.
10.4. Operational Issues
The DIRC was successfully commissioned and
attained performance close to that expected from
Monte Carlo simulation. The DIRC has been
robust and stable, and indeed, serves also as a
background detector for PEP-II tuning. Figure 49 shows a typical di-muon (e+ e; ! + ; )
event. In addition to the hits caused by the
Cherenkov light from the two tracks, about 500
backgrounds hits can be seen in the readout window of 300 ns. This background is dominated
by low energy photons from the PEP-II machine
hitting the stando box. Some care in machine
tuning is required to stay under a noise limit of
about 200 kHz/tube imposed by limited DAQ
throughput. Lead shielding has been installed
around the beamline components just outside the
backward endcap, and has substantially reduced
this background.
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966
3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981
3982
3983
3984
3985
71
After about 2 years of running, about 99.7% of
all PMTs and electronic channels are still operating with nominal performance.
Corrosion of the PMT glass face plates that are
immersed in the pure water of the stando box
has been observed. For most of the tubes, the observable e ect is typically a slight cloudiness, but
for 50 of the tubes, it is much more pronounced.
Extensive R&D has demonstrated that the corrosion is associated with a loss of sodium and boron
from the surface of the glass. For most tubes, the
corrosion rate is a few microns per year, and is
expected to be acceptable for the full projected
10 year lifetime of the experiment. However, for
the 50 tubes, the wrong glass was used by the
PMT manufacturer. This glass did not contain
zinc, which makes it much more susceptible to
rapid leaching. This leaching may eventually lead
to either a loss of performance, or some risk of mechanical failure of the face plates, for these tubes.
Direct measurements of the number of Cherenkov
photons observed in di-muon events as a function
of time suggest that the total loss of photons from
all sources is less than 2%/year, although the accuracy of this number is limited by a number of
systematic e ects at this time.
10.5. Data Analysis and Performance
Figure 49 shows the pattern of Cherenkov
photons in a di-muon event, before and after
reconstruction. The time distribution of real
Cherenkov photons from a single event is of order
50 ns wide, and during normal data taking they
are accompanied by hundreds of random photons
in a at background within the trigger acceptance
window. Given a track pointing at a particular
fused silica bar and a candidate signal in a PMT
within the optical phase space of that bar, the
Cherenkov angle is uniquely determined up to a
sixteen fold ambiguity: top/bottom, left/right,
forward/backward and wedge/no-wedge re ections. The goal of the reconstruction program
is to associate the correct track with the candidate PMT signal, with the requirement that the
transit time of the photon from its creation in the
bar to its detection at the PMT is consistent with
the measurement error of 1:5 ns.
72
DRAFT
February 6, 2001 { 10 : 40
Figure 49. Display of one e+ e; ! + ; event reconstructed in BABAR with two di erent time cuts. On
the left, all DIRC PMTs that were hit within the 300 ns trigger window are shown. On the right, only
those PMTs that were hit within 8 ns of the expected Cherenkov photon arrival time are displayed.
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
10.5.1. Reconstruction
We use an unbinned maximum likelihood formalism to incorporate all information provided by
the space and time measurements from the DIRC.
The emission angle and the arrival time of
the Cherenkov photons are reconstructed from
the observed space-time coordinates of the PMT
hits, transformed into the Cherenkov coordinate
system. The known spatial position of the bar
through which the track passed and the PMTs
hit within the readout window of 300 ns of
the trigger signal is used to calculate the threedimensional vector pointing from the center of
the end of the bar to the center of each tube.
This vector is then extrapolated into the synthetic fused silica bar (using Snell's law). This
procedure de nes, up to the 16 fold ambiguity
described above, the Cherenkov angles c and c
of a photon.
The DIRC time measurement represents the
third dimension of the photomultiplier hit reconstruction. The timing resolution is not
competitive with the positional information for
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
Cherenkov angle reconstruction, but timing information is used to suppress background hits from
the accelerator and, more importantly, exclude
other tracks in the same event as the source of
the photon. Timing information is also used to
resolve the forward-backward and wedge ambiguities in the hit-to-track association.
The relevant observable to distinguish between
signal and background photons is the di erence
between the measured and expected photon arrival time, t . It is calculated for each photon
using the track time-of- ight (assuming it to be
a charged pion), the measured time of the candidate signal in the PMT and the photon propagation time within the bar and the water lled
stando box. The time information and the requirement of using only physically possible photon propagation paths that are within the region
of total internal re ection reduces the number of
ambiguities from 16 to typically 3. Applying the
time information also substantially improves the
correct matching of photons with tracks and reduces the number of accelerator induced back-
February 6, 2001 { 10 : 40
4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
entries per mrad
4034
ground hits by approximately a factor 40, as can
be seen in Figure 49.
The reconstruction routine currently provides a
likelihood value for each of the ve stable particle
types (e,,,K,p) if the track passes through the
active volume of the DIRC. These likelihoods are
calculated in an iterative process by maximizing
the likelihood value for the entire event while testing di erent hypotheses for each track. If enough
photons are found, the result of a t of c and
the number of observed signal and background
photons are calculated for each track.
80000
(a)
60000
40000
20000
0
-100
-50
0
50
100
∆ θC,γ (mrad)
10.5.2. Results
The parameters of expected DIRC performance
were derived from extensive studies with a variety
of prototypes, culminating with a full-size prototype in a series of test beam runs at CERN [15].
The results were well-described by Monte Carlo
simulations of the detector. The present results
are close to expectations, and additional oine
work, particularly on geometrical alignment, is
expected to lead to further improvements.
In the absence of correlated systematic errors,
the resolution (C;track ) on the track Cherenkov
angle should scale as
p
C;track = C; = Npe ;
(5)
where C; is the single photon Cherenkov angle
resolution, and Npe is the number of photons detected. Figure 50(a) shows the single photon resolution obtained for photoelectrons from di-muon
events, e+ e; ! + ; . The average single photon resolution obtained is about 10.2 mrad, about
10% worse than the expected value of about
9 mrad. There is a broad background of less than
10% relative height under the peak, that originates mostly from track-associated sources. The
time resolution obtained, shown in Figure 50(b),
is 1.7 ns, close to the 1.5 ns value expected from
the single-photon resolution of the PMTs.
The number of photoelectrons shown in Figure 51 varies from a minimum of about 20 for
small polar angles at the center of the barrel to
well over 50 at large polar angles. This is in
good agreement with the value expected from the
Monte Carlo simulation at all angles. The shape
of the distribution can be understood as follows:
73
entries per 0.2ns
4033
80000
(b)
60000
40000
20000
0
-5
0
5
∆ tγ (ns)
Figure 50. Resolution of (a) the reconstructed
Cherenkov polar angle for single photons and
(b) the di erence between measured and expected
arrival time.
80
60
<Nγ>
4032
DRAFT
Data
Monte Carlo Simulation
40
20
0
-1
-0.5
0
0.5
1
cos(θtrack)
Figure 51. Number of detected photoelectrons
vs. track polar angle for reconstructed di-muon
events in data and simulation.
DRAFT
74
+ –
+ –
e e →µ µ
σ(θC) = 2.5 mrad
(σ)
0
0
θC(measured) – θC(µ)
10
4080
4081
4082
4083
4084
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
4100
4101
4102
4103
4104
4105
4106
4107
Using tracks at cos c = 0 as a reference, the number of photons initially decreases as the track angle moves in the forward (or backward) direction
because fewer Cherenkov photons are trapped by
total internal re ection in the bar. At larger values of j cos j, the number of photons increases
for two reasons: 1) the path length of the track,
and therefore the number of Cherenkov photons,
increases in the fused silica, and 2) the fraction
of photons trapped in the silica by total internal
re ection increases.
This spectrum also demonstrates a very useful feature of the DIRC in the BABAR environment, namely, the performance improves (see
equation 1) in the forward direction, as is needed
to cope with the angle-momentum correlation of
particles from the boost.
With the present alignment, the average track
Cherenkov angle resolution for di-muon events is
shown in Figure 52. The width of the tted Gaussian is 2.5 mrad. This is about 15% worse than
the 2.2 mrad expected from simulation. From
the measured single track resolution vs: momentum in di-muon events and the di erence between
the expected Cherenkov angles of charged pions
and kaons, the pion-kaon separation power of the
DIRC can be inferred. As shown in Figure 53, the
separation between kaons and pions at 3 GeV/c
is about 4.2 , approximately 15% worse than the
design goal.
+ –
4
2
0
2
(mrad)
Figure 52. Resolution of the reconstructed
Cherenkov polar angle per track for di-muons.
4079
0
B →π π
6
3
Momentum
4
(GeV/c )
Figure 53. -K separation in B 0 ! + ;
events vs: track momentum inferred from the
measured Cherenkov angle resolution and number of Cherenkov photons per track in di-muon
events.
Without DIRC
x 10 2
1500
entries per 5 MeV/c
-10
8
Expected π-K Separation
40000
20000
4078
10
2
Tracks
60000
February 6, 2001 { 10 : 40
1000
With DIRC
500
0
1.75
1.8
1.85
1.9
1.95
2
Kπ mass (GeV/c )
Figure 54. Invariant K inclusive mass spectrum
without and with the use of the DIRC for kaon
identi cation. The mass peak corresponds to the
decay of the D0 particle.
DRAFT
π Mis-ID as K Kaon Efficiency
February 6, 2001 { 10 : 40
1
4137
0.9
4138
0.8
4139
4140
0.7
4141
0.6
4142
0.2
4143
4144
0.1
0
4145
1
2
3
Track Momentum (GeV/c)
Figure 55. Eciency and mis-identi cation probability for the selection of charged kaons as a function of track momentum determined using D0 decays selected kinematically from inclusive D production.
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
4119
4120
4121
4122
4123
4124
4125
4126
4127
4128
4129
4130
4131
4132
4133
4134
4135
4136
Figure 54 shows an example of the use of the
DIRC for sample selection. The K invariant two
body mass spectra are shown without and with
the use of the DIRC for kaon identi cation. The
mass peak corresponds to the decay of the D0
particle.
The eciency for correctly identifying a
charged kaon that hit a radiator bar and the probability to wrongly identify a pion as kaon are determined using D0 decays selected kinematically
from inclusive D production and are shown as
a function of the track momentum in Figure 55
for a particular choice of particle selection criteria. The mean kaon selection eciency and
pion mis-identi cation are 96:2 0:2% (stat.) and
2:1 0:1% (stat.), respectively.
10.6. Conclusions
The DIRC is a novel ring imaging Cherenkov
detector that is well-matched to the hadronic particle identi cation requirements of BABAR. The
DIRC has been robust and stable and, two years
after installation, about 99.7% of all PMTs and
electronic channels are still operating with nominal performance. Additional shielding in the
stando box tunnel region should reduce the sensitivity to beam related backgrounds, as should
faster front-end electronics, both installed during
the winter 2000-2001 shutdown. At luminosities
around 1 1034=cm2 sec, the TDC chip will have
4146
4147
4148
4149
4150
4151
4152
4153
4154
4155
4156
4157
4158
4159
4160
4161
4162
4163
4164
4165
4166
4167
4168
4169
4170
4171
4172
4173
4174
4175
4176
4177
4178
4179
4180
4181
4182
75
to be replaced with a faster version and deeper
bu ering. The design process for this is underway.
The initial detector performance obtained is already rather close to that predicted by the Monte
Carlo simulations. Alignment and further code
developments are underway which are expected
to further improve performance.
REFERENCES
1. The BABAR Collaboration, The BABAR
Physics Book, SLAC-R-504 (1998).
2. B. N. Ratcli , SLAC-PUB-5946 (1992); B. N.
Ratcli , SLAC-PUB-6067 (1993); P. Coyle et
al., Nucl. Instr. and Methods A343 (1994)
292.
3. Spectrosil is a trademark of TSL Group PCL,
Wallsend, Tyne on Wear, NE28 6DG, England; Sold in the USA by Quartz Products
Co., 160 W. Lee Street, Louisville, Kentucky
40201.
4. I. Adam et al., IEEE Trans. Nucl. Sci., Vol.
45, No. 3 (June 1998) 657; I. Adam et al.,
ibid 450; J. Cohen-Tanugi, M. C. Convery,
B. N. Ratcli , X. Sarazin, J. Schwiening,
J. Va'vra, SLAC-JOURNAL-ICFA-21, ICFA
Instrumentation Bulletin, Fall 2000 Issue.
5. M. Benkebil et al., Nucl. Instr. and Methods A442, 364 (2000).
6. Boeing Optical Fabrication, Albuquerque,
NM, USA.
7. E. Antokhin et al., Nucl. Instr. and Methods A432, 24 (1999).
8. Electron Tubes Limited (formerly: Thorn
EMI Electron Tubes), Ruislip, Middlesex
HA47TA, England.
9. P. Bourgeois, M. Karolak and G. Vasseur,
Nucl. Instr. and Methods A442, 105 (2000).
10. Epoxy Technology, Inc., MA, USA.
11. J. Va'vra, Nucl. Instr. and Methods A453,
262 (2000).
12. P. Bailly et al., Nucl. Instr. and Methods A433, 450 (1999).
13. P. Bailly et al., Nucl. Instr. and Methods A433, 432 (1999).
14. The micro-controller is a MC68HC05x32, Motorola Inc., Schaumburg, IL, USA.
76
4183
4184
DRAFT
15. R. Aleksan et al., Nucl. Instr. and Methods A397 (1997) 261.
4185
February 6, 2001 { 10 : 40
11. Electromagnetic Calorimeter
4186
4187
4188
4189
4190
4191
4192
4193
4194
4195
4196
4197
4198
4199
4200
4201
4202
4203
4204
4205
4206
4207
4208
4209
4210
4211
4212
4213
4214
4215
4216
4217
4218
4219
4220
4221
4222
4223
4224
4225
4226
4227
4228
4229
4230
11.1. Requirements and Design
11.1.1. Requirements
The electromagnetic calorimeter (EMC) is designed to measure electromagnetic showers with
excellent eciency, and energy and angular resolution over the energy range from 20 MeV to
9 GeV. This capability allows the detection of
photons from 0 and decays as well as from electromagnetic and radiative processes. By identifying electrons, the EMC contributes to the avor
tagging of neutral B mesons via semi-leptonic decays, to the reconstruction of vector mesons like
J= , and the study of semi-leptonic and rare decays B and D mesons and leptons. The upper
bound of the energy range is set by the need to
measure QED processes, like e+ e; ! e+ e; ( )
and e+ e; ! , for calibration and luminosity
determination. The lower bound is set by the
need for highly ecient reconstruction of B meson decays containing multiple 0 and .
The measurement of extremely rare decays of
B mesons containing 0 s (e.g. B 0 ! 0 0 ) poses
the most stringent requirements on energy resolution, namely of order 1% to 2%. Below energies
of 2 GeV; the 0 mass resolution is dominated by
the energy resolution. At higher energies, the angular resolution becomes dominant, and therefore
it is required to be of the order of a few mrad.
Furthermore, the EMC has to be compatible
with the 1.5 T eld of the solenoid and operate reliably over the anticipated ten-year lifetime of the
experiment. To achieve excellent resolution, stable operating conditions have to be maintained.
Temperatures and the radiation exposure must
be closely monitored, and precise calibrations of
the electronics and energy response over the full
dynamic range must be performed frequently.
11.1.2. Design Considerations
The requirements stated above lead to the
choice of a hermetic, total-absorption calorimeter,
composed of a nely segmented array of thalliumiodide-doped cesium iodide (CsI(Tl)) crystals.
The crystals are read out with silicon photodiodes that are matched to the spectrum of scin-
February 6, 2001 { 10 : 40
4231
4232
4233
4234
4235
4236
4237
4238
4239
4240
4241
4242
4243
4244
4245
4246
4247
4248
4249
4250
4251
4252
4253
4254
4255
4256
4257
4258
4259
4260
4261
4262
4263
4264
4265
4266
4267
4268
4269
4270
4271
4272
4273
4274
DRAFT
tillation light. Recent experience at CLEO [1]
has demonstrated the suitability of this choice for
physics at the (4S ) resonance.
The energy resolution of a homogeneous crystal
calorimeter can be described empirically in terms
of a sum of two terms added in quadrature
E = p a
(6)
E 4 E ( GeV) b;
where E and E refer to the energy of a photon
and its rms error, measured in GeV. The energy dependent term a arises primarily from the
uctuations in photon statistics, but it is also impacted by electronic noise of the photon detector
and electronics. Furthermore, beam-generated
background will lead to large numbers of additional photons that add to the noise. This term
is dominant at low energies. The constant term,
b, is dominant at higher energies (> 1 GeV) and
it arises from non-uniformity in light collection,
from leakage or absorption in the material between and in front of the crystals, and from uncertainties in the calibrations. Most of these effects can be in uenced by design choices, and they
are stable with time. Others will be impacted by
changes in the operating conditions, like variations in temperature, electronics gain, and noise,
as well as by radiation damage caused by beamgenerated radiation.
The angular resolution is determined by the
transverse crystal size and the distance from the
interaction point. It can also be empirically parameterized as a sum of an energy dependent and
a constant term,
= = p c
d;
(7)
E ( GeV)
where the energy E is measureed in GeV. The
design of the EMC required a careful optimization of a wide range of choices, from the crystal
material and dimensions to the choice of the photon detector and readout electronics to the design
of a calibration and monitoring system. These
choices were made on the basis of extensive studies, prototyping and beam tests [2], and Monte
Carlo simulation, taking into account limitations
of space and the impact of other BABAR detector
systems.
4275
4276
4277
4278
4279
4280
4281
4282
4283
4284
4285
4286
4287
4288
4289
4290
4291
4292
4293
4294
4295
4296
4297
4298
4299
4300
4301
4302
4303
4304
4305
4306
4307
4308
4309
4310
4311
4312
4313
4314
4315
4316
4317
4318
4319
4320
77
Under ideal conditions, values for the energy
resolution parameters a and b close to 1% to 2%
could be obtained. A position resolution of a mm
will translate into an angular resolution of a few
mrad, i.e., values of parameters c 3 mrad and
d 1 mrad.
However in practice, for a large system with
a small, but unavoidable amount of inert material and gaps, limitations of electronics, and
background in multi-particle events, plus contributions from beam-generated background, such
performance is very dicult to achieve.
Though in CsI(Tl) the intrinsic eciency for
the detection of photons is close to 100% down
to a few MeV, the minimum measurable energy in colliding beam data is expected to be
about 20 MeV, a limit that is largely determined
by beam- and event-related background and the
amount of material in front of the calorimeter.
Because of the sensitivity of the 0 eciency to
the minimum detectable photon energy it is extremely important to keep the amount of material
in front of the EMC to the lowest possible level.
11.1.3. CsI(Tl) Crystals
Thallium-doped CsI meets the needs of BABAR
in several ways. Its properties are listed in Table 11. The high light yield and small Moliere
radius allow for excellent energy and angular resolution, while the short radiation length allows
for shower containment at BABAR energies with a
relatively compact design. Furthermore, the high
light yield and the emission spectrum permit efcient use of a silicon photo-diodes which operate well in high magnetic elds. The transverse
size of the crystals is chosen to be comparable to
the Moliere radius achieving the required angular resolution at low energies while appropriately
limiting the total number of crystals (and readout
channels).
11.2. Layout and Assembly
11.2.1. Overall Layout
The EMC consists of a cylindrical barrel and
a conical forward endcap. It has full coverage in azimuth and extends in polar angle from
cos = 0:96 to cos = ;0:77 for a solid-angle
coverage of 90% in the c.m. system (Figure 56
DRAFT
78
February 6, 2001 { 10 : 40
2359
1555
1375
38.2˚
2295
1127
920
1801
558
Interaction Point
1979
26.8˚
22.7˚
15.8˚
11-2000
8572A03
Figure 56. A longitudinal cross-section of the EMC (only the top half is shown) indicating the arrangement
of the 56 crystal rings. The detector is axially symmetric around the z axis. All dimensions are given in
mm.
Table 11
Properties of CsI(Tl) .
Parameter
Radiation Length
Moliere Radius
Density
Light Yield
Light Yield Temp. Coe .
Peak Emission max
Refractive Index (max )
Signal Decay Time
4321
4322
4323
4324
4325
4326
4327
4328
4329
4330
Table 12
Layout of the EMC, composed of 56 axially symmetric rings, each consisting of CsI crystals of
identical dimensions.
cos Length # Crystals
Interval (X0 ) Rings /Ring
Barrel
;0:774 ; 0:349 16.0
27
120
0:350 ; 0:620 16.5
7
120
0:621 ; 0:793 17.0
7
120
0:794 ; 0:890 17.5
7
120
Endcap
0:892 ; 0:822 17.5
3
120
0:923 ; 0:947 17.5
3
100
0:948 ; 0:955 17.5
1
80
0:956 ; 0:962 16.5
1
80
Values
1.85 cm
3.8 cm
4.53 g=cm3
50,000 = MeV
0.28%/C
565 nm
1.80
680 ns (64%)
3.34 s (36%)
and Table 12). The barrel contains 5,760 crystals
arranged in 48 distinct rings with 120 identical
crystals each. The endcap holds 820 crystals arranged in 8 rings, adding up to a total of 6,580
crystals. The crystals have a tapered trapezoidal
cross-section. The length of the crystals increases
from 29:6 cm in the backward to 32:4 cm in the
forward direction to limit the e ects of shower
leakage from the increasing average particle energy.
4331
4332
4333
4334
4335
4336
To minimize the probability of pre-showering,
the crystals are supported at the outer radius,
with only a thin gas seal at the front. The barrel
and outer ve rings of the endcap have less than
0:3;0:6Xo of material in front of the crystal faces.
The SVT support structure and electronics, as
February 6, 2001 { 10 : 40
4337
4338
4339
4340
4341
4342
4343
4344
4345
4346
4347
4348
4349
4350
4351
4352
4353
4354
4355
4356
4357
4358
4359
4360
4361
4362
4363
4364
4365
4366
4367
4368
4369
4370
4371
4372
4373
4374
4375
4376
4377
4378
4379
4380
4381
4382
DRAFT
well as the B1 dipole shadow the inner three rings
of the endcap, resulting in up to 3.0 Xo for the
innermost ring. The principal purpose of the two
innermost rings is to enhance shower containment
for particles incident at small polar angles.
Output
Cable
Fiber Optical Cable
to Light Pulser
11.2.3. Photo-Diodes and Preampli er Assembly
The photon detector consists of two 2 1 cm2
silicon PIN diodes glued to a transparent 1.2 mm-
Preamplifier
Board
Diode
Carrier
Plate
11.2.2. Crystal Fabrication and Assembly
The crystals were grown in boules from a melt
of CsI salt doped with 0.1% Thallium [3]. They
were cut from the boules, machined into tapered
trapezoids (Figure 57) to a tolerance of 150 m
and then polished [4]. The transverse dimensions
of the crystals for each of the 56 rings vary to
achieve the required hermetic coverage. The typical area of the front face is 4:7 4:7 cm, while the
back face area is typically 6:1 6:0 cm. The crystals act not only as a total-absorption scintillating
medium, but also as a light guide to collect light
at the photo-diodes that are mounted on the rear
surface. At the polished crystal surface light is
internally re ected, and a small fraction is transmitted. The transmitted light is recovered in part
by wrapping the crystal with two layers of di use
white re ector [5] [6], each 165 m thick . The
uniformity of light yield along the wrapped crystal was measured by recording the signal from a
highly collimated radioactive source at 20 points
along the length of the crystal. The light yield
was required to be uniform to within 2% in the
front half of the crystal; the limit increased linearly up to a maximum of 5% at the rear face.
Adjustments were made on individual crystals to
meet these criteria by selectively roughing or polishing the crystal surface to reduce or increase its
re ectivity.
