E895 p- correlation analysis - Status Report
Mike Lisa, The Ohio State University
• E895 Motivation and Measurement
• Status of HBT analysis
• Summary and plans
1
Lawrence Berkeley Lab
D. Best, T. Case, K. Crowe, D. Olson, G. Rai, H.-G. Ritter,
L. Schroeder, J. Symons, T. Wienold
Brookhaven National Lab
S. Gushue, N. Stone
Carnegie Mellon University M. Kaplan, Z. Milosevich, J. Whitfield
Columbia University
I. Chemakin, B. Cole, H. Hiejima, X. Yang, Y. Zhang
U.C. Davis
P. Brady, B. Caskey, D. Cebra, J. Chance, J. Draper, M. Heffner,
J. Romero, L. Wood
St. Mary’s College
J. Kintner
Harbin Institute (China)
L. Huo, Y. Liu, W. Zhang
Kent State Univeristy
M. Justice, D. Keane, H. Liu, S. Panitkin, S. Wang, R. Witt
Lawrence Livermore Lab
V. Cianciolo, R. Sotlz
Ohio State University
A. Das, M. Lisa, R. Wells
University of Auckland (NZ) D. Krofcheck
Purdue University
M. Gilkes, A. Hirsch, E. Hjort, N. Porile, R. Scharenberg, B.
Srivastava
S.U.N.Y. Stony Brook
N.N. Ajitanand, J. Alexander, P. Chung, R. Lacey, J. Lauret, E.
2
LeBras, B. McGrath, C. Pinkenburg
Systematics/meta-analysis suggest approach
to maximum AGS energy interesting...
P. Braun-Munzinger and J. Stachel,
NPA606, 320 (1996)
3
Perhaps some signals only apparent near threshold
D. Rischke, NPA 610, c88 (1996)
E895 flow status
discussed by R. Lacey
4
Ideally, HBT gives a measure of source size
 
 
P( k , k )
C2 ( k1 , k 2 )   1 2
P( k1 )  P ( k 2 )
   


i( k 2  k1 )( x 2  x1 )
d x1  d x 2  ( x1 )  ( x 2 )  e
 1 


3
3
d
x

d
x


(
x
)


(
x

1
2
1
2)
3
 2
~
 1   ( Q)
3
C (Qinv)
Pion
Source  (x)
Width ~ 1/R
2
1
0.05
0.10
Qinv (GeV/c)
5
HBT systematics at AGS interesting in themselves,
& can look for suprises
Rischke & Gyulassy
NPA 608, 479 (1996)
“ec”
“e”
May miss signal at “too high” Ebeam
6
HBT - another handle on
mean field effects at AGS
7
Generated by H. Liu and S. Panitkin
AGS  Bevalac
Particle reconstruction upgrades have taken huge effort,
but have born fruit...
8
PID via dE/dx
for primaries
2 AGeV
4 AGeV
8 AGeV
(negative particles cleaner)
9
Strange neutrals reconstructed
(& provide sensitive diagnostic of data quality)
L p + p
K0 p+ +p
plots from P. Chung, SUNY-SB
10
Non-uniform trigger in dataset analysed
Will be possible to
select top ~5% for
all energies offline
Current analysis:
2 GeV: b  0-8 fm
4 GeV: b  0-8 fm
8 GeV: b  0-3 fm
Otherwise seems OK
e.g. log increase of
multiplicity with Ebeam:
Ebeam Ebeam
Mmax  Mmax + 50
Mpmax  Mpmax + 15
11
Singles coverage for pions
12
Large acceptance many p-
But...phase space means
most are at large Q
Background (denominator)
generated with standard
event-mixing (15 previous)
4 GeV central
13
Pairwise cuts to remove track splitting effects
“Raw” correlation function
shows encouraging structure
at low Qinv
Simulations: requirement that
> 50% of track is seen
kills truly found pairs.
(Au+Be event)
14
Pairwise cuts, cont’
Track-splitting virtually eliminated by pairwise cut:
require that sum of % track seen > 100% (applied to “real” and “mixed” pairs)
Next low-Q problem:
track-merging.
15
Merging effect reduced by cut on projected
seperation at exit of TPC.
Real pairs
Mixed pairs
16
Require particles to exit TPC 10 cm apart.
0 cm cut
5 cm cut
10 cm cut
15 cm cut
17
Overview of E895 HBT Analysis
fit of singles distribution
raw data
generation of MC pairs
(kuip macro files)
(pass1)
TRKS
• Embed MC pairs into raw data
• perform pass1
• correlate embedded, extracted particles
EMBED_PARTS
TRKS
AM_PID
AM_HBT
HBT_SW
HBT_EVENT_CUT
HBT_TRK_CUT
HBT_PAIR_CUT
histograms
acceptance corrections
coulomb correction
diagnostic ntuples
correlation functions
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Corrections - I
Ideally...
 