Following these checks, the crystals were further wrapped in 25 m thick aluminum foil which
was electrically connected to the metal housing
of the photo-diodes/preampli er assembly to provide a Faraday shield. The crystals were covered
on the outside with a 13 m thick layer of mylar to assure electrical isolation from the external
support.
79
Aluminum
Frame
Silicon
Photo-diodes
TYVEK
(Reflector)
Aluminum
Foil
(R.F. Shield)
CsI(Tl) Crystal
Mylar
(Electrical
Insulation)
CFC
Compartments
(Mechanical
Support)
11-2000
8572A02
Figure 57. A schematic of the wrapped
CsI(Tl) crystal and the front-end readout package
mounted on the rear face. Also indicated is the
tapered, trapezoidal CFC compartment, which is
open at the front. This drawing is not to scale.
4383
4384
4385
4386
4387
4388
4389
4390
4391
4392
4393
4394
thick substrate of polysterene that in turn is glued
to the center of the rear face of the crystal by
an optical epoxy [7] to maximize light transmission [8]. The surrounding area of the crystal face
is covered by a plastic plate coated with white
re ective paint [9]. The plate has two 3- mmdiameter penetrations for the bers of the light
pulser monitoring system.
As part of the quality control process, the
1.836 MeV photon line from a 88 Y radioactive
source was used to measure the light yield of every crystal-diode assembly, employing a Canberra
80
4395
4396
4397
4398
4399
4400
4401
4402
4403
4404
4405
4406
4407
4408
4409
4410
4411
4412
4413
4414
4415
4416
4417
4418
4419
4420
4421
4422
4423
4424
4425
4426
4427
4428
4429
4430
4431
4432
4433
4434
4435
4436
DRAFT
2003T preampli er with 2 s Gaussian shaping.
The resulting signal distribution has a mean and
rms of 7300 photo-electrons/ MeV and 890 photoelectrons/ MeV, respectively, with no crystal below 4600 photo-electrons/ MeV [8]. 19
Each of the diodes is directly connected to a
low-noise preampli er. The entire assembly is enclosed by an aluminium xture as shown in Figure 57. This xture is electrically coupled to the
aluminium foil wrapped around the crystal and
thermally coupled to the support frame to dissipate the heat load from the preampli ers.
Extensive aging tests were performed to ascertain that the diodes and the preampli ers met the
ten-year life time requirements. In addition, daily
thermal cycles of 5oC were run for many months
to assure that the diode-crystal epoxy joint could
sustain modest temperature variations.
February 6, 2001 { 10 : 40
Aluminum
Strongback
Aluminum Support Cylinder
Electronic Mini-Crates
CFC Crystal
Support Tubes
0.9 m
0.48 m
Aluminum
RF Shield
4.04 m
10-2000
8572A1
11.2.4. Crystal Support Structure
The crystals are inserted into modules that are
supported individually from an external support
structure. This structure is built in three sections, a cylinder for the barrel and two semicircular structures for the forward endcap. The
barrel support cylinder carries the load of the barrel modules plus the forward endcap to the magnet iron through four exible supports. These
supports decouple and dampen any acceleration
induced by movements of the magnet iron during
a potential earthquake.
The modules are built from tapered, trapezoidal compartments made from carbon- berepoxy composite (CFC) with 300 m thick walls
(Figure 58). Each compartment loosely holds
a single wrapped and instrumented crystal and
thus assures that the forces on the crystal surfaces never exceed its own weight. Each module
is surrounded by an additional layer of 300 m
CFC to provide additional strength. The modules are bonded to an aluminum strong-back that
is mounted on the external support. This scheme
minimizes inter-crystal materials while exerting
19 The calibration procedure employed in this measure-
ment introduces a dependency of the light yield on the
shaping time of the preampli er. When connected to the
actual front-end electronics in the BABAR detector, the signal is reduced by a factor 1.29.
Figure 58. The support structure and assembly
of the EMC barrel.
4437
4438
4439
4440
4441
4442
4443
4444
4445
4446
4447
4448
4449
4450
4451
4452
4453
4454
4455
4456
4457
4458
4459
minimal force on the crystal surfaces; this prevents deformations and surface degradation that
could compromise performance. By supporting
the modules at the back, the material in front of
the crystals is kept to a minimum.
The barrel section is divided into 280 separate modules, each holding 21 crystals (7 3 in
). After the insertion of the crystals, the
aluminum readout frames, which also sti en the
module, are attached with thermally-conducting
epoxy to each of the CFC compartments. The
entire 100 kg-module is then bolted and again
thermally epoxied to an aluminum strong-back.
The strong-back contains alignment features as
well as channels to couple into the cooling system.
Each module was installed into the 2.5 cm-thick,
4 m-long aluminum support cylinder, and subsequently aligned. On each of the thick annular
end- anges this cylinder contains access ports for
digitizing electronics crates with associated cooling channels, as well as mounting features and
alignment dowels for the forward endcap.
The endcap is constructed from 20 identical
February 6, 2001 { 10 : 40
4460
4461
4462
4463
4464
4465
4466
4467
4468
4469
4470
4471
4472
4473
4474
4475
4476
4477
4478
4479
4480
4481
4482
4483
4484
4485
4486
4487
4488
4489
4490
4491
4492
4493
4494
4495
4496
4497
4498
4499
4500
4501
4502
4503
4504
4505
DRAFT
CFC modules (each with 41 crystals), individually aligned and bolted to one of two semi-circular
support structures. This vertical split into two
halves was necessary to facilitate access to the
central detector components.
The entire calorimeter is surrounded by a double Faraday shield composed of two 1 mm-thick
aluminum sheets so that the diodes and preampli ers are further shielded from external noise.
This cage also serves as the environmental barrier, allowing the slightly hygroscopic crystals to
reside in a dry, temperature controlled nitrogen
atmosphere.
11.2.5. Cooling System
The EMC is maintained at constant, accurately monitored temperature. Of particular concern are the stability of the photo-diode leakage
current which rises exponentially with temperature, and the large number of diode-crystal epoxy
joints that could experience stress due to di erential thermal expansion. In addition, the light
yield of CsI(Tl) is weakly temperature dependent.
The primary heat sources internal to
the calorimeter are the preampli ers (2 50 mW/crystal) and the digitizing electronics
(3 kW per end- ange). In the barrel, the preampli er heat is removed by conduction to the
module strong backs which are directly cooled
by Fluorinert (polychlorotri uoro-ethylene) [10].
The digitizing electronics are housed in 80 minicrates, each in contact with the end- anges of
the cylindrical support structure. These crates
are indirectly cooled by chilled water pumped
through channels milled into the end anges
close to the inner and outer radii. A separate
Fluorinert system in the endcap cools both the
20 mini-crates of digitizing electronics and the
preampli ers.
11.3. Electronics
The EMC electronics system, shown schematically in Figure 59, is required to have negligible
impact on the energy resolution of electromagnetic showers from 20 MeV to 9 GeV, while accommodating the use of a 6.13 MeV radioactive
source for calibration. These requirements set a
limit of less than 250 keV equivalent noise energy
4506
4507
4508
4509
4510
4511
4512
4513
4514
4515
4516
4517
4518
4519
4520
4521
4522
4523
4524
4525
4526
4527
4528
4529
4530
4531
4532
4533
4534
4535
4536
4537
4538
4539
4540
4541
4542
4543
4544
4545
4546
4547
4548
4549
4550
4551
4552
81
(ENE) per crystal and de ne an 18-bit e ective
dynamic range of the digitization scheme. For
source calibrations, the least signi cant bit is set
to 50 keV, while for colliding beam data it is set
to 200 keV. To reach the required energy resolution at high energies, the coherent component
has to be signi cantly smaller than the incoherent noise component. In addition, it is important that the impact of high rates of low energy
(<5 MeV) beam-induced photon background be
minimized.
11.3.1. Photo-Diode Readout and Preampli ers
The ENE is minimized by maximizing the light
yield and collection, employing a highly ecient
photon detector, and a low-noise electronic readout. The PIN silicon photo-diodes [11] have a
quantum eciency of 85% for the CsI(Tl) scintillation light [12]. At a depletion voltage of 70 V,
their typical dark current wre measured to be
4 nA for an average capacitance of 85 pF; the
diodes are operated at a votage of 50 V. The
input capacitance to the preampli er is minimized by connecting the diodes to the preamplier with a very short cable. The preampli er is a
low-noise charge-sensitive ampli er implemented
as a custom application speci c integrated circuit (ASIC) [13]. It shapes the signal and acts
as a band-pass lter to remove high- and lowfrequency noise components. The optimum shaping time for the CsI(Tl)-photodiode readout is
2 ; 3 s, but a shorter time was chosen to reduce
the probability of overlap with low-energy photons from beam background. The commensurate
degradation in noise performance is recovered by
implementing a real-time digital signal-processing
algorithm following digitization.
To achieve the required operational reliability [14] for the inaccessible front-end readout components, two photo-diodes were installed, each
connected to an preampli er. In addition, all
components were carefully selected and subjected
to rigorous tests, including a 72-hour burn-in of
the preampli ers at 70 C to avoid infant mortality. The dual signals are combined in the postampli cation/digitization circuits, installed in minicrates at the end- anges, a location that is acces-
DRAFT
82
on Crystal
Amplifier
Crystal
with
two
Diodes
A
February 6, 2001 { 10 : 40
on-detector
x1
x32
CARE
Chip
off-detector
Rangebits
ROM
Link
x1
Mem
x4
B
x1
x32
ADC
select, x32 10-bit
A,B, or
ABavg x256
Optical
Fiber
Link
Event
Data
Sum
Trigger
Data
11-2000
8572A04
Figure 59. Schematic diagram of the EMC readout electronics.
4554
4555
4556
4557
4558
4559
4560
4561
4562
4563
4564
4565
4566
4567
4568
4569
4570
4571
4572
4573
4574
4575
4576
4577
4578
4579
4580
4581
4582
sible for maintenance.
Filtered
11.3.2. Postampli cation, Digitization and
Readout
The two preampli ers on each crystal, A and
B, each provide ampli cation factors of 1 and 32
and thus reduce the dynamic range of the signal that is transmitted to the mini-crates to 13bits. A custom auto-range encoding (CARE) circuit [13] further ampli es the signal to arrive at
a total gain of 256, 32, 4 or 1 for four energy
ranges, 0-50 MeV, 50-400 MeV, 0.4-3.2 GeV, and
3.2-13.0 GeV, respectively. The appropriate range
is identi ed by a comparator and the signal is digitized by a 10-bit, 3.7 MHz ADC. Data from 24
crystals are multiplexed onto a ber-optic driver
and sent serially at a rate of 1.5 Gbits/s across a
30 m-long optical ber to the ROM. In the ROM,
the continuous data stream is entered into a digital pipeline. A correction for pedestal and gain
is applied to each sample. The pipeline is then
tapped to extract the input to the calorimeter
trigger.
Upon receipt of the L1 Accept signal, data
samples within a time window of 1 s are selected for the feature extraction. Up to now, the
calorimeter feature extraction algorithm performs
a parabolic t to the peak of the signal waveform
to derive its energy and time. In the future, it is
planned to employ a digital lter prior to the signal t to further reduce noise. For this lter algo-
Channels / 0.01 (MeV)
4553
800
Unfiltered
400
0
0
1-2001
8583A9
0.4
Electronics Noise (MeV)
0.8
Figure 60. The distribution of equivalent noise
energy for all channels of the EMC with/without
digital ltering. The data were recorded in the
absence of beams by a random trigger.
4583
4584
4585
4586
4587
4588
4589
rithm, the frequency decomposition of an average
signal pulse and the typical noise spectrum are
measured for all channels and subsequently used
to derive an optimum set of weights that maximizes the signal-to-noise ratio. These weights are
then applied to individual samples to obtain a
ltered waveform.
February 6, 2001 { 10 : 40
4590
4591
4592
4593
4594
4595
4596
4597
4598
4599
4600
4601
4602
4603
4604
4605
4606
4607
4608
4609
4610
4611
4612
4613
DRAFT
The magnitude of the electronic noise is measured as the rms width of the pedestal distribution as shown in Figure 60. The observed distribution for all channels translates to an ENE of
230 keV and 440 keV with and without digital
ltering; this result is comparable to design expectations. Measurements of the auto-correlation
function indicate that the coherent noise component is negligible compared to the incoherent
noise, except for regions where the preampli ers
saturate (see below).
During data taking, the data acquisition imposes a single-crystal readout threshold in order
to keep the data volume at an acceptable level.
This energy threshold is currently set to 1 MeV
and during stable colliding beam conditions on
average 1,000 crystals are read out (measured
with 600 mA of e; and 1100 mA of e+ and a
random clock trigger), corresponding to an average occupancy of 16%. The electronic noise accounts for about 10%, while the remaining signals originate from beam-generated background
(see Chapter 3). In addition, a typical hadronic
event contributes signals in xxx crystals.
4636
4637
4638
4639
4640
4641
4642
4643
4644
4645
4646
4647
4648
4649
4650
4651
4652
4653
4654
4655
4656
4657
4658
4659
4614
4615
4616
4617
4618
4619
4620
4621
4622
4623
4624
4625
4626
4627
4628
4629
4630
4631
4632
4633
4634
4635
11.3.3. Electronics Calibration and Linearity
To measure pedestal o sets, determine the
overall gain, and to remove non-linearities the
front-end electronics are calibrated by precision
charge injection into the preampli er input. Initially, residual non-linearities of up to 12% in limited regions near each of the range changes were
observed and corrected for oine [15]. These nonlinearities were traced to oscillations on the ADC
cards that have since been corrected. The correction resulted in markedly improved energy resolution at high energies. Residual non-linearities
(typically 2 ; 4%) arise primarily from cross-talk,
impacting both the electronics calibrations and
the colliding-beam data. The e ect is largest at
about 630 MeV (950 MeV) in a high (low) gain
preampli er channel. The implementation of an
energy dependent correction is expected to signi cantly reduce this small, remaining e ect, and
lead to a further improvement of the energy resolution.
4660
4661
4662
4663
4664
4665
4666
4667
4668
4669
4670
4671
4672
4673
4674
4675
4676
4677
4678
4679
4680
83
11.3.4. Electronics Reliability
With the exception of minor cable damage
during installation (leaving two channels inoperative), the system of 13,160 readout channels
has met its reliability requirements. After the
replacement of a batch of failing optical- ber
drivers, the reliability of the digitizing electronics
improved substantially, averaging channel losses
of less than 0:1%.
11.4. Energy Calibration
The energy calibration of the EMC proceeds
in two steps: First, the measured pulse height in
each crystal has to be translated to the actual energy deposited. Second, the energy deposited in
a shower spreading over several adjacent crystals
has to be related to the energy of the incident
photon or electron by correcting for energy loss
mostly due to leakage at the front and the rear,
and absorption in the material between and in
front of the crystals, as well as shower energy not
associated with the cluster.
The oine pattern recognition algorithm that
groups adjacent crystals into \clusters" is described in detail in section 11.6.
11.4.1. Individual Crystal Calibration
In spite of the careful selection and tuning of
the individual crystals, their light yield varies signi cantly and is generally non-uniform. It also
changes with time under the impact of beamgenerated radiation. The absorbed dose results
is largest at the front of the crystal and results
in increased attenuation of the transmitted scintillation light. The light yield must therefore be
calibrated at di erent energies, corresponding to
di erent average shower penetration, to track the
e ects of the radiation damage.
The calibration of the deposited energies is
performed at two energies at opposite ends of
the dynamic range, and these two measurements
are combined by a logarithmic interpolation. A
6.13 MeV radioactive photon source [17] provides
an absolute calibration at low energy, while at
higher energies (3 - 9 GeV) the relation between
polar angle and energy of e from Bhabha events
is exploited [18].
DRAFT
84
Events / 0.047 MeV
4700
4701
4702
4703
200
4704
4705
4706
4707
4708
100
4709
4710
4711
4712
4713
0
4
6
8
Energy (MeV)
4714
4715
4716
4717
4718
4719
Figure 61. A typical pulse-height spectrum
recorded with the radioactive source to calibrate
the single-crystal energy scale of the EMC. The
spectrum shows the primary 6.13 MeV peak and
two associated escape peaks at 0.511 MeV and
1.022 MeV below.
4681
4682
4683
4684
4685
4686
4687
4688
4689
4690
4691
4692
4693
4694
4695
4696
4697
4698
4699
A ux of low-energy neutrons (4 108=s) is
used to irradiate Fluorinert [10] to produce photons of 6.13 MeV via the reaction 19 F + n !16
N + ; 16 N !16 O + ; 16 O !16 O + . The
activated 16 N has a half-life of 7 seconds and
thus does not cause radiation damage or longterm activation. The uid is pumped at a rate
of 125 `/s from the neutron generator to a manifold of thin-walled (0.5 mm) aluminum pipes that
are mounted immediately in front of the crystals.
At this location, the typical rate of photons is
40 Hz/crystal.
Figure 61 shows a typical source spectrum that
was derived from the raw data by employing a
digital lter algorithm. For a 30-minute exposure,
a statistical error of 0.35% is obtained, compared
to a systematic uncertainty of less than 0.1%.
This calibration is performed weekly.
At high energies, single crystal calibration
4720
4721
4722
4723
4724
4725
4726
4727
4728
4729
4730
4731
4732
4733
4734
4735
4736
4737
4738
4739
4740
4741
4742
4743
4744
4745
February 6, 2001 { 10 : 40
is performed with a pure sample of Bhabha
events [18]. As a function of the polar angle of the
e , the deposited cluster energy is constrained
to equal the prediction of a GEANT [19] based
Monte Carlo simulation. For a large number of
energy clusters, a set of simultaneous linear equations relates the measured to the expected energy
and thus permits the determination of a gain constant for each crystal. In a 12-hour run at a luminosity of 3 1033cm;1sec;2 some 200 hits per
crystal can be accumulated, leading to a statistical error of 0.35%. This calibration has been performed about once per month, and will be fully
automated in the future.
11.4.2. Cluster Energy Correction
The correction for energy loss due to shower
leakage and absorption is performed as a function
of cluster energy and polar angle. At low energy
(E < 0.8 GeV), it is derived from 0 decays [20].
The true energy of the photon is expressed as a
product of the measured deposited energy and a
correction function which depends on ln E and
cos . The algorithm constrains the two-photon
mass to the nominal 0 mass and iteratively nds
the coecients of the correction function. The
typical corrections are of order 6 1%. The uncertainty in the correction is due to systematic
uncertainties in the background estimation and
the tting technique.
At higher energy (0:8 GeV < E < 9 GeV) the
correction is estimated from single-photon Monte
Carlo simulations. A second technique using radiative Bhabha events [21] is being developed.
The beam energy and the precise track momenta
of the e+ and e; , together with the direction of
the radiative photon, are used to t the photon
energy. This tted value is compared to the measured photon energy to extract correction coecients, again as a function of ln E and cos .
11.5. Monitoring
11.5.1. Environmental Monitoring
The temperature is monitored by 256 thermal
sensors that are distributed over the calorimeter,
and has been maintained at 20 0:5 C. Dry nitrogen is circulated throughout the detector to
stabilize the relative humidity at 1 0:5%.
February 6, 2001 { 10 : 40
4748
4749
4750
4751
4752
4753
4754
4755
4756
4757
4758
4759
4760
4761
4762
4763
4764
4765
4766
4767
4768
4769
4770
4771
4772
4773
4774
4775
4776
4777
4778
4779
4780
4781
4782
4783
4784
4785
4786
4787
4788
4789
4790
4791
11.5.2. Light-Pulser System
The light response of the individual crystals is measured daily using a light-pulser system [16] [22]. Spectrally ltered light from a
xenon ash lamp is transmitted through optical bers to the rear of each crystal. The light
pulse is similar in spectrum, rise-time and shape
to the scintillation light in the CsI(Tl) crystals.
The pulses are varied in intensity by neutraldensity lters, allowing a precise measurement
of the linearity of light collection, conversion to
charge, ampli cation and digitization. The intensity is monitored pulse-to-pulse by comparison to
a reference system with two radioactive sources,
241 Am and 148 Gd, that are attached to a small
CsI(Tl) crystal that is read out by both a photodiode and a photo-multiplier tube. The system
is stable to 0.15% over a period of one week and
has proven to be very valuable in diagnosing problems. For example, the ability to accurately vary
the light intensity has lead to the detection of
non-linearities in the electronics [16].
11.5.3. Radiation Monitoring and Damage
The radiation exposure is monitored by 60/56
RadFETs placed in front of the barrel/endcap
crystals. RadFETs [23] are real-time integrating dosimeters based on solid-state Metal Oxide
Semiconductor (MOS) technology. In Figure 62
the accumulated dose is compared to the observed
loss in scintillation light, separately for the endcap, the forward, and the backward barrel. The
dose appears to follow the integrated luminosity,
approximately linearly. The light loss is greatest
in the forward region corresponding to the area of
highest integrated radiation dose. The size of the
observed light loss is close to expectations, based
on extensive irradiation tests.
11.6. Reconstruction Algorithms
A typical electromagnetic shower spreads over
many adjacent crystals, forming a cluster of energy deposits. Pattern recognition algorithms
have been developed to eciently identify these
clusters and to di erentiate single clusters with
one energy maximum from merged clusters with
more than one local energy maximum, referred
to as a bumps. Furthermore, the algorithms have
200
Forward Barrel
Backward Barrel
160
Endcap
Dose (Rad)
4747
85
120
80
40
0
Gain Change (%)
4746
DRAFT
–4
Forward Barrel
Backward Barrel
Endcap
–8
0
1-2001
8583A10
10
20
Integrated Luminosity (fb-1)
Figure 62. Impact of beam-generated radiation
on the CsI(Tl) crystals: a) the integrated dose
measured with RadFETs placed in front of the
crystals, b) the degradation in light yield measured with the radioactive-source calibration system.
86
4792
4793
4794
4795
4796
4797
4798
4799
4800
4801
4802
4803
4804
4805
4806
4807
4808
4809
4810
4811
4812
4813
4814
4815
4816
4817
4818
4819
4820
4821
4822
4823
4824
4825
4826
4827
4828
4829
4830
4831
4832
4833
4834
4835
4836
4837
DRAFT
to determine whether a bump is generated by a
charged or a neutral particle.
Clusters are required to contain at least one
seed crystal with an energy above 10 MeV. Surrounding crystals are considered as part of the
cluster if their energy exceeds a threshold of
1 MeV, or if they are contiguous neighbors (including corners) of a crystal with at least 3 MeV.
The value of the single crystal threshold is set by
the data acquisition system in order to keep the
data volume at an acceptable level, given the current level of electronics noise and beam-generated
background. It is highly desirable to reduce this
threshold since uctuations in the e ective energy
loss at the edges of a shower cause a degradation
in resolution, particularly at low energies.
Local energy maxima are identi ed within a
cluster by requiring that the candidate crystal have an energy, ELocalMax , which exceeds
the energy of each of its neighbors, and satisfy the following condition: 0:5(N ; 2:5) >
ENMax =ELocalMax, where ENMax is the highest
energy of the neighboring N crystals with an energy above 2 MeV.
Clusters are divided into as many bumps as
there are local maxima. An iterative algorithm
is used to determine the energy of the bumps.
Each crystal is given a weight,Pwi , and the bump
energy is de ned as Ebump = i wi Ei , where the
sum runs over all crystals in the cluster. For a
cluster with a single bump, the result is wi 1.
For a cluster with multiple bumps, the crystal
weight for each bump is calculated as
;2:5ri =rM ) ;
wi = Ei P exp(
j Ej exp(;2:5rj =rM )
where the index j runs over all crystals in the
cluster. rM refers to the Moliere radius, and ri
is the distance of the ith crystal from the centroid of the bump. At the outset, all weights are
set to one. The process is then iterated, whereby
the centroid position used in calculating ri is determined from the weights of the previous iteration, until the bump centroid position is stable to
within a tolerance of 1 mm.
The position of a bump is calculated using a center-of-gravity method with logarithmic,
rather than linear weights [24] [25], Wi = 4:0 +
4838
4839
4840
4841
4842
4843
4844
4845
4846
4847
4848
4849
4850
4851
4852
4853
4854
4855
4856
4857
4858
4859
4860
4861
4862
4863
4864
4865
4866
4867
4868
4869
4870
4871
4872
4873
4874
4875
4876
4877
4878
4879
4880
4881
February 6, 2001 { 10 : 40
ln Ei =Ebump , where only crystals with positive
weights (i.e., Ei > 0:0184 Ebump, are used in the
calculation. This procedure emphasizes lowerenergy crystals, while utilizing only those crystals
that make up the core of the cluster. A systematic bias of the calculated polar angle originates
from the non-projectivity of the crystals. This
bias is corrected by a simple o set of ;2:6 mrad
for cos < 0 and +2:6 mrad for cos > 0.
A bump is associated with a charged particle
by projecting a track to the inner face of the
calorimeter. The distance between the track impact point and the bump centroid is calculated
and if it is consistent with the angle and momentum of the track, the bump is associated with
this charged particle. Otherwise, it is assumed to
originate from a neutral particle.
On average, xxx clusters are detected per
hadronic event, of which yyy are identi ed as neutral particles. At current operating conditions,
beam-induced background contributes on average on 1.4 neutral clusters with energies above
20 MeV. This number is signi cantly smaller
than the average number of crystals with energies above 10 MeV(see Chapter 3.
11.7. Performance
11.7.1. Energy Resolution
At low energy, the energy resolution of the
EMC is measured directly with the radioactive
source yielding E =E = (5:0 0:8)% at 6.13 MeV
(see Figure 61). At high energy, the resolution
is derived from Bhabha scattering, where the energy of the detected shower can be predicted from
the polar angle of the e . The measured resolution is E =E = (1:9 0:07)% at 7.5 GeV (Figure 63). Figure 64 shows the energy resolution
extracted from a variety of processes as a function of energy. Below 2 GeV, the mass resolution
of 0 and mesons decaying into two photons of
approximately equal energy is used to infer the
EMC energy resolution. The decay c1 ! J=
provides a measurement at an average energy of
500 MeV, and measurements at high energy
are derived from Bhabha scattering. A t to the
February 6, 2001 { 10 : 40
DRAFT
σ (E) / E
500
400
Entries/0.007
87
0.07
π0 → γ γ
η → γγ
0.06
Bhabhas
χ c → J/ψ γ
MonteCarlo
0.05
300
0.04
200
0.03
0.02
100
0.01
0
0.6
1-2001
8583A32
0.8
1.0
EMeasured/EExpected
0 -1
10
Figure 63. The ratio of the measured energy to
the expected energy for electrons from Bhabha
scattering. The solid line indicates a t to a Gaussian distribution.
4882
4883
4884
4885
4886
4887
4888
4889
4890
4891
4892
4893
4894
4895
4896
4897
4898
4899
energy dependence results in
32 0:30)%
E = (2:p
(8)
4 E ( GeV) (1:85 0:12)%;
E
These values of these tted parameters are higher
than the somewhat optimistic earlier estimates,
but they agree with detailed Monte Carlo simulations which include the contributions from electronic noise and beam background, as well as the
impact of the material and the energy thresholds.
11.7.2. Angular Resolution
The measurement of the angular resolution is
based on the analysis of 0 and decays to two
photons of approximately equal energy. The result is presented in Figure 65. The resolution
varies between about 12 mrad at low energies and
3 mrad at high energies. A t to the empirical parameterization of the energy dependence results
in
= = ( 3p:87 0:07 0:00 0:04) mrad):(9)
E ( GeV)
1
Photon Energy (GeV)
Figure 64. The energy resolution for the electromagnetic calorimeter measured for photons and
electrons from various processes. The solid curve
is a t to Equation 6and the shaded area denotes
the r.m.s. error on the t.
4900
4901
4902
4903
4904
4905
4906
4907
4908
4909
4910
4911
4912
4913
4914
4915
4916
4917
4918
These tted values are slightly lower than one
would expect from detailed Monte Carlo simulations.
11.7.3. 0 Mass and Width
Figure 66 shows the two-photon invariant mass
in BB events. The reconstructed 0 mass is measured to be 135:1 MeV=c2 and is stable to better
than 1% over the full photon energy range. The
width of 6:9 MeV=c2 agrees well with the prediction obtained from detailed Monte-Carlo simulations. In low-occupancy + ; events the width
is slightly smaller, 6:5 MeV=c2 , for 0 energies below 1 GeV. A similar improvement is also observed in analyses using selected isolated photons
in hadronic events.
11.7.4. Electron Identi cation
Electrons are separated from charged hadrons
primarily on the basis of the shower energy, lateral shower moments, and track momentum [26].
σ (θ ) (mrad)
Entries / 0.001 GeV
DRAFT
88
14
π0 → γ γ
12
η → γγ
10
MonteCarlo
8
February 6, 2001 { 10 : 40
12000
10000
8000
6000
6
4000
4
2000
2
0
0
0.5
1
1.5
2
2.5
0
0.05
3
0.1
4920
4921
4922
4923
4924
4925
4926
4927
4928
4929
4930
4931
4932
4933
4934
4935
4936
4937
4938
In addition, the dE/dx energy loss in the DCH
and the DIRC Cherenkov angle are required to
be consistent with an electron. The most important variable for the discrimination of hadrons
is the ratio of the shower energy to the track
momentum (E=p). Figure 67 shows the eciency for electron identi cation and the pion misidenti cation probability as a function of momentum for two sets of selection criteria. The electron eciency is measured using radiative Bhabhas and e+ e; ! e+ e;e+ e; events. The pion
misidenti cation probability is measured for selected charged pions from KS0 decays and threeprong decays. A tight (very tight) selector results in an eciency plateau at 94.8% (88.1%) in
the momentum range 0:5 GeV=c < p < 2 GeV=c.
The pion misidenti cation probability is of order
0.3% for the very tight criteria. The selection criteria can, of course, be tailored to meet the needs
of speci c physics analyses.
0.2
0.25
Figure 66. Invariant mass of two photons in BB
events. The energies of the photons and the 0
are required to exceed 30 MeV and 300 MeV,
respectively. The solid line is a t to the data [20].
Figure 65. The angular resolution of the EMC for
photons from 0 and decays. The solid curve is
a t to Equation 7.
4919
0.15
mγ γ (GeV)
Photon Energy (GeV)
4939
4940
4941
4942
4943
4944
4945
4946
4947
4948
4949
4950
4951
4952
4953
4954
4955
4956
4957
11.8. Summary
The EMC is presently performing close to design expectations. Improvements in the energy
resolution are expected from the optimization of
the feature-extraction algorithms designed to further reduce the electronics noise. Modi cations to
the electronics should allow for more precise calibrations. The expected noise reduction should
permit a lower single-crystal readout threshold.
However, this decrease in noise might be o -set
by an increase in the beam background that is expected for higher luminosities and beam currents.
REFERENCES
1. T. Swarnicki, Performance of the CLEOII CsI(Tl)Calorimeter in Proceedings of
Workshop on B Factories, Stanford, CA,
USA (1992).
2. R.J. Barlow et al., Results from the BABAR
Electromagnetic Calorimeter Beam Test,
Electron Identification Efficiency
1
0.1
0.08
0.75
e±
π±
0.5
tight
very tight
tight
very tight
0.06
0.04
0.25
0.02
Pion Misidentification Probability
DRAFT
February 6, 2001 { 10 : 40
4978
4979
4980
4981
4982
4983
4984
4985
4986
4987
4988
4989
4990
4991
4992
0
0
0.5
0
1
1.5
2
2.5
Momentum [GeV/c]
4993
4994
4995
4996
4997
4998
4999
Figure 67. The electron eciency and pion misidenti cation probability as a function of the particle momentum.
4958
4959
4960
3.
4.
4961
4962
4963
4964
4965
4966
4967
4968
4969
5.
6.
4970
4971
4972
4973
4974
4975
4976
4977
7.
8.
Nucl. Instr. and Methods A420 (1999) 162.
Chemetall and Aldrich-APL.
Shanghai Institute of Ceramics, Shanghai,
P.R.China;
Beijing Glass Research Institute, Beijing,
P.R.China;
Hilger Analytical, United Kingdom;
Crismatec, France;
Amcrys-H, Ukraine.
TYVEK, registered trademark of E.I. DuPont
de Nemours & Co.
G. Dahlinger, Aufbau und Test eines
Kalorimeter-Prototyps aus CsI(Tl) zur
Energie- und Ortsmessung hochenergetischer Photonen, PhD Thesis, Technische
Universitat Dresden, Germany (1998).
A17-01 manufactured by EPILOX, Germany.
J. Brose, G. Dahlinger, K.R. Schubert, Nucl.
Instr. and Methods A417 (1998) 311; C. Jessop et al., Development of Front End Readout
5000
5001
5002
5003
5004
5005
5006
5007
5008
5009
5010
5011
5012
5013
5014
5015
5016
5017
5018
5019
5020
5021
5022
5023
5024
5025
89
for the BABAR Electromagnetic Calorimeter,
BABAR Note-216 (199x).
9. NE-561 manufactured by Nuclear Enterprises, USA.
10. Fluorinert (polychlorotri uoro-ethylene) is
manufactured by 3M Corporation.
11. S-2744-08 PIN diode by Hamamatsu Photonics, K. K., Hamamatsu City, Japan.
12. C. Jessop et al. Performance Tests of
Hamamatsu 2774-08 Photo-Diodes for the
BABAR Electromagnetic Calorimeter Front
End Readout, BABAR Note-236 (199x).
13. G. Haller, D. Freytag, IEEE Trans.Nucl.Sci.
43, 1610 (1996)
14. C. Jessop, Reliability Issues for the FrontEnd Readout of the BaBar Electromagnetic
Calorimeter, BABAR Note-217 (199x).
15. S. Menke, Oine Correction of NonLinearities in the BaBar Electromagnetic
Calorimeter, BABAR Note-527 (199x).
16. M. Kocian, Das Lichtpulsersystem des
elektromagnetischen CsI(Tl)-Kalorimeters
des BABAR-Detektors, PhD Thesis, TUDIKTP/00-03, Technische Universitat Dresden, Germany (2000).
17. F. Gaede, D. Hitlin, M. Weaver, The Radioactive Source Calibration of the BABAR Electromagnetic Calorimeter, BABAR Note-531 199x.
J. Button-Shafer et al., Use of Radioactive
Sources for Calibrating and Monitoring the
BABAR Electromagnetic Calorimeter, BABAR
Note-322 (199x).
18. R. Muller-Pfe erkorn, Die Kalibration des
elektromagnetischen CsI(Tl)-Kalorimeters
des Babar-Detektors mit Ereignissen der
Bhabha-Streuung, PhD Thesis, TUDIKTP/01-01, Technische Universitat Dresden, Germany 2001.
19. GEANT Detector Description and Simulation
tool, CERN Program Library, Long Writeup
W5013, (1994.
20. S. Menke et al. Calibration of the BABAR Electromagnetic Calorimeter with 0 s , BABAR
Note-528 (199x).
21. J. Bauer, Radiative Bhabha Calibration
for the BABAR Electromagnetic Calorimeter,
BABAR Note-521 (2000).
22. P.J. Clark, The BaBar Light Pulser System,
90
5026
5027
5028
5029
5030
5031
5032
5033
5034
5035
5036
5037
5038
5039
5040
DRAFT
PhD Thesis, University of Edinburgh, UK
(2000);
B. Lewandowski, Entwicklung und Aufbau
eines Lichtpulsersystems fr das Kalorimeter
des BABAR-Detektors, PhD Thesis, RuhrUniversitat Bochum, Germany (2000).
23. RADFET
24. B. Brabson et al. Nucl. Instr. and Methods A332 (1993) 419.
25. S. Otto, Untersuchungen zur Ortsrekonstruktion electromagnetischer Schauer, Diplomarbeit, Technische Universitat Dresden, Germany 2000.
26. U. Langenegger, Electron Identi cation with
BABAR, BABAR Note-530 (2000).
5041
5042
5043
5044
5045
5046
5047
5048
5049
5050
5051
5052
5053
5054
5055
5056
5057
5058
5059
5060
5061
5062
5063
5064
5065
5066
5067
5068
5069
5070
5071
5072
5073
5074
5075
5076
5077
5078
5079
5080
5081
5082
5083
5084
5085
5086
February 6, 2001 { 10 : 40
12. Detector for Muons and Neutral
Hadrons
12.1. Physics Requirements and Goals
The Instrumented Flux Return (IFR) was designed to identify muons with high eciency and
good purity, and to detect neutral hadrons (primarily KL0 and neutrons) over a wide range of
momenta and angles. Muons are important for
tagging the avor of neutral B mesons via semileptonic decays, for the reconstruction of vector
mesons, like the J= , and the study of semileptonic and rare decays involving leptons of B
and D mesons and leptons. KL0 detection allows
for the study of exclusive B decays, in particular
CP eigenstates. The IFR could also help in vetoing charm decays and improve the reconstruction
of neutrinos.
The principal requirements for IFR are large
solid angle coverage, good eciency, and high
background rejection for muons down to momenta below 1 GeV=c. For neutral hadrons, high
eciency and good angular resolution are most
important. Because this system is very large and
dicult to access, high reliability and extensive
monitoring of the detector performance and the
associated electronics plus the voltage distribution are necessary.
12.2. Overview and RPC Concept
The IFR uses the steel ux return of the magnet as muon lter and hadron absorber. Single
gap resistive plate chambers (RPCs) [1] with twocoordinate readout have been chosen as detectors.
The RPCs are installed in the gaps in the nely
segmented steel (see Chapter 4) of the six barrel
sectors and the two end-doors of the ux return,
as illustrated in Figure 68. The steel segmentation has been optimized on the basis of Monte
Carlo studies of muon penetration and charged
and neutral hadron interactions. The steel is
segmented into 18 plates, increasing in thickness
from 2 cm of the inner nine plates to 10 cm of
outermost plate(s). The nominal gap width is
3.5 cm in the inner layers of the barrel and 3.2 cm
elsewhere. There are 19 RPC layers in the barrel
and 18 in the endcaps. In addition, two layers of
cylindrical RPCs are installed between the EMC
DRAFT
February 6, 2001 { 10 : 40
91
3200
0
320
FW
342 RPC
Modules
920
19 Layers
1250 1940
18 Layers
BW
432 RPC
Modules
1-2001
8583A3
5087
5088
5089
5090
5091
5092
5093
5094
5095
5096
5097
5098
5099
5100
5101
5102
5103
5104
5105
5106
5107
and the magnet cryostat to detect particles exiting the EMC.
RPCs detect streamers from ionizing particles
via capacitive readout strips. They o er several
advantages: simple, low cost construction and the
possibility of covering odd shapes with minimum
dead space. Further bene ts are large signals and
fast response allowing for simple and robust front
end electronics and good time resolution, typically 1-2 ns. The position resolution depends on
the segmentation of the readout, a few mm are
achievable.
A cross section of an RPC is shown schematically in Figure 69. The construction of the planar
and the cylindrical RPCs di er in detail, but are
based on the same concept.
The planar RPCs consist of two bakelite (phenolic polymer) sheets, 2 mmthick and separated
by a gap of 2 mm. The gap is enclosed at the edge
by a 7 mm wide frame. The gap width is kept uniform by polycarbonate spacers (0.8 cm2 ) that are
H.V.
Figure 68. Overview of the IFR: Barrel sectors and forward (FW) and backward (BW) end-doors; the
shape of the RPC modules and their dimensions are indicated.
Foam
Bakelite
Gas
Bakelite
Foam
Aluminum
X Strips
Insulator
Graphite
2 mm
2 mm
2 mm
Graphite
Insulator
Y Strips
Spacers
Aluminum
8-2000
8564A4
Figure 69. Cross section of a planar RPC with the
schematics of the high voltage (HV) connection.
92
5108
5109
5110
5111
5112
5113
5114
5115
5116
5117
5118
5119
5120
5121
5122
5123
5124
5125
5126
5127
5128
5129
5130
5131
5132
5133
5134
5135
5136
5137
5138
5139
5140
5141
5142
5143
5144
5145
5146
5147
5148
5149
5150
5151
5152
5153
5154
DRAFT
glued to the bakelite, spaced at distances of about
10 cm. The bulk resistivity of the bakelite sheets
has been especially tuned to 1011 ; 1012 cm.
The external surfaces are coated with graphite to
achieve a surface resistivity of 100 k /square.
These two graphite surfaces are connected to high
voltage ( 8 kV) and ground, and protected by
an insulating mylar lm. The bakelite surfaces
facing the gap are treated with linseed oil to improve performance. The modules are operated in
limited streamer mode and the signals are read
out capacitively, on both sides of the gap, by external electrodes made of aluminium strips on a
mylar substrate.
The cylindrical RPCs have resistive electrodes
made of a special plastic composed of a conducting polymer and ABS. The gap thickness and the
spacers are identical to the planar RPCs. No linseed oil or any other surface treatments have been
applied. The very thin and exible electrodes are
laminated to berglass boards and foam to form
a rigid structure. The copper readout strips are
attached to the berglass boards.
12.3. RPC Design and Construction
The IFR detectors cover a total active area of
about 2,000 m2 There are a total of 806 RPC
modules, 57 in each of the 6 barrel sectors, 108
in each of the four half end-doors, and 32 in the
two cylindrical layers. The size and the shape of
the modules are matched to the steel dimensions
with very little dead space. More than 25 di erent shapes and sizes were built. Because the size
of a module is limited by the maximum size of
the material available, i.e. for the bakelite sheets
(320 130 cm2 ), two or three RPC modules are
joined to form a gap-size chamber. The modules
of each chamber are connected to the gas system in series, the gas enters in two corners on the
same side a short side and exit from two outlets
on the opposite side. The high voltage is supplied
separately to each module.
In the barrel sectors, the gaps between the steel
plates extend 375 cm in the z direction and vary
in width from 180 cm to 320 cm. Three modules
are needed to cover the whole area of the gap, as
shown in Figure 68. Each barrel module has 32
strips running perpendicular to the beam axis to
5155
5156
5157
5158
5159
5160
5161
5162
5163
5164
5165
5166
5167
5168
5169
5170
5171
5172
5173
5174
5175
5176
5177
5178
5179
5180
5181
5182
5183
5184
5185
5186
5187
5188
5189
5190
5191
5192
5193
5194
5195
5196
5197
5198
5199
5200
5201
5202
February 6, 2001 { 10 : 40
measure the z coordinate and 96 strips in the orthogonal direction extending over three modules
to measure .
Each of the four half end-doors is divided into
three sections by steel spacers that are needed
for mechanical strength. Each of these sections is
covered by two RPC modules that are joined to
form a larger chamber with orthogonal readout
strips.
The readout strips are separated from the
ground aluminium plane by 4 mm thick foam
sheet and form strip lines of 33 impedance.
The strips are connected to the readout electronics at one end and terminated with a 2 k resistor
at the other. Even and odd numbered strips are
connected to di erent front-end cards, so that the
failure of a card does not result in a total loss of
signal, since a particle crossing the gap typically
generates signals in two or more adjacent strips.
The cylindrical RPC is divided into four sections, each covering a quarter of the circumference. Each of these sections has four sets of two
single gap RPCs with orthogonal readout strips,
the inner with helical u ; v strips that run parallel to the diagonals, and the outer with and
z strips. Within each section, the strips in a
given readout plane from di erent modules are
connected to form long strips extending over the
whole chamber. Details of the segmentation and
dimensions can be found in Table 13.
Prior to shipment to SLAC, all RPC modules (equipped with only one readout plane) were
tested with cosmic rays. The single rates, dark
currents and eciency were measured as a function of HV. In addition, detailed studies of the
eciency, spatial resolution, and strip multiplicity were performed [2],[3].
After the assembly of RPC modules into gap
size chambers, a new series of cosmic rays tests
was performed to assure stable and ecient operation. Before the installation of the iron ux return, the planar chambers were then inserted horizontally into the gaps. The cylindrical chambers
were inserted after the installation of the solenoid
and the EMC.
For each module, test results and conditions
are retained in a database, together with records
of the critical parameters of the components, the
February 6, 2001 { 10 : 40
DRAFT
93
Table 13
IFR Readout segmentation. The total number of channels is close to 53,000. There are a few deviations
from the numbers quoted here, in particular, in layer 19 (due to the external steel structure), and in layer
18 (due to the reduced length). The central chambers in the forward end-door have more vertical strips
because of the central hole.
section
# of coordinate # of layers # strips strip length strip width total #
sectors
layer/sect
(cm)
(mm)
channels
barrel
6
19
96
350
19.7-32.8 11; 000
z
19
96
190-318
38.5
11; 000
endcap
4
y
18
6x32
124-262
28.3
13,824
x
18
3x64
10-180
38.0
15; 000
cylinder
4
1
128
370
16.0
512
z
1
128
211
29.0
512
u
1
128
10-422
29.0
512
v
1
128
10-423
29.0
512
5203
5204
5205
5206
5207
5208
5209
5210
5211
5212
5213
5214
5215
5216
5217
5218
5219
5220
5221
5222
5223
5224
5225
5226
5227
5228
5229
5230
5231
assembly and cabling. In addition, operational
data are stored, such as the results of the weekly
eciency measurements that are used in the reconstruction and simulation software.
12.4. Power and Utilities
Once the return ux assembly was completed,
the front-end cards (FECs) [4], were installed and
services were connected, the low (LV) and high
voltage (HV), and the gas system. There are
approximately 3,300 FECs, most of them were
placed right on top of the RPC modules, inside
the steel gap. The remainder were installed in
custom crates mounted on the outside of the steel.
Each FEC is individually connected to the LV
power distribution. The total power required by
the entire system is about 8 kW at +7.0 V and
2.5 kW at -5.2 V. The LV power is supplied by
custom built switching devices with load and line
regulation to better than 1%. Additional features
are precision shunts to measure output currents
and TTL logic to inhibit output.
The HV power system is custom adaptation by
CAEN [5]. Each HV mainframe can hold up to
10 pods, each carrying two independent 10 kV
outputs at 1 mA and 2 mA. The modules are connected to the HV supply which is located in the
electronics building via a distribution box. Each
distribution box services six RPC modules and
up to six distribution boxes are daisy-chained to
5232
5233
5234
5235
5236
5237
5238
5239
5240
5241
5242
5243
5244
5245
5246
5247
5248
5249
5250
5251
5252
5253
5254
5255
5256
5257
5258
5259
5260
5261
one pod output. Provisions are made for monitoring the currents drawn by each module. To
reduce noise, the RPC ground plane is decoupled
from the HV power supply ground by a 100 k
resistor.
The RPCs operate with a non- ammable gas
mixture containing approximately 56.7% Argon,
38.8% Freon 134a (1,1,1,2 tetra uoroethane), and
4.5% isobutane. This mixture is drawn from a
760-liter tank that is maintained at an absolute
pressure of 1500 ; 1600 Torr. The mixing tank is
lled on demand with the three component gases
under control of mass- ow meters, each adjusted
to provide the desired amount to the mixture.
Samples are extracted from the mixing tank periodically and analyzed to verify the correct mixture.
The mixed gas is distributed at a gauge pressure of approximately 6.5 Torr through a parallel manifold system of 12.7-mm-diameter copper
tubing. Each chamber is connected to the manifold through several meters of 6 mm-diameter
plastic tubing (polyamide or Te on). The ow to
each of these is adjusted individually with a small
multi-turn metering valve. Protection against
overpressure is provided by an oil bubbler to atmosphere in parallel with each chamber (after the
valve), limiting the gauge pressure in the chamber
to a maximum of about 1 Torr. Return ow of
gas from each chamber is monitored by a second
DRAFT
94
Inside the
IFR Iron
or in Minicrates
(1 - 4 m)
FEC
Outside the
Detector
(1 - 8 m)
Counting
House
Data
5291
5292
FIFO Board
Data
Clock
5290
ITB
ITB
Calibration
IFB
Board
ITB
ITB
ICB
TDC
ITB
ITB
ITB
ICC
5293
5294
Readout Module
5295
5296
ROM
PDB
5297
5298
5299
IFR Crate
Controller
5300
Front End Cards
Front End Crate
DAQ
Discrimination and
Noise Reduction
Fast Buffering
on Trigger Basis
Zero Suppression
and Encoding
8-2000
8564A3
5301
5302
5303
5304
5305
Figure 70. Block diagram of the IFR electronics.
5262
5263
5264
5265
5266
5267
5268
5269
5270
5271
5272
5273
5274
5275
5276
5277
5278
5279
5280
5281
5282
5283
5284
5285
5286
5287
5288
5289
oil bubbler which creates a back pressure of about
0.2 Torr. The total ow through the entire system
is approximately 5 `/minute and corresponds on
average to two gas exchanges per day.
12.5. Electronics
A block diagram of the IFR electronics system
[6] is shown in Figure 70. It includes the front-end
cards, the data acquisition, and the trigger.
The detector-mounted FECs service 16 channels. They shape and discriminate the input signals and set a bit for each strip with a signal above
a xed threshold. The input stage operates continuously and is connected directly to the strips
which act as transmission lines. A fast OR of all
FEC input signals provides time information and
is also used for diagnostic purposes. Two types
of FECs are employed to handle the inputs of different polarity for signals from the opposite side
of the gap. Because of the very low occupancy
there is no provision for bu ering during the trigger latency [4].
Signals from 3,300 FECs are transmitted to
eight custom IFR front-end crates that are located near the detector and from there via a standard G link to four ROMs in the electronics building. Each front-end crate houses up to 16 data
handling cards, four trigger cards and a crate
controller card (ICC) that collects data from the
5306
5307
5308
5309
5310
5311
5312
5313
5314
5315
5316
5317
5318
5319
5320
5321
5322
5323
5324
5325
5326
5327
5328
5329
5330
5331
5332
5333
5334
February 6, 2001 { 10 : 40
DAQ cards and forwards them to a ROM. There
are three kinds of data cards: the FIFO boards
(IFBs) that bu er strip hits, the TDC boards
(ITBs) that provide time information, and the
calibration boards (ICBs) that inject test pulses
into the FECs. To deliver the data and clock signals within the jitter limit to all the boards in the
front-end crate, a custom backplane (PDB) for a
standard 6U Eurocard crate was designed using
9-layers \strip line" technology. Each board is
connected to the ICC via 3 point-to point lines
for three single-end signals (data-in, data-out and
clock), all of the same length and impedance
(50 ).
The IFB reads the digital hit patterns from the
FECs in less than 2.2 ms, stores the data into FIFOs and transfers FIFO contents into one of the
ROMs. Each IFB handles 64 FECs acting as an
acquisition master: it receives commands via the
PDB, and transmits and receives data patterns
from the ROM (via GLINK and ICC). This card
operates with the system clock frequency of 59.5
MHz.
The ICB is used for front-end test and calibrations. A signal with programmable amplitude
and width is injected into the FEC input stage.
To provide timing calibration and to determine
the correct readout delay, the board is also used
together with the TDCs.
The ICC interfaces the crate backplane with
the G link. The physical interface is the FINISAR
transceiver, a low cost and highly reliable data
link for applications up to 1.5 Gbit/s.
The TDC board exploits the excellent time resolution of the RPCs. Each board has 96 ECL
di erential input channels, reading fast OR signals from the FECs. Time digitization is achieved
by three custom TDCs, designed at CERN [7].
Upon receipt of a L1 trigger, data are selected
and stored until readout by the ROM. The intrinsic resolution of the board is better than 200 ps.
The 59.5 MHz clock signal is synchronized with
the data and distributed to the 16 boards. High
performance drivers are used to provide a reliable
clock distribution with a jitter of less than 0.5 ns.
February 6, 2001 { 10 : 40
5336
5337
5338
5339
5340
5341
5342
5343
5344
5345
5346
5347
5348
5349
5350
5351
5352
5353
5354
5355
5356
5357
5358
5359
5360
5361
5362
5363
5364
5365
5366
5367
5368
5369
5370
5371
5372
5373
5374
5375
5376
5377
5378
5379
5380
5381
95
12.6. Slow Controls and On-line Monitoring
The IFR is a system with a large number of
components and electronics distributed all over
the BABAR detector. To assure safe and stable
operation, an extensive monitoring and control
system was installed and has been in operation
from the start. The IFR On-line Detector Control
IODC) monitors the performance of the RPCs
by measuring the singles counting rate and the
dark current of every module. It also controls
and monitors the operation of the electronics, the
DAQ and trigger, as well as the LV, the HV, and
the gas system. The total number of hardware
channels is close to 2,500, and the hardware is
installed in 8 custom slave DAQ crates [8]).
The system has been easy to operate. HV trips
are rare. Temperature monitoring in the steel
structure and the electronics crates has proven
very useful for the diagnosis of operational problems. The occupancy is extremely low everywhere
but in layer 18 of the forward end-door which
lacks adequate shielding from machine generated
background. On average, there are about 100
strip hits per event.
12.7. Eciency Measurements and Performance
The eciency of the RPCs is evaluated for
both normal collision data and cosmic ray muons
recorded with the IFR trigger. Every week cosmic
ray data are recorded at di erent voltage settings
and the eciency is measured chamber by chamber as a function of the applied voltage. The absolute eciency at the nominal working voltage
(typically 7.6 kV) is stored in the database for
use in the event reconstruction software.
To calculate the eciency in a given chamber,
nearby hits in a given layer and hits in di erent
layers are combined to form clusters. Two di erent algorithms are used: The rst is based solely
on the IFR information and uses data recorded
with a dedicated FIR trigger; the second matches
the FIR clusters with the tracks reconstructed in
the drift chamber. Both these algorithms start
from one-dimensional IFR clusters de ned as a
group of adjacent hits in one of the two readout coordinates. The cluster position is de ned
400
# of Modules
5335
DRAFT
200
1-2001
8583A4
0
0.0
0.5
Efficiency
1.0
Figure 71. Distribution of the eciency for all
RPC modules measured with cosmic rays in June
1999. Some 50 modules were not operational at
that time.
5382
5383
5384
5385
5386
5387
5388
5389
5390
5391
5392
5393
5394
5395
5396
5397
5398
5399
5400
5401
5402
5403
5404
5405
5406
5407
5408
5409
5410
as the centroid of the strips in the cluster. In
the rst algorithm, two-dimensional clusters are
formed by joining one-dimensional clusters (of the
same readout coordinate) in di erent layers, provided the distance between their coordinate centroids is less then a given value. In each sector, two-dimensional clusters in di erent coordinates are combined to three-dimensional clusters
as long as there are less than three layers missing
one of the two coordinates. The second algorithm
extrapolates charged tracks reconstructed by the
drift chamber into the FIR. FIR clusters which
are less than 12 cm from the extrapolated track
are combined to form three-dimensional or twodimensional clusters. A detailed discussion of the
clustering algorithm can be found elsewhere [9].
The residual distributions from straight line ts
to two-dimensional clusters typically have an rms
width of less than 1 cm. An RPC is considered
ecient if a signal is detected at a distance of
less than 10 cm from the tted straight line in
either of the two readout planes. Following the
installation and commissioning of the IFR system, all the PC modules were tested with cosmic
rays and their eciency was measured. The results are presented in Figure 71. Of the active
RPC modules, 75% exceed an eciency of 90%.
Early tests indicated that the RPC dark current was very temperature dependent, speci -
DRAFT
96
1.0
32
Hall
0.5
24
a)
a)
IFR
16
0
1.0
0.5
10000
200
0.5
5411
5412
5413
5414
5415
5416
5417
5418
5419
5420
5421
5422
5423
5424
5425
5426
5427
5428
5429
5430
5431
5432
5433
5434
cally, the current increases 14 ; 20% per C. Because the IR experimental hall does not have temperatures regulation this presents a serious problem. The FECs that are installed in the steel gaps
each produce 3 W, adding up to a total power dissipation of 3.3 kW in the barrel and 1.3 kW in
the forward end-door.
During the rst summer of operation the daily
average temperature in the IR hall was 28C
and the maximum hall temperature frequently
exceeded 31C. The temperature inside the steel
rose to more than 37C and the dark currents
in many modules exceeded the capabilities of the
HV system and some RPCs had to be temporarily
disconnected.
To overcome this problem, water cooling was
installed on the barrel and end-door steel, removing 10 kW of heat and stabilizing the temperature to 20 ; 21C in the barrel, 22C in the backward and 24C in the forward end-doors. Figure 72 shows the history of temperature in the
hall and temperature and total dark current in
the backward end-door. While the current closely
follow the temperature variations, the range of
Jun
Figure 72. History of the temperature and dark
current in the RPC modules since January 2000.
top: temperature in the IR-2 hall and in the backward end-door; bottom: total dark current in the
216 modules of the backward end-door.
Apr
c)
0
Feb
100
150
Day of Year 2000
Dec
50
Oct
0
b)
Aug
2000
8-2000
8564A1
b)
0
1.0
Jun
Current ( µA ) Temp. ( °C )
February 6, 2001 { 10 : 40
12-2000
8577A8
Figure 73. Eciency history for 12 months starting in June 1999 for RPC modules showing different performance: a) highly ecient and stable;
b) continuous slow decrease in eciency; c) more
recent, faster decrease in eciency.
5435
5436
5437
5438
5439
5440
5441
5442
5443
5444
5445
5446
5447
5448
5449
5450
5451
5452
5453
5454
change is now limited to a few degrees.
During the operation at high temperatures a
large fraction of the RPCs (> 50%) showed not
only very high dark currents but also a some reduction in eciency compared to earlier measurements [10]. After the cooling was installed and
the RPCs were reconnected, some of them continued to deteriorate while others remained stable,
some of them (> 30%) at full eciency. (see Figure 73). Detailed studies revealed large regions of
very low eciency in these modules, but no clear
pattern was identi ed.
The cause of the eciency loss remains under
investigation. Several possible causes have been
excluded as the primary source of the problem,
such as the change in the bakelite bulk resistivity, loosened spacers, gas ow or composition. A
number of prototype RPCs developed similar efciency problems after being operated above a
temperature 36C for a period of two weeks. In
February 6, 2001 { 10 : 40
5456
5457
5458
5459
5460
5461
5462
5463
5464
5465
5466
5467
5468
5469
5470
5471
5472
5473
5474
5475
5476
5477
5478
5479
5480
5481
5482
5483
5484
5485
5486
5487
5488
5489
5490
5491
5492
5493
5494
5495
5496
5497
5498
5499
5500
5501
97
some of these modules, evidence was found that
the linseed oil had failed to cure and had accumulated at various spots under the impact of the
high voltage.
1.0
0.8
12.8. Muon Identi cation
While muon identi cation relies almost entirely on the IFR, other detector systems provide complementary information. Charged particles are reconstructed in the SVT and DCH and
muon candidates are required to meet the criteria for minimum ionizing particles in the EMC,
i.e. tracks depositing large amounts of energy are
rejected. Charged tracks that are reconstructed
in the tracking systems are extrapolated to the
IFR taking into account the non-uniform magnetic eld, multiple scattering and the average
energy loss. The projected intersections with the
RPC planes are computed and for each readout
plane all clusters detected within a prede ned distance from the predicted intersection are associated with the track.
For each cluster in the IFR associated with a
charged track a number of variables are de ned to
discriminate muons from charged hadrons: 1) the
total number of interaction lengths traversed from
the IP to the last RPC layer with an associated
cluster, 2) the di erence between this measured
number of interaction lengths and the number of
interaction lengths predicted for a muon of the
same momentum and angle, 3) the average number and the rms of the distribution of RPC strips
per layer, 4) the 2 for the geometric match between the projected track and the centroids of
clusters in di erent RPC layers, and 5) the 2 of
a polynomial t to the 2-dimensional IFR clusters. These variables will be combined into global
likelihood analysis to optimize the eciency and
purity of muons. At present, cuts on individual
variables are employed.
The performance of muon selection has been
tested on samples of muons from ee and nal states and pions from 3-prong decays and
KS ! + ; decays. The selection of these control samples is based on kinematic variables, and
not on variables used for muon selection. As illustrated in Figure 74, a muon detection eciency
of close to 90% has been achieved in the momen-
Efficiency
5455
DRAFT
0.6
Muons
Pions
0.4
0.2
0.0
0
8-2000
8564A7
1
2
3
4
Momentum (GeV/c)
5
Figure 74. Muon eciency and pion misidenti cation probability as a function of the track momentum, otained with loose selection criteria.
5502
5503
5504
5505
5506
5507
5508
5509
5510
5511
5512
5513
5514
5515
5516
5517
5518
5519
5520
5521
5522
5523
5524
tum range of 1 < p < 3 GeV=c with a fake rate
for pions of about 5%. The pion misidenti cation
can be reduced by a factor of two by tighter selection cuts which lower the detection eciency
to about 80%. Above 1 GeV=c decays in ight
contribution 1.5% to the pion misidenti cation.
12.9. KL0 and Neutral Hadron Detection
KL0 and other neutral hadrons interact in the
iron of the IFR and can be identi ed as clusters that are not associated with a charged track.
Monte Carlo simulations predict that about 64%
of KL's above a momentum of 1 GeV=c produce
a cluster in the cylindrical RPC, and/or a cluster
with hits in two or more planar RPC layers.
Unassociated clusters that have an angular separation of 0:3 rad are combined into a composite cluster, joining clusters that originate from
showers that spread into adjacent sectors of the
barrel, several sections of the end-doors and/or
the cylindrical RPC. This procedure also combines multiple clusters from large uctuations in
the hadronic showers. The direction of the neutral hadron is determined from the event vertex
DRAFT
98
February 6, 2001 { 10 : 40
Neutral Clusters
160
Events
100
50
MC
Data
Background
120
80
40
0
0.98
1-2001
8583A5
0
0.99
Cos ∆ξ
-100
1
1-2001
8583A6
Figure 75. Angular di erence, cos , between
the direction of the missing momentum and the
closest neutral IFR cluster for a sample of mesons produced in the reaction e+ e; ! with
! KL0 KS0 .
5525
5526
5527
5528
5529
5530
5531
5532
5533
5534
5535
5536
5537
5538
5539
5540
5541
5542
5543
5544
5545
5546
and the centroid of the neutral cluster. No information on the energy of the cluster can be obtained.
Since a signi cant fraction of the hadrons interact before reaching the IFR, information from the
EMC and the cylindrical RPCs is combined with
the IFR cluster information. Neutral showers in
the EMC are associated with the neutral hadrons
detected in the IFR, based on a match in production angles. For a good match a 2 probability of
1% is required.
An estimate of the angular resolution of the
neutral hadron cluster can be derived from a sample of KL0 produced in the reaction e+ e; ! !
KL0 KS0 . The KL0 direction is infered from the
missing momentum computed from the measured
particles in the nal state, and KS0 . The data
in Figure 75 indicate that the angular resolution
is of the order of xxx .
For multi-hadron events with a reconstructed
J= decay, Figure 76 shows the angular di erence, , between the missing momentum and
0
∆φ (Degrees)
100
Figure 76. Di erence between the direction of
the reconstructed neutral hadron cluster and the
transverse missing momentum of the event. The
Monte Carlo simulation is nomalized to the luminosity of the data; the background is obtained
using neutral hadrons and the missing momentum from di erent events.
5547
5548
5549
5550
5551
5552
5553
5554
5555
5556
5557
5558
5559
5560
5561
5562
5563
5564
5565
the direction of the nearest neutral hadron cluster. The observed peak demonstrates clearly that
the missing momentum can be associated with a
neutral hadron, assumed to be a KL0 .
????? estimate of eciency and resolution for
KL ???
12.10. Summary and Outlook
The IFR is the largest RPC system built so
far. It provides ecient muon identi cation and
allows for the detection of KL0 interacting in the
steel and the calorimeter. During the rst year
of operation a large fraction of the RPC modules
have su ered signi cant losses in eciency. This
e ect appears to be correlated with high temperatures, but the full extent of the problem and its
cause remain under study. Thanks to the large
number of RPC layers, this problem has not yet
impacted the overall performance too severely.
But current extraplolations, even after installa-
February 6, 2001 { 10 : 40
5566
5567
5568
5569
5570
5571
5572
5573
5574
DRAFT
tion of water cooling on the steel, indicate a severe problem for the future operation. Recently,
24 end-door modules have been replaced by new
RPCs with improved treatment of the bakelite
surfaces that should lead to full curing of the linseed oil coating. Results from this new production and other tests will need to be evaluated before decisions on future improvements of the IFR
can be made.
5607
5608
5609
5610
5611
5612
5613
5614
5615
5616
5617
5575
5576
5577
5578
5579
5580
5581
5582
5583
5584
5585
5586
5587
5588
5589
5590
5591
5592
5593
5594
5595
5596
5597
5598
5599
5600
5601
5602
5603
5604
5605
5606
REFERENCES
1. R. Santonico, R. Cardarelli, Nucl. Instr. and
Methods A187 (1981) 377.
2. F. Anulli et al. Nucl. Instr. and Methods A409 (1998) 542.
3. A. Calcaterra, et al. Performance of the
BABAR RPCs in a Cosmic Ray Test in Proceedings of the International Workshop on
Resistive Plate Chambers and Related Detectors, Naples, Italy (1997).
4. N.Cavallo, et al. Nucl. Phys. B(Proc. Suppl.)
61B (1998) 545; N. Cavallo, et al., Nucl. Instr.
and Methods A 409 (1998) 297.
5. HV system SY-127, Pod Models A300-P and
A300-N, by CAEN, Viareggio, Italy.
6. G. Crosetti et al. Data Acquisition System
for the RPC Detector of BABAR Experiment
in Proceedings of the International Workshop
on Resistive Plate Chambers and Related Detectors, Naples, Italy (1997).
7. TDC custom chip INFO NEEDED!
8. P. Paolucci, The IFR Online Detector Control
System, SLAC PUB-8167 (1999).
9. L. Lista, Object Oriented Reconstruction
Software for the IFR Detector of BABAR Experiment in Proceedings of the Conference on
Computing in High Energy Physics, Padova,
Italy (2000).
10. A. Zallo et al. The BABAR RPC System in
Proceedings of the 5th International Workshop on Resistive Plate Chambers and Related
Detectors, Bari, Italy (1999).
5618
5619
5620
5621
5622
5623
5624
5625
5626
5627
5628
5629
5630
5631
5632
5633
5634
5635
5636
5637
5638
99
13. Trigger
13.1. Trigger design requirements
The most basic requirements for the trigger
system are to select the physics events of interest (Table 14) with a high, stable and wellunderstood eciency while keeping the total
event output rate to permanent storage under 120
Hz at the design luminosity of 3 1033 cm;2 s;1 .
This total includes some prescaled samples of
events, such as ones failing the trigger conditions
or from random beam crossings, which are required for diagnostic and background studies.
The actual eciency requirement depends on
the physics channel being considered. The total
trigger eciency is required to be at least 99%
for all BB events and at least 95% for continuum
events, which are required for background subtraction. Less stringent requirements are put on
the eciencies of other physics channels of interest, e.g. the eciencies for ducial pair events
are required to be 90-95%, depending on decay
channels.
The trigger is required to be robust and exible in order to achieve stable operation even under unexpected situations. It is designed to be
able to operate at up to ten times the expected
nominal PEP-II background rates and to degrade
slowly for backgrounds above that level. It is also
required to be able to operate when some of the
detector channels are dead or noisy. Additionally, the trigger should not cause more than 1%
deadtime.
Table 14
Physics event production and trigger rates at
(4S ) and a luminosity of L = 3 1033 cm;2 s;1 .
The e+ e; cross section refers to ducial events
with either e+ or e; inside EMC detection volume. [Numbers need check]
Event type
Cross- Production Level 1 Trigger
section Rate ( Hz)
Rate ( Hz)
( nb)
BB
1.05
3.2
3.2
uu+dd+cc+ss
3.39
10.2
10.1
e + e;
~ 53
159
156
+ ;
1.16
3.5
3.1
+ ;
0.94
2.8
2.4
100
5639
5640
5641
5642
5643
5644
5645
5646
5647
5648
5649
5650
5651
5652
5653
5654
5655
5656
5657
5658
5659
5660
5661
5662
5663
5664
5665
5666
5667
5668
5669
5670
5671
5672
5673
5674
5675
5676
5677
5678
5679
5680
5681
5682
5683
5684
5685
DRAFT
The trigger is implemented in two levels; the
Level 1 (L1) hardware and Level 3 (L3) software triggers. The underlying concept is that the
Level 1 is as open as possible to physics, after
which Level 3 selects the events of most interest,
consistent with the desired output rate.
As an indication for the level of background the
Level 1 need to reduce from, the expected rate of
background interactions producing at least one
track in the drift chamber with pt >120 MeV=c
or at least one cluster in the calorimeter with
E >100 MeV is 10 kHz each at a luminosity of
3 1033. The Level 1 is required to have an output rate of less than 2 kHz. It needs to operate
with a latency xed within a de ned range; for all
events which are triggered, 99% of the trigger signals have to be delivered to the Fast Control and
Timing System (FCTS) within the time window
between 11 and 12 s after the event occured. All
parts of the Level 1 are also required to read out
sucient trigger data for each event to allow ofine calculation of the eciencies from the data
themselves.
The Level 3 performs the second stage of rate
reduction from the maximum Level 1 output rate
to a maximum nal rate of 120 Hz. It must reject
events from a number of background processes
which are not of physics interest. In addition, it
must ag events needed for luminosity, diagnostics and calibration purposes. It is required that
only 90 Hz of the output rate is used by physics
events, with the remaining 30 Hz used for this
other information. Finally, Level 3 software is
required to comply with the BABAR software conventions and standards; in particular, the code
has to usable in both online and oine systems
to reduce duplication of e ort and also provide
consistency.
13.2. Level 1 trigger system
The Level 1 trigger selects events and achieves
its rate reduction based on a fraction of the detector data processed by specialized hardware. Hits
in the drift chamber, energy deposition in the
electromagnetic calorimeter and hit occupancy in
the instrumented ux return systems are used to
quickly identify particle tracks, the presence of
deposited energy, and the presence of muons, re-
5686
5687
5688
5689
5690
5691
5692
5693
5694
5695
5696
5697
5698
5699
5700
5701
5702
5703
5704
5705
5706
5707
5708
5709
5710
5711
5712
5713
5714
5715
5716
5717
5718
5719
5720
5721
5722
5723
5724
5725
5726
5727
5728
5729
5730
5731
5732
5733
February 6, 2001 { 10 : 40
spectively. The drift chamber trigger (DCT) and
electromagnetic calorimeter trigger (EMT) are
both designed to independently satisfy all triggering requirements with high eciency, thereby
providing the needed degree of redundancy. The
instrumented ux return trigger (IFT) is used for
triggering -pair and cosmic ray events, and for
diagnostic purposes.
Each of the three Level 1 trigger processors generates trigger \primitives" (summary data on the
position and energy of particles) that is sent to
the global trigger (GLT) which then forms 24 discreet triggers (which may be \true" or \false").
The FCTS, receiving these 24 signals, can optionally select a subset of triggers with a mask,
prescale some of the triggers and | if a valid trigger remains | issue a Level 1 Accept to initiate
event readout. The trigger de nition logic, masks
and prescale values are all con gurable on a per
run basis.
The overall structure of the Level 1 system is
illustrated in Figure 77. All the Level 1 hardware shown in this diagram is hosted in ve 9u
VME crates. The Level 1 system operates in
a continuous sampling mode of processing input
data and generating output trigger information
at xed time intervals. The DCH frontend electronics and the EMC UPCs send raw data to the
DCT and EMT 2 s after the event time. The
DCT and EMT event processing time are 4{5 s
and it is followed by another 3 s in the GLT
to issue an Level 1 trigger. The Level 1 trigger
takes 1 s to propagate through the fast control system and the ROMs to generate a readout
just before the event data reaching the end of the
12.8 s detector frontend bu er.
The basic trigger primitives generated by the
DCT, EMT and IFT are mostly only the azimuthal angle () projections of the particle raw
trigger signals, with only a few cases of very
coarse polar angle () information, such as barrel and endcap distinctions. The trigger primitives are processed in the GLT to generate up to
24 Level 1 trigger lines which are passed on to
the FCTS. The DCT and EMT primitives sent
to the GLT are maps signalling tracks or energy deposits in the various regions. The IFT
primitive is a three-bit pattern representing sex-
DRAFT
February 6, 2001 { 10 : 40
101
Table 15
Trigger primitives. Most energy thresholds are adjustable and the listed thresholds are typical values.
The detailed de ntion for the IFT U pattern can be found in Table 16.
Description
Origin No. of bits Threshold
B Short track reaching DCH superlayer 5 BLT
16 120 MeV=c
A Long track reaching DCH superlayer 10 BLT
16 180 MeV=c
A0 High pt track
PTD
16 800 MeV=c
M All- MIP energy
TPB
20 100 MeV
G All- intermediate energy
TPB
20 250 MeV
E All- high energy
TPB
20 700 MeV
X Forward endcap MIP
TPB
20 100 MeV
Y Backward barrel high energy
TPB
10
1 GeV
U Muon IFR sextant hit pattern
IFS
3
5734
5735
5736
3 bits/134 ns
IFR FEE
FastOR
IFR Muon Trigger
Synch Board (IFS)
[1]
Calorimeter Trigger
EMC ROM Processor Board (TPB)
[10]
24 bits/67 ns
5738
5739
5740
5741
5742
5743
32 bits/134 ns
DCH FEE
5737
Global Level 1
Trigger (GLT)
90 bits/
[1]
134 ns
Drift Chamber Track
Segment Finder (TSF)
[24]
16 bits/134 ns
5744
5745
Fast Control
5746
5747
320 bits/134 ns
Drift Chamber Binary
Link Tracker (BLT)
[1]
176x16 bits/269 ns
Drift Chamber PT
Discriminator (PTD)
[8]
5748
5749
5750
5751
5752
5753
5754
5755
5756
Figure 77. Level 1 system schematics for the major L1 components. The numbers on the links
between di erent components are the tansmission
rates of total signal bits.
5757
5758
5759
5760
5761
5762
5763
5764
tant hit topology in the IFR. The meanings of
the various trigger primitive inputs to the GLT
are summarized in Table 15. The DCT, EMT
and IFT primitives are all time-stretched to reect the uncertainty in their time determination
before being sent to the GLT.
The DCT, EMT and GLT contain 4-event
bu er DAQ readout capabilities similar to other
detector system front-end DAQ designs. The
DAQ data contain information from various
stages of the trigger for each event and are used
for monitoring and determining trigger eciencies, as well as providing seed information for
Level 3.
13.2.1. Level 1 drift chamber trigger
The input data from the DCH front-end electronics (FEE) to the DCT consist of one bit, updated every 269 ns, for each of the 7104 DCH
cells (see section 6.4 for details). These bits convey time information from amplitude discriminators whose input is the wire signal for that cell.
The DCT outputs primitives consisting of three maps of 16 bits each, as listed in Table 15, which
are sent to the GLT every 134 ns.
From the input signal bits, the DCT continuously links wire hits into \segments", a series
of associated hits in one DCH superlayer. It
then chains segments into tracks, and nds the
position at superlayer 5 (10) for a short (long)
track. It also determines whether the tracks represent particles having transverse momentum (pt )
DRAFT
102
Track
6
5799
2
5800
4
Super
layer
5
7
5798
Pivot cell layer
1
3
5801
5802
5803
0
5804
5805
5806
8 Cell Template
5807
5808
5809
Figure 78. Track Segment Finder pivot cell group.
5765
5766
5767
5768
5769
5770
5771
5772
5773
5774
5775
5776
5777
5778
5779
5780
5781
5782
5783
5784
5785
5786
5787
5788
5789
5790
5791
5792
5793
5794
5795
5796
5797
greater than a preset value. The algorithms of
the DCT are executed in three types of modules. First, the track segments and positions are found using a set of 24 Track Segment
Finder (TSF) modules [1]. The segments are
then passed to the Binary Link Tracker (BLT)
module [2], where they are linked into complete
tracks. In parallel, detailed information for segments found in axial superlayers is transmitted to
the eight PT Discriminator (PTD) modules [3],
which determine if the segments are consistent
with tracks of particles having a pt greater than
a user-speci ed minimum.
Each TSF is responsible for processing a subset of the DCH input data and extracting track
segments from a set of contiguous hits within
a \pivot group", see Figure 78. For any pivot
group, the cells are numbered 0 through 7, with
cell 4 being the pivot cell. The shape of a
pivot group was chosen such that only reasonably straight tracks originating from the interaction point can produce a segment. Each of the
24 TSF modules consists of 72-75 track segment
engines, one for each pivot group, which amounts
to a total of 1776 pivot groups. The engine processes the data from the eight cells in its assigned
pivot group to determine whether a grouping of
hit cells can produce a track segment. Depending on where a track passes through a particular
cell, the resulting ionization will take one to four
269 ns clock ticks to drift to the signal wire. It
is this time delay, or drift time, that the TSF
uses to more accurately establish the position of
5810
5811
5812
5813
5814
5815
5816
5817
5818
5819
5820
5821
5822
5823
5824
5825
5826
5827
5828
5829
5830
5831
5832
5833
5834
5835
5836
5837
5838
5839
5840
5841
5842
5843
5844
5845
February 6, 2001 { 10 : 40
the track as well as the event time. Typically,
valid segment patterns consist of hits, close together in time and in at least three out of four
layers within the superlayer (to account for cell
ineciencies). At the hardware level, this is implemented using a self-triggered two-bit counter
for each of the eight cells in the pivot group. A
counter is enabled when a hit is registered. By
incrementing the counter every 269 ns, a 16-bit
address, corresponding to 65,536 possibilities, is
obtained at any given clock tick. Each non-zero
address is then translated by the pre-loaded lookup table (LUT) into a two-bit weight indicating
whether there is a four-layer segment, a threelayer segment, or a low-quality (un-calibrated)
segment. A three-layer segment with the pivot
cell hit missing is also allowed. The pivot group
is monitored to determine which of the three subsequent clock ticks produces the highest weight,
or \best" pattern. This time information is used
to align each segment in time and to create a
coincidence window for all segments related to
the same event. The contents of the LUT are
derived from an oine calibration and consist
of high-precision position and event time information. The segment arrival time is estimated
mainly based on the variation of weight in time.
The resulting time-adjusted weight and position
are then passed on to the BLT and PTD's. The
data sent to the BLT are the `coarse-' data with
just the wire address, while the ` ne-' data sent
to the PTD's contain the high resolution position
information. Since the TSF segment arrival time
estimate combining information of several hits is
considerably more precise than the 269 ns input
time interval, the TSF data for BLT is transmitted at 134 ns intervals to gain on timing resolution.
The position resolution as measured from the
data after calibration, is 600 m for a 4-layer
segment and 900 m for a 3-layer segment typically. For tracks from the IP, the eciency for all
TSF segments is 97%, and the eciency for calibrated 3-layer or 4-layer TSF segments is 94%.
The BLT receives segment hit information from
all 24 TSF's, corresponding to the entire DCH, at
a rate of 320 bits every 134 ns and links them into
complete tracks. The segment hits are mapped
5846
5847
5848
5849
5850
5851
5852
5853
5854
5855
5856
5857
5858
5859
5860
5861
5862
5863
5864
5865
5866
5867
5868
5869
5870
5871
5872
5873
5874
5875
5876
5877
5878
5879
5880
5881
5882
5883
5884
5885
5886
5887
5888
5889
5890
5891
5892
5893
DRAFT
onto the DCH geometry in terms of \supercells",
with 10 radial superlayers and 32 sectors in ,
and each bit indicates whether a segment is found
in that supercell or not. The input data to the
BLT are combined using a logical OR with a programmable mask pattern. The masking capability allows the system to activate track segments
that correspond to dead or highly inecient cells
so that the track eciency does not degrade. The
linking algorithm uses an extension of a simple
method, developed for the CLEO-II trigger [5],
to link the segments into a continuous track. It
starts from the innermost superlayer, A1, and
moves radially outward. A track is found if there
is a segment hit in every layer and if the segments
in two consecutive superlayers are within a certain number of supercells (three or ve depending on the superlayer type) of each other. This
allows for track curvature in the magnetic eld
and dip angle variations. Up to two superlayers are allowed to be missing. Tracks that reach
the outer layer of the DCH (superlayer A10) are
classi ed as type A. Tracks that reach the middle
layer (superlayer U5) are classi ed as type B. The
data are then compressed and output to the GLT
in the form of two words of 16 bits each corresponding to A and B tracks. Each bit in a word
represents the \track hit" state of a supercell in
the designated superlayer.
The eight PTD's receive the ne- information on track segments from axial superlayers
only, and determine if the segments are consistent with tracks having a pt greater than some
con gurable minimum. An envelope for tracks
above the minumum pt is de ned using the interaction point (IP) and a track segment position in
one of the \seed" superlayers, A7 or A10. A high
pt A0 candidate is identi ed when there are sucient track segments with accurate information
lie inside this envelope.
The processing on each PTD is subdivided in
eight processing engines, one for each supercell
in each of the two superlayers A7 and A10. The
principal components in each engine are an algorithmic processor and LUT's containing the limits for each individual seed position. The contents
of the LUT's thus specify the allowed track segment positions for each of the three other axial
103
Efficiency
February 6, 2001 { 10 : 40
BABAR
1
0.75
0.5
0.25
0
0
0.25
0.5
0.75
1
Track transverse momentum (GeV/c)
Figure 79. DCT track eciency vs. pt , where the
A0 threshold is set to 800 MeV=c.
5894
5895
5896
5897
5898
5899
5900
5901
5902
5903
5904
5905
5906
5907
5908
5909
5910
5911
5912
5913
5914
5915
5916
superlayers and consequently de ne the e ective
pt discrimination threshold.
The resulting pt turn-on for the PTD A' tracks
is shown in Figure 79 together with the BLT A,B
track eciency.
Each of the three main boards relies heavily on
multiple FPGA's [4] which perform the on-board
control and algorithmic functions. All cabling is
handled by a small (6u) back-of-crate interface
behind each main board.
13.2.2. Level 1 calorimeter trigger
The input data for the EMT are 280 \tower
sums" in the calorimeter. The EMC barrel consists of an array of 48 120 ( ) crystals. These
are grouped into \towers" of 8 3, giving 6 towers in by 40 in . Each barrel tower therefore
corresponds to 24 crystals. The endcap is divided
into 40 wedges in , making a further 40 towers,
each containing between 19 and 22 crystals. The
towers thus form an array of 7 40 covering the
whole of the EMC. The energies of crystals in
each tower, if above a threshold of 10 MeV, are
summed and sent to the EMT as an unsigned 16-
5917
5918
5919
5920
5921
5922
5923
5924
5925
5926
5927
5928
5929
5930
5931
5932
5933
5934
5935
5936
5937
5938
5939
5940
5941
5942
5943
5944
5945
5946
5947
5948
5949
5950
5951
5952
5953
5954
5955
5956
5957
5958
5959
5960
5961
5962
5963
5964
DRAFT
bit word every 269 ns on dedicated cables from
the EMC UPC's.
The EMT primitives consist of ve types of maps, as listed Table 15, which are sent to the
GLT every 134 ns. Further details of the EMT
system can be found in [6].
For each of the 40 positions, the EMT algorithm sums the seven tower energies in and
sends the resulting sum to a eight-tap nite
impulse response (FIR) lter [reference needed].
The FIR output zero-crossing time is used as the
estimate of the time of the deposit. The energy is
determined from this zero-crossing by a time o set, tuned to coincide with the peak of the shaped
EMC pulse. This energy is compared with ve
thresholds, one for each of the maps, and the
corresponding primitive bit is set if the energy is
above the threshold.
The conversion of the tower data into the GLT
bits is performed by ten Trigger Processor
Boards (TPB). Each TPB receives data from 28
towers, corresponding to a strip of 7 4 in ,
and so performs the algorithm for four positions. In addition, data from the towers bordering this strip are needed. Each TPB receives summary data from another TPB for the seven towers in which are at the position immediately
next to its 28 towers. Each TPB also outputs the
equivalent data to another TPB for the seven towers in at the highest position of its 28 towers.
These \nearest neighbour" data allow energy deposits which overlap two TPB's to be accurately
reconstructed. The complete algorithm is implemented in one FPGA [7] for each position, with
four identical components per TPB.
The basic performance of the EMT can be expressed through the eciency and timing jitter of
the trigger primitives. The eciency of the primitives can be measured by the number of times a
trigger bit is set as a function of the actual energy reconstructed oine in the EMC. Figure 80
shows this eciency for energies around the M
threshold. The trigger bit changes from 10% to
90% eciency over a range from 110 to 145 MeV
and is at the full eciency of 99% by the average minimum ionizing particle (MIP) energy of
180 MeV.
The EMT time jitter with respect to the en-
February 6, 2001 { 10 : 40
efficiency
104
1
0.75
0.5
0.25
0
0.05
0.1
0.15
0.2
EMC cluster energy (GeV)
Figure 80. EMT M eciency vs. EMC cluster
energy for an M threshold setting of 120 MeV.
5965
5966
5967
5968
5969
5970
5971
5972
5973
5974
5975
5976
5977
5978
5979
5980
5981
5982
5983
5984
5985
5986
5987
ergy deposit time is measured by comparing the
FIR output time oine with the DCH track start
time determination, t0 . The di erence has an
RMS of 150 ns, and 97% are within the 500 ns
window. The event timing is considerably more
precise when averaging over many hits.
13.2.3. Level 1 IFR trigger
The IFT is used for triggering on pairs and
for diagnostics. For the purposes of the trigger,
the IFR is divided in ten sectors, namely the six
barrel sextants and the four half-endcap doors.
The inputs to the IFT are the \Fast OR" signals
which cover all strips from eight selected layers
in each sector. The output to the GLT consists
of three bits every 134 ns.
A majority logic algorithm de nes trigger objects for every sector if at least four of the eight
trigger layers have hits inside a time window of
134 ns. The IFR Trigger Synchronization (IFS)
module processes the trigger objects from the ten
sectors and generates the three-bit trigger word
(U) encoding seven possible exclusive trigger conditions, as de ned in Table 16. The trigger U5,
February 6, 2001 { 10 : 40
5988
DRAFT
for example, covers all the 2-prong -pair topologies.
Table 16
IFR trigger pattern (U) de nition.
U
1
2
3
4
5
6
7
5989
5990
5991
5992
5993
5994
5995
5996
5997
5998
5999
6000
6001
6002
6003
6004
6005
6006
6007
6008
6009
6010
6011
6012
6013
6014
6015
6016
6017
6018
6019
6020
6021
6022
Trigger condition
other 2 topologies not in U=5-7
1 in backward endcap
1 in forward endcap
1 in barrel
2 back-back 's in barrel + 1 forward 1 in barrel + 1 forward 2 back-back 's in barrel
The eciency of the IFT has been evaluated
with cosmic rays triggered by the DCT and crossing the detector close to the interaction point.
For these events, 98% have been triggered by the
IFT as events with at least one track, and 73%
as events with two tracks, inside the geometrical
region of the IFR. Most of the ineciency is concentrated in the angular regions between two adjacent sextants and between the barrel and endcaps.
13.2.4. Level 1 global trigger
The GLT receives inputs consisting of the primitives from the DCT, EMT and IFT, as listed in
Table 15, every 134 ns. Due to the di erent latencies of the primitives, the GLT rst aligns the
input signals using con gurable delays. It then
forms some additional combined maps from the
DCT and EMT data. A total of 16 maps are input to a LUT which treats each map as an input
address and a three-bit output for each trigger object which counts, e.g., the number of B tracks or
number of M clusters. To count as distinct trigger objects, the map bits are typically required to
have a separation of more than one bin.
These 16 counts not only include the standard
inputs of A, B, A0 , M, G, E and Y, but also
matched objects such as BM for B tracks matched
to an M cluster in . B and M are also used to derive additional back-to-back objects, B and M ,
where the count requires a pair of bits separated by a con gurable angle of typically 120.
E and M are also used to make an EM object
for back-to-back EM pairs.
6023
6024
6025
6026
6027
6028
6029
6030
6031
6032
6033
6034
6035
6036
6037
6038
6039
6040
6041
6042
6043
6044
6045
6046
6047
6048
6049
6050
6051
6052
6053
6054
6055
6056
6057
6058
6059
6060
6061
6062
6063
6064
6065
6066
6067
6068
6069
105
The 16 counts plus the IFT hit pattern are
then used to form 24 raw trigger lines. The nal stage logic selects the highest priority trigger
to determine the Level 1 trigger time and latches
the other triggers compatible in time for the 24
line trigger output. The trigger decision logic for
each trigger line is a logical AND of the 17 outputs from operations applied to the object counts.
The decision operations on each object can be:
always-pass, or , = or < to a con gurable cut
parameter. The trigger logic evaluates the timecentroid of the highest priority line with a 67 ns
sampling period based on the last 1 s of history. This faster sampling rate can improve output trigger timing resolution in the case of sending output at the time-centroid of a trigger which
is on for an odd number of 134 ns intervals. If a
higher priority trigger line starts within this time,
the process resets to follow the higher priority line
instead. There is an optional con gurable output
delay for each line to control the Level 1 trigger
to be set as late as allowed by its timing jitter
so as to minimize the chance of early background
hits poisoning real signals in the SVT single hit
electronics.
The 24 bit GLT output signal is sent to the fast
control system every 67 ns, which handles trigger mask selection and prescaling. The achieved
timing resolution for hadronic data events has an
RMS of 52 ns and 99% of the events are within
77 ns.
The GLT consists of a single 9u VME module. Most of the logic, including diagnostic and
DAQ memories, are implemented in FPGA's [4].
The LUT section is an array of 16 memory chips
with 8 Mbytes of VME downloadable con guration data.
13.3. Level 1 trigger performance and operational experience
The Level 1 trigger con guration consists of
pure DCT, pure EMT, mixed and prescaled triggers, not only aimed for maximum eciency and
background suppression, but also for the convenience of trigger eciency determination. The
trigger lines are designed to collectively preserve
the many types of physics interactions. Although
most trigger lines have as their primary target a
106
6070
6071
6072
6073
6074
6075
6076
6077
6078
6079
6080
6081
6082
6083
6084
6085
6086
6087
6088
6089
6090
6091
6092
6093
6094
6095
6096
6097
6098
6099
6100
6101
6102
6103
6104
6105
6106
6107
6108
6109
6110
6111
6112
6113
6114
6115
6116
6117
DRAFT
speci c physics source, they are not strictly classied to only serve that source. For example, some
of the two-particle triggers are not only ecient
for Bhabha, and events, but are also useful
for selecting jetlike hadronic events and some rare
B decays.
The eciencies and rates of some selected
Level 1 trigger lines are listed in Table 17 and for
various physics processes. Although triggering on
generic BB events is relatively easy, it is essential
to make sure high eciencies are also maintained
for the important rare B decay processes which
are typically less distinctive in decay multiplicity.
We therefore include eciencies for events containing two bench mark processes of B 0 ! 0 0
and B ; ! (while the other B in the event
decay generically) in Table 17.
The eciencies for the hadronic events are
absolute eciencies including acceptance losses,
based on MC simulation including local ineciency e ects calibrated using single particle triggering eciencies from data. The eciencies for
-pair events are for MC \ ducial" events, having
two or more tracks with pt > 120 MeV=c originating close to the IP and with in the range to reach
at least DCH superlayer U5. The Bhabha and pair eciencies are determined from the data, for
events with 2 high momentum particles back to
back in e+ e; center of mass frame, and within
the EMC ducial volume. It can be seen that
with the exception of events, there are ecient orthogonal triggers using only DCT or only
EMT/IFT for these processes. The eciencies
predicted by the MC are generally in good agreement with data when tested using events passing typical analysis selections and based on orthogonal triggers. Prescaled triggers with a very
open acceptance of physics events, such as (B2
& A1) or (M2) are also used to measure the
trigger eciencies.
The trigger rates listed in Table 17 are for
a typical run with HER (LER) currents at
650 mA (1350 mA) and a luminosity of 2:2 1033
cm;2 s;1. These are stable to within 20% for the
same PEP-II setup. Longer term they can be
50% higher after a major shutdown, when starting with a poor vacuum, and gradually improve in
the following months. There are also occasional
February 6, 2001 { 10 : 40
4000
2000
0
-50
0
50
L3 track z0 (cm)
Figure 81. Track z0 for all Level 1 triggers.
6118
6119
6120
6121
6122
6123
6124
6125
6126
6127
6128
6129
6130
6131
6132
6133
6134
6135
6136
6137
6138
6139
6140
6141
6142
background spikes which can double the Level 1
rate. However, even many of these spikes cannot
induce signi cant dead time due to the Level 1
2 kHz capability. The expected Level 1 trigger
rate for the design luminosity of 3 1033 cm;2 s;1 ,
based on current observations, will be well below
2 kHz.
Within the typical Level 1 rate of 970 Hz,
Bhabha and annihilation physics events amount
to 130 Hz. There is also 100 Hz of cosmics and
20 Hz of random beam crossing triggers. The
dominant source of background which causes the
remaining triggers is due to lost particles interacting with the beam line components. The distribution of track z0 (as reconstructed by Level 3) for
all input Level 1 triggers are shown in Figure 81.
The most prominent peaks at z = 20 cm correspond to the end ange of the beam pipe. The
peak at z = ;55 cm corresponds to a step in the
synchrotron mask.
The Level 1 trigger hardware operation has
been very stable. For the rst one and half years
of operation, there have been only 4 hardware
failures in the whole Level 1 system, mainly auxilliary or communication boards. The only occa-
February 6, 2001 { 10 : 40
DRAFT
107
Table 17
Level 1 Trigger eciency (%) and rates (Hz) at a luminosity of trigger 2:2 1033 cm;2 s;1 for some
selected triggers lines applied to various physics processes. The symbols refer to the counts for each
object. The un lled \;" entries are cases where the line is not intended for this physics source.
Level 1 Trigger
BB B!0 0 B! cc uds
ee Rate
A3 & B 1
97.1
66.4
81.8 88.9 81.1
; ; 17.7 180
A1 & B 1 & A'1
95.0
63.0
83.2 89.2 85.2 98.6 99.1 79.9 410
All pure DCT OR
99.1
79.7
92.2 95.3 90.6 98.9 99.1 80.6 560
M3 & M 1
99.7
98.6
93.7 98.5 94.7
; ; 53.7 160
EM 1
71.4
94.9
55.5 77.1 79.5 97.8
; 65.8 150
All pure EMT OR
99.8
99.2
95.5 98.8 95.6 99.2
; 77.6 340
B3 & A2 & M2
99.4
81.2
90.3 94.8 87.8
; ; 19.7 170
M 1 & A1 & A'1 95.1
68.8
83.7 90.1 87.0 97.8 95.9 78.2 250
E1 & B2 & A1
72.1
92.4
60.2 77.7 79.2 99.3
; 72.8 140
M 1 & U5 (-pair)
;
;
; ; ;
; 60.3 ; 70
All Level 1 triggers
>99.9
99.8
99.7 99.9 98.2 >99.9 99.6 94.5 970
6143
6144
6145
6146
6147
6148
6149
6150
6151
6152
6153
6154
6155
6156
6157
6158
6159
6160
6161
6162
6163
6164
6165
6166
6167
6168
6169
6170
sional online adjustment needed was for the EMT
tower mask to temporarily suppress noisy channels in the EMC electronics.
13.4. Level 3 trigger system
The Level 3 trigger makes its selection based
upon the complete event data from BABAR processed by a farm of 32 commercial Unix processors. The Level 3 also has access to the output of
the Level 1 trigger processors and FCTS trigger
scalers. Level 3 operates by improving upon the
themes de ned in Level 1. For example, better
DCH tracking (vertex resolution) and EMC clustering allow for greater rejection of beam backgrounds and more e ective prescaling of Bhabhas. Filters to select speci c rates of luminosity
and calibration events are also implemented.
The Level 3 system runs within the Online
Event Processing (OEP) framework (see section ??). Along with other responsibilities, OEP
hands events to the Level 3 algorithm, then
prescales and logs those which trigger. The execution time budget for the complete Level 3 code
is 10 ms per event on a 333 MHz Sun Ultra5 node.
To provide optimum exibility under di erent
running conditions, Level 3 was designed according to a general logic model that can be con gured to support an unlimited variety of event selection mechanisms. The logic model provides for
6171
6172
6173
6174
6175
6176
6177
6178
6179
6180
6181
6182
6183
6184
6185
6186
6187
6188
6189
6190
6191
6192
6193
6194
6195
6196
6197
6198
6199
a number of di erent, independent classi cation
tests, called \paths", that are executed in parallel, together with a mechanism for combining
these tests into the nal set of classi cation decisions. Thus, modi ed or new algorithms can be
handled with no changes to the underlying logic.
The Level 3 design is based on a three-phase
logic model. In the rst phase, events are classied on the basis of the Level 1 and FCTS decisions (the 32 FCTS output lines). These classi cation decisions are referred to as Level 3 \input
lines". Any number of Level 3 input lines may
be de ned, but it is required that each Level 1
output line must be used in de ning at least one
Level 3 input line, thereby ensuring that any interesting physics events selected by Level 1 are
viewed by Level 3.
In the second phase, events are tested using a
series of \scripts", each of which requires the logical AND of various event properties. The decision
of which scripts to execute is determined by the
values of the Level 3 input lines. Each script constructs objects of interest from the raw event data
and tests the event for the logical AND of various properties, e.g., charged particle multiplicity,
EMC energy, etc. Each script returns a pass/fail
decision in the form of a ag.
In the nal phase, the Level 3 output lines
are formed by considering combinations of the
108
6200
6201
6202
6203
6204
6205
6206
6207
6208
6209
6210
6211
6212
6213
6214
6215
6216
6217
6218
6219
6220
6221
6222
6223
6224
6225
6226
6227
6228
6229
6230
6231
6232
6233
6234
6235
6236
6237
6238
6239
6240
6241
6242
6243
6244
6245
6246
DRAFT
results of the scripts. Each output line is dened as the logical OR of a subset of the script
ags. The Level 3 design allows output lines to
consider certain script ags as vetoes, rejecting
events that otherwise satisfy the line's requirement if the veto is true. As an example, Bhabhascattering events, a particularly high-rate background for scripts which select events based on
charged tracks or EMC energy deposits, are rejected by a Bhabha veto from lines that correspond to interesting physics and therefore may
not be prescaled by OEP.
The Level 3 implementation depends crucially
on several aspects of Framework. Any code in
the form of \modules" can be included and runtime con guration using a standard Tcl interface is allowed. Framework supports multiple
execution paths and Level 3 exploits this feature. When multiple paths are used, the same
instance of a module may be included in multiple paths. Framework ensures that the event
processing for such modules is only invoked once
per event. Hence calculations done by a module are not repeated even if the module is called
more than once. Thus, the same module may be
used on multiple paths without incurring signi cant CPU overhead. Finally, Framework provides
the ability to terminate processing of an event
before all modules have been executed. Thus,
a \ lter module" can deliver a pass/fail indicator to Framework for a given event. If the lter
module fails the event, Framework terminates the
path and proceeds immediately to either the next
path, if any paths remain to be executed, or to
the next event.
13.4.1. Level 3 drift chamber tracking algorithm
A large fraction of the events which passes
Level 1 but must be rejected by Level 3, is background events in which charged particles are produced in material located close to the IP. Level 1
does not currently have sucient tracking resolution to identify these background tracks but
Level 3 can do full three-dimensional track nding
and tting. The DCH-based algorithm, L3Dch,
consists of fast pattern recognition, that is ecient for tracks coming from the IP, and a track
6247
6248
6249
6250
6251
6252
6253
6254
6255
6256
6257
6258
6259
6260
6261
6262
6263
6264
6265
6266
6267
6268
6269
6270
6271
6272
6273
6274
6275
6276
6277
6278
6279
6280
6281
6282
6283
6284
6285
6286
6287
6288
6289
6290
6291
6292
6293
6294
February 6, 2001 { 10 : 40
tter, which determines the ve helix track parameters for tracks with pt above 250 MeV=c.
To speed up the process of pattern recognition, L3Dch starts with the track segments from
the TSF system. The TSF provides an address
that can be decoded to give information about
which wires were hit in the pivot cell and an estimated position. To improve the resolution
L3Dch makes use of the actual DCH hit information for hits which were reported as used on the
TSF segments.
For those TSF segments that have a staggered
pattern indicating a simple solution to the leftright ambiguity for each layer, a track t0 is determined. The t0 values for each segment in an event
are binned and the average value in the most
populated bin is used as the estimated event t0 .
Almost all events, physics or background, which
pass Level 1 have enough segments to form a t0
estimate. The rms resolution on this estimate is
1.8 ns for Bhabha events and 3.8 ns for hadronic
events.
The actual pattern recognition for L3Dch is
done with a look up table. This track table is
made by considering the DCH as divided into 120
sectors, corresponding to the number of cells in
the innermost layers. The track table is made up
of the sectors for all tracks originating within
2 cm of the IP in the xy plane and within 10 cm
in z and having a pt above 250 MeV=c. The pattern recognition consists of looping over the table
entries looking for matches to segments found by
the TSF's. The located set of segments for a given
track is then passed to the track tter. In order
to account for ineciencies, the track table allows
for up to two missing DCH layers.
The track tter is provided with both the track
segments and the \seed" track which made the
entry in the lookup table. From this information, the track tter ts the ve helix parameters, adding segments not found in the original
pattern recognition, if needed. Hits which have
large residuals are dropped from the t. The t
is iterated so that segments close to the initial
track t can be added to the track before a new
t is performed. The nal t does not demand
that the track originate from the IP.
The two-track miss distances near the IP for
DRAFT
February 6, 2001 { 10 : 40
1000
6325
6326
800
6327
6328
600
6329
400
6330
6331
200
6332
0
6333
-1
0
1 -8
d0 miss (cm)
0
8
z0 miss (cm)
6334
6335
6336
6337
Figure 82. Level 3 track d0 and z0 miss distance
for Bhabha events.
6295
6296
6297
6298
6299
6300
6301
6302
6303
6304
6305
6306
6307
6308
6309
6310
6311
6312
6313
6314
6315
6316
6317
6318
6319
6320
6321
6322
6323
6324
Bhabha events are plotted in Figure 82. The individual track impact parameter resolution can be
derived from the p
width of these distributions by
multiplying by 1= 2. This gives an Level 3 track
d0 resolution of 0.83 mm and and an average z0
resolution of 6.9 mm. Similarly, tting the 1=pt
di erence between the 2 tracks in -pair events
yields a pt resolution of pt =pt 0:019 pt= GeV=c
for Level 3 tracks.
13.4.2. Level 3 calorimeter clustering algorithm
A number of events which are interesting for
CP physics, calibration or luminosity measurements consist essentially of only neutral particles.
Hence an all-neutral trigger for Level 3 based on
information from the EMC is essential. In addition, calorimeter information is vital to complement the drift chamber data for the identi cation of Bhabha events, in particular for the veto
algorithm. Finally, partial orthogonality in triggering based on information from the DCH and
the EMC is necessary to obtain a thorough understanding of the eciency of triggers based on
either of these systems.
The Level 3 EMC-based trigger algorithm,
L3Emc, operates by nding EMC clusters at energies down to the MIP level. The clustering is done
in two steps, the processing of the EMC data to
assemble lists of adjacent crystals with signi cant
energy deposits and the formation of clusters and
6338
6339
6340
6341
6342
6343
6344
6345
6346
6347
6348
6349
6350
6351
6352
6353
6354
6355
6356
6357
6358
6359
6360
6361
6362
6363
6364
6365
6366
6367
6368
6369
6370
6371
109
computation of integrated quantities, such as the
total energy, centroid position, and cluster moments.
Out of the total 6580 crystals in the calorimeter, the EMC at present sends typically 1900 socalled \EMC digi's" per event, the vast majority
of these being caused by electronics noise. An
EMC digi encodes the peak energy and time of
the crystal waveform as determined by the EMC
feature extraction. To lter out noise, L3Emc
applies an energy threshold of currently 30 MeV
and and a time-window cut of t > 5:7 s on the
raw data on a per crystal basis. For the remaining crystals, raw energies and times are converted
into physical units to create \L3Emc digi's".
The L3Emc digis are lled into a linear list and
fed into a fast clustering algorithm that performs
a single iteration to split it into sub-lists of neighboring crystals. The neighboring information is
not buried in the algorithm but is con gured for
each crystal by a lookup table. The lookup table addresses are mapped to detector coordinates
by a hash algorithm using module, ber, channel id's. From the resulting lists of adjacent hits,
clusters are formed by summing contiguous crystal energies. If the total energy is above 100 MeV,
the Level 3 clusters are stored with an energy
weighted centroid and average time, the number
of crystals and a lateral and Zernicke moment
[reference needed] describing the shower shape for
particle identi cation.
13.4.3. Level 3 lters
Based on the Level 3 tracks and clusters that
are computed with the tools described above, a
variety of lter algorithms is implemented, which
perform the event classi cation and background
reduction.
The logging decision is essentially represented
by two orthogonal sets of lters which form the
open physics trigger lines and which are based on
pure drift chamber and pure calorimeter information, respectively.
The drift chamber algorithms comprise two IP
track lters which select events with one \tight"
(high pt ) track or two \loose" tracks originating
from the interaction point, respectively. The high
pt track is required to have a transverse momen-
110
6372
6373
6374
6375
6376
6377
6378
6379
6380
6381
6382
6383
6384
6385
6386
6387
6388
6389
6390
6391
6392
6393
6394
6395
6396
6397
6398
6399
6400
6401
6402
6403
6404
6405
6406
6407
6408
6409
6410
6411
6412
6413
6414
6415
6416
6417
6418
6419
DRAFT
tum of pt >800 MeV=c and to satisfy tight vertex
cuts of jd0 j <1.0 cm, jz0 j <7.0 cm. Two tracks
are accepted with pt>250 MeV=c and somewhat
looser vertex cuts of jd0 j <1.5 cm, jz0j <10.0 cm.
The calorimeter algorithm uses two cluster lters which select events with either high energy
deposits or high cluster multiplicity and with a
high (pseudo) event mass. The event mass is calculated from the cluster energy sums and the energy weighted centroid positions assuming massless particles. The lters require either 2 clusters
of ECM > 350 MeV in the center-of-mass or 4
clusters, which form an event mass greater than
1.5 GeV in both cases.
At current luminosities the output of these
open lters is dominated by Bhabha events, which
therefore need to be rejected from the respective
physics lines. This is accomplished by a Bhabha
veto algorithm, which makes use of a very pure
identi cation based on tight track-cluster matching criteria. The veto algorithm is separated into
the 1-prong (with only a positron in the backward
part of the detector) and 2-prong category (with
both particles in the detector). Both algorithms
impose stringent criteria on EMC energy deposits
that are consistent with Bhabha events, while relying on both the track momenta and on E=p to
allow for nal state radiation. The 2-prong veto
requires either a small acolinearity between the
tracks in the center-of-mass or one that is consistent with initial state radiation (ISR), where a
photon is radiated along the beam direction leading to a missing energy close to the observed missing track momentum.
For purposes of calibration and oine luminosity measurements, a fraction of Bhabha events are
agged on a separate trigger line by an eciencyoriented algorithm. The output event distribution is ` attened' using a binned prescaling mechanism in order to obtain similar calibration statistics for all EMC crystals. In addition, calibration
samples include radiative Bhabha events,
nal
state events and cosmic rays. Radiative Bhabha
events are identi ed by selecting two-prong events
with missing energy and requiring a calorimeter
cluster in a cone around the missing momentum
vector with no requirement on the cluster energy.
e+ e; ! events are selected with no tracks
6420
6421
6422
6423
6424
6425
6426
6427
6428
6429
6430
6431
6432
6433
6434
6435
6436
6437
6438
6439
6440
6441
6442
6443
6444
6445
6446
6447
6448
6449
6450
6451
6452
6453
6454
6455
6456
6457
6458
6459
6460
6461
6462
6463
6464
6465
6466
February 6, 2001 { 10 : 40
and two high energy clusters back-to-back in the
center-of-mass. The cosmics selection is driftchamber-based and requires two tracks back-toback in the laboratory frame with a small di erence in impact parameters and curvature, where
a signi cant background from ISR Bhabha events
faking this topology is removed based on the relation between the visible energy and the track
momenta (as used by the 2-prong veto).
As an important part of online monitoring and
diagnostics, there are lters for the online luminosity measurement, which is served by a trackbased lepton-pair selection with a well known efciency, and hadronic lters for selection of continuum and BB -enriched samples. The latter two
are distinguished by an event shape cut using a
ratio of Fox-Wolfram moments [8]. Both selections are combined for an online determination
of the hadron to luminosity ratio, which provides
sensitivity to the (4S ) line shape and is used to
monitor the center-of-mass energy of PEP-II.
13.5. Level 3 performance and operational
experience
The Level 3 trigger eciencies for Level 1 accepted events are tabulated in Table 18, for various physics processes with similar de nitions as
in Table 17, based on production Level 3 processing applied to MC simulation events. High
eciencies are achieved for the DCH and EMC
based lters independently for hadronic events.
The comparison between data and MC Level 3
trigger pass fractions for the various lters also
show good agreement when requiring tracking
and EMC based hadronic event selections in turn.
An example of the actual Level 3 event display
used for online monitoring with Level 3 recontructed tracks and Level 3 EMC clusters is shown
in Fig. 83, together with the Level 1 and Level 3
trigger line status for the event.
For a typical run on the (4s) peak with
an average luminosity of 2.61033 cm;2 s;1 , the
Level 3 total output rate is 120 Hz and the event
composition is tabulated in table 19. The desired
physics sources contribute 13% of the total output
and the calibration and diagnostic samples take
up 40% of the total. An improved version of the
Level 3 IP track lter to be deployed in the 2001
February 6, 2001 { 10 : 40
DRAFT
111
Figure 83. An Level 3 event display. The small circles and small crosses in the DCH volume are DCH hits
and TSF segment hit wires respectively. The lled EMC crystals represent energy deposit (full crystal
depth=2 GeV) from Level 3 EMC clusters.
112
DRAFT
February 6, 2001 { 10 : 40
Table 18
Level 3 Trigger eciency (%) for various physics processes.
Level 3 Trigger
BB B!0 0 B! cc uds 1 track lter
89.9
69.9
86.5 89.2 88.2 94.1
2 track lter
98.9
84.1
94.5 96.1 93.2 87.6
Combined DCH lters
99.4
89.1
96.6 97.1 95.4 95.5
2 cluster lter
25.8
91.2
14.5 39.2 48.7 34.3
4 cluster lter
93.5
95.2
62.3 87.4 85.5 37.8
Combined EMC lters
93.5
95.7
62.3 87.4 85.6 46.3
Combined DCH+EMC lters >99.9
99.3
98.1 99.0 97.6 97.3
Combined L1+L3
>99.9
99.1
97.8 98.9 95.8 92.0
Table 19
Level 3 output event composition at a luminosity
of 2.61033 cm;2 s;1 .
Event type
Rate (Hz)
L1,L3 pass through diagnostics
7
Random triggers, cosmics
2
Calibration Bhabhas
30
, Radiative Bhabha
10
Total calibration/diagnostics
49
Hadrons and ; 16
Unidenti ed Bhabhas
18
Other QED, 2-photon
13
Beam wall interactions
26
Total physics accept
73
6483
6484
6485
6486
6487
6488
6489
6490
6491
6492
6493
6494
6495
6496
6497
6467
6468
6469
6470
6471
6472
6473
6474
6475
6476
6477
6478
6479
6480
6481
6482
run is expected to reduce the beam wall background by a factor of 2, which will be sucient
to meet the 120Hz maximum total output rate for
the design luminosity of 31033 cm;2 s;1 , despite
the somewhat larger calibration sample demand
than the original design.
At the current performance, the Level 3 executable takes an average processing time of 8.5 ms
per event on a current 333 MHz Sun Ultra-5 farm
node. At the highest Level 1 input rates, the
Level 3 process runs at 72 % CPU usage, while
the rest is spent in OEP (including the network
event builder) and in the operating system kernel.
This results in an estimated maximum Level 1 input rate of about 2700 Hz with 32 nodes, which
is well above the design value of 2 kHz.
6498
6499
6500
6501
6502
6503
6504
6505
6506
6507
6508
6509
6510
6511
6512
6513
6514
6515
6516
13.6. Summary and outlook
Both the Level 1 and Level 3 trigger systems
have met the eciency and the maximum rate
requirements for the original design luminosity of
3 1033 cm;2 s;1 . The triggering eciencies for
BB events are generally well over the 99% design
goal for both Level 1 and Level 3. The orthogonal
trigger setup based on DCH only and EMC only
information, for both Level 1 and Level 3, has successfully delivered stability and measurability of
the overall trigger eciency. The current system
also provides a solid foundation with a clear upgrade path to higher luminosities of 1034 cm;2 s;1
or more.
Future Level 1 trigger improvements will
mostly come from further background rejection
capability a orded by algorithm re nements and
upgrades of the DCT. This is essential for reducing the load on the DAQ and Level 3 at much
higher luminosities. Re ned algorithms for the
PTD and BLT are expected to be deployed in
the 2001 run. The new PTD algorithm will effectively narrow the track d0 acceptance window,
while new BLT algorithm will narrow the track z0
acceptance. For the longer term, a major DCT
upgrade design is underway for a new set of trigger boards with the capability of selecting tracks
with a narrow z0 window around the IP. The TSF
segment spatial resolution for the stereo layers
can be utilized to provide segment z coordinates
at 2 cm resolution. The implied track z0 resolution of 4 cm show good promise for rejecting
majority of the beam wall background events at
z = 20 cm.
February 6, 2001 { 10 : 40
6517
6518
6519
6520
6521
6522
6523
6524
6525
6526
6527
6528
6529
6530
6531
6532
6533
6534
6535
6536
6537
6538
6539
6540
6541
6542
6543
6544
6545
6546
6547
6548
6549
6550
6551
6552
6553
6554
6555
6556
6557
6558
6559
6560
6561
DRAFT
The future improvement for Level 3 will also
emphasize background rejection. The current
physics lter algorithms are still rather simple
and there is room for better eciency at the same
background rejection rate. The remaining beamwall background event fraction are expected to
be signi cantly reduced at higher luminosities.
The main tasks will be more aggressive rejection of Bhabha events and suppression of uninteresting QED and two-photon events. To enhance the physics topology recognition capability, a major source of improvement is expected to
be L3Dch tracking for low momentum tracks at
pt<250 MeV=c. Given the rapid growth of computing technology, a rather moderate CPU upgrade for the Level 3 online farm in the near future will be sucient to keep up with higher luminosities of greater than 1034 cm;2 s;1 , as well
as having the CPU capacity for improved Level 3
algorithms.
REFERENCES
1. A. Berenyi et al., \Continuously Live Image
Processor for Drift Chamber Track Segment
Triggering", IEEE Trans. Nucl. Sci. 46 (1999)
348.
2. A. Berenyi et al., \A Binary Link Tracker for
the BABAR Level 1 Trigger System", IEEE
Trans. Nucl. Sci. 46 (1999) 928.
3. A. Berenyi et al., \A Real-Time Transverse
Momentum Discriminator for the BABAR
Level 1 Trigger System", submitted to IEEE
Transactions on Nuclear Science.
4. The FPGA's used in the DCT and GLT are
from ORCA 2C series manufactured by Lucent Technologies.
5. K. Kinoshita, Nucl. Instr. and Methods A276
(1989) 242.
6. P. D. Dauncey et al., \Design and Performance of the Level 1 Calorimeter Trigger
for the BaBar Detector", submitted to IEEE
Transactions on Nuclear Science.
7. The FPGA's used for EMT algorithm logic
are Xilinx 4020E.
8. G. C. Fox and S. Wolfram, Nucl. Phys. B149
(1979) 413.
6562
6563
6564
6565
6566
6567
6568
6569
6570
6571
6572
6573
6574
6575
6576
6577
6578
6579
6580
6581
6582
6583
6584
6585
6586
6587
6588
6589
6590
6591
6592
6593
6594
6595
6596
6597
6598
6599
6600
6601
6602
6603
6604
6605
6606
6607
113
14. The Online Computing System
14.1. Overview
The BABAR online computing system comprises
the data acquisition chain from the common
front-end electronics, through the embedded processors in the data acquisition system and the
Level 3 trigger, to the logging of event data. It
also includes those components required for detector and data acquisition control and monitoring, immediate data quality monitoring, and online calibration.
14.1.1. Design requirements
The data acquisition chain was designed to
meet the following basic performance requirements. It must support a Level 1 trigger accept
rate of up to 2000 Hz, with an average event size
of 32 kbytesand a maximum output (Level 3
trigger accept) rate of 120 Hz. While performing
these functions it must not contribute more than
a time-averaged 3% to dead time during normal
data acquisition.
The online system is also required to be capable of performing data acquisition simultaneously
on independent partitions | sets of detector system components | to support calibrations and
diagnostics.
The nal design embodies many other basic requirements to ensure its e ective functioning as
described in the following paragraphs.
Normal detector operation, data acquisition
and routine calibrations are performed eciently
and under the control of a simple user interface
with facilities for detecting, diagnosing, and recovering from common error conditions.
Following standard practice, the event data acquired by the system is subjected to monitoring.
Such monitoring is con gurable by experts and
designed to detect anomalies in the detector systems which, if present, are reported to operators
for rapid assessment and, if necessary, corrective
action.
Environmental conditions of the detector, such
as the state of low and high voltage power, high
purity gas supplies, and of the accelerator, such
as beam luminosity and currents, are measured
and recorded in a fashion that permits its asso-
114
6608
6609
6610
6611
6612
6613
6614
6615
6616
6617
6618
6619
6620
6621
6622
6623
6624
6625
6626
6627
6628
6629
6630
6631
6632
6633
6634
6635
6636
6637
6638
6639
6640
6641
6642
6643
6644
6645
6646
6647
6648
6649
6650
6651
DRAFT
ciation with the event data logged. Conditions
relevant to data quality are monitored for consistency with speci ed standards. Operators are
alerted if these are not met. Data-taking is inhibited or otherwise agged if conditions are incompatible with maintaining the quality of the
data.
Operational con gurations, calibration results,
software versions in use, and routine and error
messages are also recorded and correlatable, in
support of the reconstruction of the conditions
of operation when analyzing data or diagnosing
problems.
14.1.2. System Components
The online computing system is designed as a
set of subsystems using elements of a common
software infrastructure running on a dedicated
collection of hardware.
The major subsystems are:
Online Data ow (ODF) { responsible for
communication with and control of the detector systems' front-end electronics, and
the acquisition and building of event data
from them
Online Event Processing (OEP) { responsible for processing of complete events, including Level 3 (software) triggering, data
quality monitoring, and the nal stages of
calibrations;
Logging Manager { responsible for receiving
selected events sent from OEP and writing
them to disk les for use as input to the
\Prompt Reconstruction" processing
Detector Control { responsible for the control and monitoring of environmental conditions of the detector systems
Run Control { which ties together all
the other components, and is responsible
for sequencing their operations, interlocking them as appropriate, and providing a
graphical user interface for operator control
Each of these components, as well as a selection
of the common tools which tie them together are
described below.
6652
6653
6654
6655
6656
6657
6658
6659
6660
6661
6662
6663
6664
6665
6666
6667
6668
6669
6670
6671
6672
6673
6674
6675
6676
6677
6678
6679
6680
6681
6682
6683
6684
6685
6686
6687
6688
6689
6690
6691
6692
6693
6694
6695
6696
6697
6698
February 6, 2001 { 10 : 40
The entire system is coded primarily in the
C++ language, with some use of Java for graphical user interfaces. Object-oriented analysis and
design techniques have been used throughout.
This has been an important factor in the success
of the project, having produced bene ts in development speed, maintainability, and extensibility.
14.1.3. Hardware infrastructure
The hardware infrastructure for the online system is shown schematically in Figure 84.
The data from the front-end electronics (FEE)
of the various detector systems are routed via optical ber links to a set of 157 custom VME Readout Modules (ROMs). These ROMs are grouped
by detector system and contained within 23 crates
controlled by the ODF software described below. One ROM in each crate aggregates the data
and forwards the result for event building into
a farm of commercial Unix workstations [1], 32
of which are used during normal operation. The
crates and farm machines communicate via fullduplex 100 Mbps Ethernet, linked by a network
switch|the event builder switch [2]. The ROMs
are supported by a boot server providing core and
system-speci c code and con guration information [3].
The farm machines host the OEP and Level 3
trigger software. The events accepted by the trigger are logged via TCP/IP to a logging server [3]
and written to a disk bu er for later reconstruction and archival storage. Various data quality
monitoring processes run on farm machines not
used for data acquisition.
Several additional le servers hold the online
databases and production software releases. A
further set of application servers host the central
functions of the various online subsystems. Operator displays are supported by a group of ten
console servers [4].
An additional set of fteen VME crates, each
with an embedded processor, contain the data acquisition hardware for the detector control subsystem.
These, the online farm, and all the application
and console servers are connected via a switched
100 Mbps Ethernet network distinct from that
used for event building, with 1 Gbps ber Ether-
February 6, 2001 { 10 : 40
6699
6700
6701
6702
6703
6704
6705
6706
6707
6708
6709
6710
6711
6712
6713
6714
6715
6716
6717
6718
6719
6720
6721
6722
6723
6724
6725
6726
6727
6728
6729
6730
6731
6732
6733
6734
6735
6736
6737
6738
6739
6740
6741
6742
6743
6744
DRAFT
net used for the le servers and inter-switch links.
14.1.4. User interaction
Operator control of the online system is
achieved primarily through a custom Motif
graphical user interface (GUI) for run control and
an extensive hierarchy of displays for detector
control, including control panels, strip charts and
an alarm handler. An electronic logbook is made
available through a Web browser interface. These
and other GUIs are organized across seventeen
displays for the use of the experiment's operators. This operator environment provides for basic control of data acquisition, the overall state of
the detector, and certain calibration tasks.
Each detector system has developed a set of
specialized calibration and diagnostic applications using the tools provided in the online system. A subset of these calibrations has been speci ed to be run once per day, during a ten minute
scheduled beam-o period. The run control logic,
combined with the capability for creating partitions, allows all detector systems' calibrations to
be run in parallel and provides the operator with
basic feedback on the success or failure of each.
6745
6746
6747
6748
6749
6750
6751
6752
6753
6754
6755
6756
6757
6758
6759
6760
6761
located in each of the 23 data acquisition VME
crates. ODF builds complete events from them,
rst collecting the data in each crate into an additional dedicated ROM, and then collecting the
data from the 23 of these, across the event builder
network switch, into the online farm Unix workstations.
The operation of the system is controlled by
ODF software running on one of the application
servers, under the direction of run control. A single ROM in the VME crate containing the central
FCTS hardware supports the software interface
to ODF. The distribution of ROMs by detector
system is shown in Table 20. The numbers of
ROMs connected directly to the detector FEE
and of those used for event building are shown
separately.
Table 20
Online Data ow Hardware Components
Detector VME Readout
System Crates Modules
SVT
5
14+5
DCH
2
4+2
DRC
2
6+2
EMC
10
100+10
IFR
1
4+1
EMT
1
1+1
DCT
1
3+1
GLT
1
1+1
FCTS
1
1
TOTAL
24
157
14.2. Online Data ow
Online Data ow (ODF) handles data acquisition and processing from the detector systems'
FEE through the delivery of complete events in
the online farm [5]. The ODF subsystem receives
the Level 1 trigger outputs, lters and distributes
them to the FEE, reads back the resulting data
and hierarchically assembles it into events. It
provides interfaces for control of data acquisition, processing and calibration of detector system data, and FEE con guration. Multiple independent partitions of the detector may be operated simultaneously.
Event data acquisition proceeds from a trigger decision formed in the Fast Control and Timing System (FCTS) hardware [6], a part of ODF,
based on inputs from the Level 1 trigger. Trigger
decisions are distributed, in the full detector conguration, to the 133 ROMs connected via optical
bers to the detector system FEE. These ROMs
read and process the data from the FEE. One to
ten such ROMs from a single detector system are
115
6762
6763
6764
6765
6766
6767
6768
6769
6770
6771
6772
6773
6774
6775
All of the ROM CPUs boot via NFS over the
event building network from the boot server described above. The entire system takes about
40 seconds to load and boot 1.5 MB of core ODF
code and, typically, 4 MB of detector-speci c
code.
The ODF software allows all the components
of this heterogeneous system to be represented as
entities in a uniform object-oriented application
framework. The components are organized into
ve levels which map closely onto the system's
physical structure.
For each component at each level, its behavior
is abstracted as a nite state machine. The com-
DRAFT
116
February 6, 2001 { 10 : 40
BABAR
detector
FEE
VME
dataflow
[24]
VME
detector
control [15]
logging
server
x16
x24
file and DB
servers
[5]
event
builder
switch
[1]
network
switches
740 GB
x78
[1]
720 GB
x10
[2]
application
servers
[10]
x78
x10
farm nodes [78]
1.2 Gbps GLINKs
console
servers
[10]
various analog/digital links
...
1 Gbps ethernet
computer
center
100 Mbps ethernet
monitors [17]
Figure 84. Physical infrastructure of the BABAR online system
6776
6777
6778
6779
6780
6781
6782
6783
6784
6785
6786
6787
6788
6789
6790
6791
6792
6793
6794
6795
6796
6797
plete set of these machines is kept coherent by
passing messages and data regarding state transitions along the chain of levels. The basic ow
of control and data is shown in Figure 85. The
mapping of levels to components is as follows:
Control { the Unix-based process controlling
the operation of each partition and the source
of all state transitions except for L1 Accept. It
transmits state transition messages over the network to the source level, waiting for acknowledgement of their successful processing by all levels.
Source { the FCTS hardware and the software running in the ROM located in the FCTS
VME crate. For each partition in existence, its
source level receives control level transitions and
Level 1 trigger outputs and distributes them via
the FCTS hardware to all ROMs in the VME
crates that are included in the partition. Level 1
triggers are modeled in the subsystem as an additional, idempotent state transition, L1 Accept,
and are treated uniformly with the others wherever possible.
6798
6799
6800
6801
6802
6803
6804
6805
6806
6807
6808
6809
6810
6811
6812
6813
6814
6815
6816
6817
6818
6819
Segment { the ROMs connected to the detector FEE, with their ODF and detector systemspeci c software. Each segment level ROM receives state transition messages from the source
level and runs appropriate core and detector
system-speci c tasks in response. These tasks include the acquisition of raw data from the FEE
in response to L1 Accepts, and feature extraction
| taking this data, eliminating uninteresting hits
and applying calibration corrections, and returning results in a reduced form. Output data resulting from this processing is attached to the transition messages, which are then forwarded over
the VME backplane to the fragment level ROM
in each crate.
Fragment { the per-crate event builder ROMs
and software. The single fragment level ROM
in each crate aggregates the messages from the
crate's segment level ROMs | the rst stage of
event building | and forwards the combined message to one of the event level Unix nodes.
Event { the processes on the online farm nodes
February 6, 2001 { 10 : 40
6820
6821
6822
6823
6824
6825
6826
6827
6828
6829
6830
6831
6832
6833
6834
6835
6836
6837
6838
6839
6840
6841
6842
6843
6844
6845
6846
6847
6848
6849
6850
6851
6852
6853
6854
6855
6856
6857
6858
6859
6860
6861
6862
6863
6864
6865
6866
DRAFT
receiving complete events and handing them over
to OEP for ltering and logging. The ODF event
level code aggregates messages, with their attached data, from all the crates in a partition
{ the second and nal stage of event building.
The resulting data may be further processed by
user code in the event level, but is normally
just passed on to OEP. The control level is noti ed of the completion of processing of all transitions other than L1 Accept. Both the fragment
and event level event builders use a data-driven
\push" model, with a back pressure mechanism
to signal when they are unable to accept more
data.
Test stands of varying complexity are supported. The simplest possible consists of a single
Unix machine which runs both control and event
level code, with two FCTS modules and a single ROM, running source, segment, and fragment
level code, in one VME crate. Subsystem con guration is detected at run-time, so the same code
that runs in the full subsystem can also run in
test stand systems.
14.2.1. Control and Source Levels
The control level sends state transition messages for a partition over the network, using the
UDP datagram protocol [10], to the source level
in the single ROM inside the FCTS crate. In the
source level, the transition message is sent over
VME to an FCTS module which forwards it as a
104-bit 59.5 MHz serial word to all VME crates
in the relevant partition. This serial word contains a 56-bit event time stamp (counting at 59.5
MHz), a 32-bit transition-speci c word and additional control bits. L1 Accept transitions and
calibration sequences originate in the source level,
but then the same mechanism is used to transmit
them through the system.
The FCTS hardware receives the 24 Level 1
trigger output lines and eight additional external trigger lines. The FCTS crate is a 9U VME
crate, with a custom P3 backplane on which all
the trigger lines are bussed. For each partition,
an FCTS module receives these lines. It is con gurable with a bit mask specifying the trigger lines
enabled for its partition, and an optional prescale
factor for each line. A trigger decision is formed
6867
6868
6869
6870
6871
6872
6873
6874
6875
6876
6877
6878
6879
6880
6881
6882
6883
6884
6885
6886
6887
6888
6889
6890
6891
6892
6893
6894
6895
6896
6897
6898
6899
6900
6901
6902
6903
6904
6905
6906
6907
6908
6909
6910
6911
6912
6913
117
for the partition by taking the logical OR of the
enabled prescaled lines. Twelve of these modules
are installed in the full system, thus setting its
maximum number of partitions.
The FCTS crate receives two timing signals
from the accelerator: a 476 MHz clock tied
to the RF bucket structure of PEP-II and a
136 kHz ducial that counts at its revolution frequency. The former is divided by eight to create
a 59.5 MHz system clock. The ducial is used to
start timing counters and to check that the clocks
have no problems.
There are two types of deadtime in the ODF
subsystem. The rst arises due to the minimum
spacing of 2.7 s required between L1 Accept
transitions. This restriction simpli ed the logic
design of the FEE readout, because each datum
in the silicon tracker and drift chamber is thus
associated with only one L1 Accept. The FCTS
hardware enforces this by ensuring that all transitions have at least the minimum separation. The
command spacing introduces an irreducible, yet
minimal deadtime: 0.54% at 2 kHz.
The second type of deadtime arises when all
FEE bu ers are full and thus unable to accept
another event. In a time required to be less than
the inter-command spacing, each VME crate in
a partition may send back a full signal indicating that it is no longer able to process further
L1 Accept transitions. The FCTS hardware detects these signals and disables triggering until
the FEE are once again prepared to accept data.
An actual L1 Accept signal is only generated
from a partition's trigger decision when neither
form of deadtime is asserted.
14.2.2. Segment and Fragment Levels
The segment and fragment levels reside in the
23 detector system VME crates. These are standard 9U crates with a custom P3 backplane.
The 104 bit serial transition messages that
leave the source level are received by a FCTS
module in each VME crate in a partition. This
module in turn forwards these messages to the
ROMs in the crate over the custom backplane,
along with the 59.5 MHz system clock.
A ROM consists of four components (Figure 86): a commercial single-board computer
DRAFT
118
Run Control
Level 1 Trig
February 6, 2001 { 10 : 40
control
Detector FEEs
control & data
Source
(FCTS)
Control
x1
Unix
x1 FCTS ROM
Segment
(1−10/crate)
x133
ROMs
Fragment
(Crate)
x23 Slot−1 ROMs
Event
x32
Unix farm
Figure 85. Schematic of the ODF levels, their mapping onto physical components, and the ow of control
and data between them.
6925
Personality Card
Single-board
Computer (SBC)
6926
P1
6927
6928
6929
6930
Two fibers to FEE
6931
i960 Card
P2
Two fibers from FEE
6932
6933
6934
6935
Controller Card
6936
P3
6937
6938
6939
Front Panel
FULL
59.5 MHz Clock
FCTS Transition Messages
6940
6941
6942
6943
Figure 86. A ROM with a triggered personality
card (TPC)
6944
6945
6946
6947
6914
6915
6916
6917
6918
6919
6920
6921
6922
6923
6924
(SBC) [7] and three custom boards: a controller
card for receiving FCTS commands and supporting FEE reads and writes; a personality card that
transmits commands to and receives data from
the FEE; and a PCI mezzanine card with an Intel i960 I/O processor. The SBCs run the VxWorks [9] operating system with custom code
written in C++ and assembly language.
There are two styles of personality cards in the
system: triggered (TPC) and untriggered (UPC).
UPCs are used only in the EMC system. These
6948
6949
6950
6951
6952
6953
6954
6955
6956
6957
6958
6959
accept data continuously from the FEE into a
bu er pipeline, at a rate of 3.7 MHz. From these
samples EMC trigger information is derived and
sent over a dedicated serial link to the trigger
hardware, providing it with a continuous data
stream. An L1 Accept causes up to 256 samples of the raw data stream to be saved to an
intermediate memory store on the UPC.
A TPC (used in all other systems) reads out
FEE data only when an L1 Accept is received,
again saving it into an intermediate memory
store. Each detector reads out data in a window
of time around the trigger signal, large enough to
allow for trigger jitter (see section [ref?] above)
and detector time resolution. For instance, this
window is about 500 ns wide for the silicon vertex
tracker. The actual event time within the window
is determined approximately in the Level 3 trigger software in OEP and then re ned o -line in
the course of full event reconstruction.
FEE commands (such as for initiating event
reading) are sent and data received by the personality cards over unidirectional 1.2 Gbps serial
optical ber links [8]. All systems' FEE provide
zero suppression in hardware except in the EMC
and IFR. Data are transferred from the personality card to SBC memory using the i960 as a direct
memory access (DMA) engine. This DMA runs
at nearly the ideal 133 MB/s rate of the PCI bus.
The FEE for various systems are able to bu er
data for three to ve L1 Accept transitions. The
ROM keeps track of the bu er occupancy and
sends, when necessary, a full signal (within the
required 2.7 s interval) back to the FCTS to suppress further triggers. The full condition is re-
February 6, 2001 { 10 : 40
6960
6961
6962
6963
6964
6965
6966
6967
6968
6969
6970
6971
6972
6973
6974
DRAFT
moved when event reads by the ROM free sucient bu er space. This mechanism handles back
pressure from any stage of the data acquisition
through to logging by OEP; when downstream
processes back up for any reason, back pressure is
applied all the way up the chain until FEE bu ers
are lled and the assertion of full throttles the
L1 Accept rate.
The ODF application framework provides uniform software entry points for the insertion of user
code at each level of the system. This capability
is used primarily at the segment level, for FEE
con guration and feature extraction. Table 21
shows detector output sizes, bytes per hit, and
overhead (e.g. addressing).
Table 21
Typical Event Sizes from Detector Systems
Detector Hit Size Total Size Overhead
SVT
2 bytes
4.9 kB
0.4 kB
DCH 10 bytes 4.8 kB
0.2 kB
DRC
4 bytes
3.1 kB
0.3 kB
EMC
4 bytes
9.1 kB
3.0 kB
IFR
??
1.2 kB
0.2 kB
EMT
;
1.2 kB
<0.1 kB
DCT
;
2.7 kB
0.1 kB
GLT
;
0.9 kB
<0.1 kB
TOTAL
27.9 kB
4.2 kB
6975
6976
6977
6978
6979
6980
6981
6982
6983
6984
6985
6986
6987
6988
6989
6990
6991
6992
Data from the segment level ROMs in a crate
are gathered by the fragment level ROM using
a chained sequence of DMA operations. The
maximum throughput of the fragment level event
builder is observed to be about 31 Mbps.
In calibrations, ODF may be operated in a
mode in which L1 Accept data are not transferred out of the segment level ROMs. This allows calibration data accumulation to occur at
high rates inside the ROMs, not limited by the
throughput of the event builders or any downstream consumers. Completed calibration results
are computed, read out of the ROMs, and written
to a database on a non-L1 Accept state transition
following the accumulation.
14.2.3. Event Level
For each L1 Accept transition passing through
the ODF subsystem, all fragment ROM data are
6993
6994
6995
6996
6997
6998
6999
7000
7001
7002
7003
7004
7005
7006
7007
7008
7009
7010
7011
7012
7013
7014
7015
7016
7017
7018
7019
7020
7021
7022
7023
7024
7025
7026
7027
7028
7029
7030
7031
7032
7033
7034
7035
7036
7037
7038
7039
119
sent to one of the Unix farm machines. The destination is chosen by a deterministic calculation
based on the L1 Accept's 56-bit time stamp, available from the FCTS in each ROM. This technique has been demonstrated to produce a uniform quasi-random distribution and to introduce
no detectable ineciency in processing. Events
sent to a farm machine still busy with a previous
event are held in a bu er to await processing.
All fragment data for an event are sent over the
switched 100 Mbps Ethernet event building network to the selected farm machine. The connectionless User Datagram Protocol (UDP/IP) [10]
was chosen as the data transport protocol [11],
allowing a ow control mechanism to be tailored
speci cally to this application. Dropped packets are minimized by the network's purely pointto-point, full duplex switched architecture, and
by careful tuning of the bu ering in the network
switch and other parameters. In the rare instance when a packet is lost this is detected by the
event builder and the resulting incomplete event
agged.
The event level provides the standard software
entry points for user code. In normal operation,
these are used only to transfer events via shared
memory to the OEP subsystem for Level 3 triggering, monitoring, and logging.
14.2.4. System Monitoring
It is important that the clocks of the FEE stay
synchronized with the rest of the system. Each
FEE module maintains a time counter which is
compared to the time stamp of each L1 Accept in
order to ensure that the system remains synchronized. If it becomes unsynchronized, a special
synch command can be sent through the FCTS,
causing all systems to reset their clocks.
To ensure that the data from the correct event
is retrieved from the FEE, a ve-bit number is
incremented and sent down from the FCTS to
the FEE with each L1 Accept. These bits are
stored in the FEE along with the data and are
compared on read-back. If they disagree, a special clear-readout command is sent which resynchronizes ROM bu er pointers with FEE bu er
pointers.
All transitions, including L1 Accept, are logged
120
7040
7041
7042
7043
7044
7045
7046
7047
7048
7049
7050
7051
7052
7053
7054
7055
7056
7057
7058
7059
7060
7061
7062
7063
7064
7065
7066
7067
7068
7069
7070
7071
7072
7073
7074
7075
7076
7077
7078
7079
7080
7081
7082
7083
7084
7085
7086
DRAFT
in a 4 kB deep by 20 byte wide FIFO as they pass
through the FCTS crate. The transition type,
the event time stamp, a bit list of the trigger
lines contributing to the decision, and the current
full bit list from all VME crates are recorded in
this FIFO. There are also scalers which record
delivered and accepted luminosity, deadtime due
to the 2.7 s minimum inter-command spacing,
deadtime caused by VME crates being full and
triggers on each line. These FIFOs and scalers
are read out over VME by the FCTS ROM, which
then transmits the data to monitoring programs
that calculate quantities such as luminosity, deadtime and trigger rates. The UDP multicast protocol [12] is used to allow ecient simultaneous
transmission of data to multiple clients.
To provide diagnostics, a system which multicasts additional performance information on demand from each CPU, typically at 1 Hz, is used.
This information is currently received by a single
client on one of the Unix application servers and
archived. It can be retrieved subsequently to investigate any unusual behaviour observed in the
system.
14.3. Online Event Processing
The online event processing (OEP) subsystem provides a framework for the processing of
complete events delivered from the ODF event
builder [13]. The Level 3 (software) trigger described in Section [trg:l3] above operates in this
framework, along with event-based data quality
monitoring and the nal stages of online calibrations.
The OEP subsystem serves as an adapter between the ODF event builder interface and the application framework originally developed for the
o -line computing system. Raw data delivered
from the ODF subsystem are put into an objectoriented form and made available through the
standard event data analysis interface.
The use of this technique permitted the Level 3
trigger and most of the data quality monitoring software to be written and debugged within
the o -line environment. This software is decomposed into small, reusable units | modules, pluggable software components in the framework |
many of which are shared among multiple appli-
7087
7088
7089
7090
7091
7092
7093
7094
7095
7096
7097
7098
7099
7100
7101
7102
7103
7104
7105
7106
7107
7108
7109
7110
7111
7112
7113
7114
7115
7116
7117
7118
7119
7120
7121
7122
7123
7124
7125
7126
7127
7128
7129
7130
7131
7132
7133
7134
February 6, 2001 { 10 : 40
cations.
The OEP interfaces allow user applications to
append new data blocks to the original raw data
from ODF. The results of Level 3 event analysis
are stored in this manner so that the trigger decision and the tracks and calorimeter clusters on
which it is based may be used in later processing,
such as reconstruction and trigger performance
studies.
In order to provide sucient CPU capacity,
with headroom, the Level 3 trigger task within
OEP is typically run on 32 of the nodes in the
online farm described above.
Histograms and other monitoring data are accumulated across the farm. In order to be able to
monitor the full resulting statistics, a \distributed
histograming package" (DHP) [14] was developed
which provides to networked clients a single view
of histograms and time history data summed
across all nodes, communicating via CORBAbased protocols [15,16].
In addition to the primary triggering and monitoring functions carried out on the 32 nodes, OEP
provides a \trickle stream" protocol that allows
networked clients to subscribe to a sampling of
the event data. This scheme provides support for
event displays and additional detailed data quality monitoring (\Fast Monitoring"). Figure 87
shows the basic ow of data in the OEP subsystem.
A system for automated comparisons of monitoring data against de ned references was developed. Statistical comparisons of live histograms,
or the results of ts to them, may be performed
at con gurable time intervals to reference histograms, analytic spectra, or speci ed nominal
values of t parameters.
Comparison failures, tagged with con gurable
severity levels based on the con dence levels of
the comparisons, are displayed to operators and
logged in the common occurrence database, described below.
The Java Analysis Studio package [17] previously developed at SLAC has been enhanced with
the ability to serve as a DHP client, and it is used
for operator viewing of monitoring data accumulated in OEP. This was implemented by devising a
Java \middleware server" which adapts the DHP
DRAFT
February 6, 2001 { 10 : 40
121
Appl. Servers
EL
DHP
Req
L3
JAS
Disp
JAS
Srvr
EL
DataFlow
(crates)
L3
JAS
Disp
32 OEP farm nodes
EL
FM
Farm
Node
Evt
Disp
L3
LM
Logging
Server
Consoles
Trickle stream event data
DHP CORBA protocol
JAS RMI protocol
(various event data protocols)
Disk array
Figure 87. Flow of data in the OEP subsystem: ODF event level (EL) and Level 3 trigger processes on
each OEP node; the Logging Manager (LM) on the logging server; the DHP \requestor" process that
combines histograms from all 32 Level 3 processes; one instance of a Fast Monitoring (FM) process with
DHP histograms; the Java server that makes DHP histograms available to JAS clients; two such clients,
and one event display for the Level 3 trigger. OEP-speci c data transport protocols are identi ed.
7135
7136
7137
7138
7139
7140
7141
7142
protocol to the native JAS data protocol.
A dedicated operator console supports the data
quality monitoring system, providing for its control and for the display of the histograms from
Fast Monitoring and the Level 3 trigger processes,
using JAS, along with any error conditions detected by the automatic histogram analysis facility.
7152
7153
7154
7155
7156
7157
7158
7159
7160
7143
7144
7145
7146
7147
7148
7149
7150
7151
14.4. Data logging
Events selected by the Level 3 trigger algorithms in OEP are retained for subsequent full
reconstruction. The events are sent from the 32
OEP nodes via TCP/IP to a single multithreaded
process, the Logging Manager (LM), running on
the logging server described above. The LM
writes them to the server's RAID arrays in a format speci c to OEP. Data from all 32 nodes are
7161
7162
7163
7164
7165
7166
7167
7168
combined into a single le for each data-taking
run (typically two to three hours of data acquisition, resulting in les of about 15{20 GB in size).
Completed les are copied to the SLAC HPSS
(High Performance Storage System) [18] system
for archiving to tape. These les are retrieved
from HPSS for reconstruction, which is typically
completed within less than eight hours of data
acquisition. The data les are also retrievable
for other tasks such as detector system hardware
diagnostics and oine tests of the Level 3 trigger
algorithms.
14.5. Detector Control
14.5.1. Design Principles
The Experimental Physics and Industrial Control System, EPICS [19], was selected to provide
the \engineering layer" for the online detector
122
7169
7170
7171
7172
7173
7174
DRAFT
control (ODC) subsystem: the direct connection
to the electrical signals of the power supplies and
other hardware, with sucient monitoring and
control to allow commissioning, fault diagnosis
and testing. A summary of monitoring and control points is presented in Table 22.
Table 22
Detector Control monitor points this is a place
holder table awaiting data from Vera
Detector Monitor Control
System Points Points
SVT
?
?
DCH
?
?
DRC
?
?
EMC
?
?
IFR
?
?
PEPII
?
?
General
?
?
TOTAL
?
?
7200
7201
7202
7203
7204
7205
7206
7207
7208
7209
7210
7211
7212
7213
7214
7215
7216
7217
7218
7219
7175
7176
7177
7178
7179
7180
7181
7182
7183
7184
7185
7186
7187
7188
7189
7190
7191
7192
7193
7194
7195
7196
7197
7198
7199
Beyond the writing of custom drivers, only minor additions or changes were required to EPICS
as distributed. EPICS and the additional software specializing it for BABAR are written in the
C language.
Detector-wide standard hardware was adopted
to ease development and maintenance. The standard ODC crate is a 6U VME chassis containing a single board computer [20] serving as an
EPICS input/output controller (IOC). Fifteen
such crates are used in the experiment. EPICS is
fully distributed: each IOC supplies its own naming service, notify-by-exception semantics, and
processing. The IOCs boot from a dedicated
server.
Analog data are either digitized by modules
within the crates or, more commonly, on digitizer boards located directly on the detector. In
the latter case, the CANbus standard [21] is used
for the transport of signals to and from the detector. A custom \general monitoring board"
(GMB) [22] was developed to interface CANbus
to the on-detector electronics. The GMB contains
a microcontroller, an ADC, multiplexors and operational ampli ers. It can digitize up to 32 sig-
7220
7221
7222
7223
7224
7225
7226
7227
7228
7229
7230
7231
7232
7233
7234
7235
7236
7237
7238
7239
7240
7241
7242
7243
7244
7245
February 6, 2001 { 10 : 40
nals.
14.5.2. User interface
The operator view of this part of the control
system is via screens controlled by the EPICS display manager, DM.
Dedicated control and display panels were developed using DM for each of the detector subsystems, using common \color rules" to show the
status of devices, aggregated up the hierarchy of
detector system components. A top-level panel
for ODC summarizes all subsystems' status and
provides access to their specialized panels.
The EPICS alarm handler (ALH) with some
BABAR-speci c modi cations is used to provide
operators with audible and color-coded alarms
and warnings in a hierarchical view of all the
subsystems and components. Conditions directly
relevant to personnel or detector safety are further enforced by hardware interlocks, the states
of which are themselves re ected in a set of uniform EPICS displays and in the ALH, and an
alarm annunciator.
14.5.3. Interfaces to other BABAR software
A custom C++ layer above EPICS consisting
of Component Proxies and Archivers provides for
device-oriented state management and archival
data collection. This is ODC's interface to the
rest of the online system.
The 27 component proxies (CPs), running on
a Unix application server, each de ne a logical
component representing some aspect of a detector
system or the experiment's central support systems, aggregated from the 105 individual EPICS
channels.
The CPs present a simple nite state machine model as their interface to Run Control.
The most important actions available are Con gure, on which the CP accesses the con guration
database described below, retrieving set points
for its component's channels, and Begin Run,
which puts the CP into the Running state, in
which setpoint changes are prohibited and readbacks are required to match settings. While in the
Running state, the CP maintains a Runnable ag
which re ects that requirement and allows Run
Control to ensure that data acquisition is per-
February 6, 2001 { 10 : 40
7246
7247
7248
7249
7250
7251
7252
7253
7254
7255
7256
7257
7258
7259
7260
7261
7262
7263
7264
7265
7266
7267
7268
7269
7270
7271
7272
7273
7274
7275
7276
7277
7278
7279
7280
7281
7282
7283
7284
7285
7286
7287
7288
7289
7290
7291
7292
DRAFT
formed only under satisfactory conditions. When
the ag is negated, indicating that readback values are out of tolerance, Run Control will automatically pause data-taking.
The CP's other principal function is to provide
an interface for the rest of the online system to
the data collected by ODC on the state of the
detector hardware and its environment | dubbed
\ambient data."
The recording of this data so that it may be
analyzed and correlated with event data is essential to the full understanding of the detector. It
is the task of the Archiver processes, each paired
with a CP, to collect the ambient data, aggregate them and write out time histories approximately every hour to the Ambient Database described below. The data are recorded associated
with times in order that they may be correlated
with the time stamps of the event data. Data
from the Archivers' current bu ers or from the
database may be viewed with a custom graphical
browser.
Most ambient data sources generally vary only
within their own noise limits, so unnecessary data
volume is avoided by limiting storage of data to
movements outside a dead-band (set separately
for each channel) or across an alarm threshold.
14.5.4. Integration with the accelerator
Close integration between the BABAR detector
and the PEP-II accelerator is essential to maintain the health of the detector components and
the eciency of data collection. Data from the
accelerator control system are transported via
EPICS channel access to BABAR for display and
storage, managed by a dedicated CP. In turn,
background signals from the detector are made
available to PEP-II to aid in injection and tuning, minimizing backgrounds and optimizing integrated luminosity. An important component
of this communication is the \injection request"
handshake. When the PEP-II operator requests
a signi cant change in the beam conditions, such
as injection or dumping of the beams, the request can only procede following con rmation
from BABAR. This complements the safety interlocks that BABAR controls to enforce protection
from radiation damage.
7293
7294
7295
7296
7297
7298
7299
7300
7301
7302
7303
7304
7305
7306
7307
7308
7309
7310
7311
7312
7313
7314
7315
7316
7317
7318
7319
7320
7321
7322
7323
7324
7325
7326
7327
7328
7329
7330
7331
7332
7333
7334
7335
7336
7337
7338
7339
123
14.5.5. Operational experience
The ODC subsystem has been operational since
the initial cosmic-ray commissioning of the detector and the beginning of data-taking with colliding beams. The core EPICS infrastructure has
proven to be very robust. The large size of the
subsystem, with its 15 IOCs and 105 data channels, produces heavy but manageable trac on
the experiment's network.
[Need some data volume and rate numbers from ODC folks here. Walt Innes is looking to this but tentatively reported a value of 4
MB/day]
14.6. Run Control
The run control subsystem is implemented as
an application of SMI++, a toolkit for designing distributed control systems [23]. Using this
software, the BABAR experiment is modeled as a
collection of objects behaving as nite state machines. These objects represent both real entities,
such as the ODF subsystem or the drift chamber
high voltage controller, and abstract subsystems
such as the \calibrator," a supervisor for the coordination of online components for calibration
runs. The behavior of the objects are described
in a specialized language (SML) which is interpreted by a generic logic engine to implement the
control system.
The SML descriptions of the objects which
make up the experiment simply specify their own
states and transitions as well as the connections
between the states of di erent objects. Objects perform actions on state transitions, which
may include explicitly commanding transitions in
other objects; objects may also be programmed to
monitor and automatically respond to changes of
state in other objects. Such connections express
the control hierarchy of the experiment. Anticipated error conditions in components of the online system are re ected in their state models, allowing many errors to be handled automatically
by the system. To reduce complexity, logically
related objects are grouped together into a hierarchy of cooperating domains.
The system is highly automated; user input
is generally required only to initialize the system, start and stop runs, and handle unusual er-
124
7340
7341
7342
7343
7344
7345
7346
7347
7348
7349
7350
7351
7352
7353
7354
7355
7356
7357
7358
7359
7360
7361
7362
7363
7364
7365
7366
7367
7368
7369
7370
7371
7372
7373
7374
7375
7376
7377
7378
7379
7380
7381
7382
7383
7384
7385
7386
DRAFT
rors. The user communicates with Run Control
via a con gurable Motif-based GUI included in
SMI++.
The states and behavior of Run Control objects representing external systems are provided
by a special class of intermediate software processes called proxies. A proxy monitors its system, provides an abstraction of it to Run Control,
and receives state transition commands. These
commands are interpreted and applied to the underlying hardware or software components, implementing the transitions' actions. The control
level of an ODF partition is an example of such
a proxy.
Communication between the various proxies
and the Run Control engines is provided by
DIM [24], a fault tolerant \publish and subscribe"
communications package based on TCP/IP sockets, allowing Run Control to be distributed transparently over a network.
Essential to the operation of the online system
is the notion of the Runnable status of its various Detector Control and data acquisition components, indicating that they are in a state suitable for production-quality data-taking. The Run
Control logic interlocks data-taking to the logical
AND of all components' Runnable status. While
this condition is not satis ed, data-taking may
not start and any existing run will be paused with
an alert sent to the operator.
14.7. Common software infrastructure
14.7.1. Databases
Five major databases are used by the online
system:
1. Con guration Database: This database, implemented using the commercial object-oriented
database management system Objectivity [25],
allows the creation of hierarchical associations of
system-speci c con guration data with a single
numeric con guration key. This key is distributed
to all online components, which can then use it
to retrieve from the database all the con guration
information they require. Convenient mnemonics
are associated with the keys for currently relevant con gurations, and may be selected for use
via the Run Control GUI [26].
2. and 3. Ambient and Conditions Databases:
7387
7388
7389
7390
7391
7392
7393
7394
7395
7396
7397
7398
7399
7400
7401
7402
7403
7404
7405
7406
7407
7408
7409
7410
7411
7412
7413
7414
7415
7416
7417
7418
7419
7420
7421
7422
7423
7424
7425
7426
7427
7428
7429
7430
7431
7432
February 6, 2001 { 10 : 40
These databases, also implemented using Objectivity, are based on the notion of time histories
of various data items associated with the experiment. The history for each item is divided into
intervals over which a speci c value of the item is
valid. The Ambient Database is used principally
by the Detector Control subsystem to record detector parameters and environmental data at the
time they are measured [26].
The Conditions Database is used to record calibration constants and alignments, a digested subset of the Ambient information, and the con guration keys in force during data taking runs. It
has the additional feature, compared to the Ambient Database, that the data for a given time
interval may be updated as they are re ned in
the course of improved understanding of the apparatus [27].
The Con guration and Conditions Databases
are exported for reconstruction and physics analysis.
4. Occurrence (Error) Log: Informational and
error messages generated in the online system are
routed through the CMLOG system [28] to a central database, from which they are available for
operators' real-time viewing or historical browsing, using a graphical tool, as well as for subscription by online components which may require noti cation of certain occurrences.
5. Electronic Logbook: An Oracle-based [29]
logbook is used to maintain the history of the
experiment's data-taking, organized by runs normally of approximately two hours' duration. It
contains information on beam parameters | instantaneous and integrated luminosity, currents,
and energies | as well as records of data acquisition parameters such as trigger rates, data volumes, and deadtimes, and the detector con guration used for a run. The logbook also contains
text comments and graphics added by the operations sta .
A number of other databases are used in the
online system for various tasks such as indexing
logged data les, the repair history of online hardware and spares, and tracking software problem
reports.
February 6, 2001 { 10 : 40
7433
7434
7435
7436
7437
7438
7439
7440
7441
7442
7443
7444
7445
7446
7447
7448
7449
7450
7451
7452
7453
7454
7455
7456
7457
7458
7459
7460
7461
7462
7463
7464
7465
7466
7467
7468
7469
7470
7471
7472
7473
7474
7475
7476
7477
7478
DRAFT
14.7.2. Software Release Control and Conguration Management
All of the online software is maintained in the
common BABAR code repository, based on the
freely available Concurrent Versions System software, CVS [30].
The online's Unix and VxWorks applications
are built and maintained with an extension of the
standard BABAR software release tools [31]. At
the start of every data-taking run, the identities
of the current production software release and any
installed patches are recorded; thus it is possible
at a later date to reconstruct the versions of online
software used to acquire data.
14.8. Summary
The online system has exceeded its data acquisition performance goals. It is capable of acquiring colliding beam events, with an average
size of 28 kB, at a O(2500 Hz) Level 1 trigger
rate and reducing this rate in Level 3 to the required O(120 Hz) limit. This provides comfortable margins, for under normal beam conditions
the Level 1 trigger rate is O(800{1000) Hz.
The system is capable of logging data from
Level 3 at much higher rates when required for
diagnostics; the nominal 120 Hz gure represents
a compromise between data volume and its consequential load on downstream processing and
archival storage, and trigger eciency for low
multiplicity nal states.
During normal data-taking the online system
routinely achieves an eciency of over 98%, taking both data acquisition livetime and the system's overall reliability into account.
14.8.1. Upgrade Path
There are several hardware options open for
scaling the existing ODF subsystem. Currently
most ROMs receive more than one ber from
the FEE. These bers could be distributed over
more ROMs to add processing power. There are
also commercial upgrade paths for the ROMs'
MVME2306 SBC boards available from Motorola. Crates can be split (up to a maximum
total of 32) to create more VME event building
bandwidth, as well as more fragment level CPU
power and network bandwidth. Gigabit Ethernet
7479
7480
7481
7482
7483
7484
7485
7486
7487
7488
7489
7490
7491
7492
7493
7494
7495
7496
7497
7498
7499
7500
7501
7502
7503
7504
7505
7506
7507
7508
7509
7510
7511
7512
7513
7514
7515
7516
7517
7518
7519
7520
7521
7522
7523
7524
125
connections could also be installed to improve the
network event builder's bandwidth.
Various software upgrade options are being
investigated, including optimizing the VxWorks
network drivers and grouping sets of events together in order to reduce the impact of per-event
overhead.
Current background projections indicate that
fragment level CPU, segment level memory bus
bandwidth and network event building bandwidth are the most likely bottlenecks we will encounter in the future.
Increases in the Level 1 trigger rate or in the
background occupancy and complexity of events
are expected to necessitate providing additional
capacity for OEP, principally for Level 3 triggering. The online farm machines could be replaced
with faster models. More machines could also be
used, at the expense of increases coherent loading
on various servers and of additional management
complexity.
No requirement for signi cant capacity upgrades to the data logging subsystem or to Detector Control is foreseen at this time.
REFERENCES
1. Sun Ultra 5, with single 333 MHz
UltraSPARC-IIi CPUs and 512 MB of
RAM, Sun Microsystems, Inc., Palo Alto,
California, USA.
2. Cisco model 6500, Cisco Systems, Inc., San
Jose, California, USA.
3. For the period of data taking covered by
this writing, a single Sun Microsystems Ultra Enterprise 450, with four ? MHz CPUs,
2 GB of RAM, and 720 GB of RAID-3 disk,
acted as the data logging server as well as the
ODF boot server. Beginning in 2001 all its
responsibilities other than data logging were
moved to a new Sun Ultra 220R machine with
dual 450 MHz UltraSPARC-II CPUs, 1 GB of
RAM, and an additional 200 GB RAID array.
4. These le and database servers (presently
ve) are primarily additional Sun Ultra 220Rs, each with about 200 GB of
RAID disk. The ten application servers are
a mix of Sun Ultra 60s, with dual 360 MHz
126
7525
7526
7527
7528
7529
7530
7531
7532
7533
7534
7535
7536
7537
7538
7539
7540
7541
7542
7543
7544
7545
7546
7547
7548
7549
7550
7551
7552
7553
7554
7555
7556
7557
7558
7559
7560
7561
7562
7563
7564
7565
7566
7567
7568
7569
7570
7571
DRAFT
UltraSPARC-II CPUs and 1 GB of RAM, and
Ultra 5s; the console servers are all Ultra 5s.
5. R. Claus et al., \Development of a Data
Acquisition System for the BABAR CP Violation Experiment", Proceedings of the 11th
IEEE NPSS RealTime Conference, 14-18
June 1999, Santa Fe, New Mexico, USA.
(http://strider.lansce.lanl.gov/rt99/index11.html)
6. P. Grosso, et al., \The BABAR Fast Control System", Proceedings of the International
Conference on Computing in High-Energy
Physics, 1998, 31 Aug-4 Sep 1998, Chicago,
Illinois, USA.
7. Motorola model MVME2306 boards, each
with a 300 MHz PowerPC 604 CPU, 32 MB
of RAM, 5 MB of non-volatile ash memory,
two PCI mezzanine card slots (only one of
which is currently used), a 100 Mbps Ethernet interface, and a Tundra Universe II
VME interface, manufactured by the Motorola Computer Group division of Motorola
Inc., Tempe, AZ 85282.
8. These use the Hewlett-Packard GLINK protocol. [need a more detailed reference here from
Gunther, perhaps]
9. The VxWorks RTOS and Tornado Development interface are products of Wind River
Systems, Inc., Alameda, CA 94501-1153.
10. J. Postel, \User Datagram Protocol", RFC0768, 28 Aug 1980. Internet documentation
including all RFCs (Requests for Comment)
are available online from the Internet Engineering Task Force, http://www.ietf.org.
11. T. J. Pavel, et al., \Network Performance
Testing for the BABAR Event Builder", Proceedings of the CHEP 1998 Conference.
12. S. Deering, \Host Extensions for IP Multicasting", RFC-1112, August 1989.
13. G. P. Dubois-Felsmann, E. Chen, Yu.
Kolomensky, et al., Flexible Processing
Framework for Online Event Data and Software Triggering, IEEE Transactions on Nuclear Science 47, 353 (2000).
14. DHP reference needed { Scott Metzler, CHEP
conference or BABAR note.....
15. CORBA (Common Object Request Broker
Architecture) Standards, Object Manage7572
7573
7574
7575
7576
7577
7578
7579
7580
7581
7582
7583
7584
7585
7586
7587
7588
7589
7590
7591
7592
7593
7594
7595
7596
7597
7598
7599
7600
7601
7602
7603
7604
7605
7606
7607
7608
7609
7610
7611
7612
7613
7614
7615
7616
7617
7618
7619
February 6, 2001 { 10 : 40
ment Group, http://www.corba.org/.
16. ACE/TAO CORBA implementation, Distributed Object Computing Group, Washington University, St. Louis, Missouri, USA
and University of California, Irvine, California, USA, http://www.cs.wustl.edu/
~schmidt/TAO.html.
17. JAS reference
18. High Performance Storage System
(HPSS), International Business Machines, Inc., Armonk, New York, USA,
http://www4.clearlake.ibm.com/
hpss/index.jsp.
19. L. Dalesio et al., \The Experimental Physics
and Industrial Control System Architecture:
past, present and future", Nuclear Instruments and Methods in Physics Research A,
vol. 352, pp.179, 1994.
20. The ODC IOCs are model MVME177 boards
with MC68060 CPUs from Motorola Computer Group, op. cit..
21. ISO standards 11519, \Road vehicles | Lowspeed serial data communication" (1994),
and 11898, \Road vehicles | Interchange of
digital information | Controller area network (CAN) for high-speed communication"
(1993). See http://www.iso.ch/ .
22. T. Meyer and R. McKay, \The BABAR General Monitoring Board", BABAR Note 366,
May 1998.
23. B. Franek and C. Gaspar, \SMI++ Object
Oriented Framework for Designing and Implementing Distributed Control Systems", IEEE
Trans. Nucl. Sci. 45 (1998) 1946. SMI++ was
adopted, with some upgrades, from a system
used in the DELPHI experiment at CERN.
24. C. Gaspar and M. Donszelman, \DIM - A
Distibuted Information Management System
for the DELPHI experiment at CERN", Proceedings of the IEEE Eight Conference REAL
TIME '93 on Computer Applications in Nuclear, Particle and Plasma Physics; Vancouver, Canada.
25. A product of Objectivity Inc.,
Mountain
View,
CA
94041;
http://www.objectivity.com/.
26. G. Zioulas et al., \Ambient and Con guration databases for the BABAR online system",
February 6, 2001 { 10 : 40
7620
7621
7622
7623
7624
7625
7626
7627
7628
7629
7630
7631
7632
7633
7634
7635
DRAFT
Proceedings of the Real Time 99 Conference,
June 14-18, 1999, Santa Fe, New Mexico.
27. [reference to be supplied, perhaps from CHEP
98].
28. The products cdev and CMLOG are software facilities produced by the Accelerator Controls group of the Thomas Je erson National Accelerator Facility. Documentation is available from their Web site,
http://www.cebaf.gov/.
29. A product of Oracle Corporation, Redwood
Shores, CA USA. http://www.oracle.com/.
30. The Concurrent Versions System is an opensource distributed version control system.
http://www.cvshome.org/.
31. Reference to Software Release Tools
7636
127
15. Summary