 
 
P ( k1 , k 2 )
Ptrue ( k1 , k 2 )


 
C2 ( k1 , k 2 ) 

P( k1 )  P( k 2 ) Pmixed ( k1 , k 2 )
“Background” pair distribution contains all physics
and detector effects except for the BE symmetrization
Well-known deviation from this is due to final-state Coulomb repulsion...
Approximate correction - Gamow factor:
2 p
mp e 2
G (Qinv )  2 p ;  
Qinv
e 1
(Better to do full Coulomb integration)
19
Corrections - II
Detector acceptance effects are more subtle, especially with a tracking detector
Original pion pair
k1
k2
MC Scattering
Pixel noise
Digitization, thresholds
Measured
particle(s)
k1’, k2’, (k3’...)
Pattern recognition
Track merging and splitting and momentum resolution and distortion.
Hit the low-Q pairs hardest, and affect the correlation signal significantly.
Effects depend on k1, k2 (six-dimensional!), as well as track and pair cuts!!!
Correcting for or minimizing these 2-particle effects requires detailed simulation.
20
Generating the Acceptance /Resolution Correction - I
for some
set of cuts:
Kacceptance =
C2(ideal)
C2(reconstructed)
only phase space (k) cut applied
(no track quality or 2-track cuts)
d6N
B(k1,k2) = 3 3
d k1d k2
d6N
R(k1,k2) = 3 3 • C2(k1,k2)
d k1d k2
=
R(k1,k2)
B(k1,k2)
R(k1,k2)
B(k1,k2)
apply same track and pair
cuts as applied to data
d6N
B(k1,k2) = 3 3
d k1d k2
d6N
R(k1,k2) = 3 3
• C2(k1,k2)
d k1d k2
(weighting by C2(k1,k2) implies foreknowledge of correlation function  iterative approach)
21
Understanding close pairs
• Close pairs are embedded into real data events at pixel level
with measured momentum distribution, to get correct noise
track density environment
• Full event reconstruction run
• Gives momentum distortions, pair loss...
22
Resolution from the 4 GeV simulations...
finite resolution + phase space
give Q distortion at low Q
10 MeV/c resolution
(includes MCS)
(Qin > 40 MeV/c)
23
Pair loss from 4 GeV simulations
Single pair in...
Lost pair
Pair loss constant above 50 MeV/c
(statistical loss of single track)
Split track
24
Finally: “Correction to the Coulomb Correction”
In the measured ratios, we apply the Coulomb correction (currently the Gamow
correction) according to the measured Q, not the true Q.
With the simulated pairs, we have the true and reconstructed momenta, so
can account for this.
Then, the full acceptance/resolution correction function is:
R(k1,k2)
B(k1,k2)
G(k1,k2)
R(k1,k2)

B(k1,k2) G(k1,k2)
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Corrections for 10 cm exit seperation
26
Corrections for 2 cm exit separation
27
Acceptance/resolution well understood & accounted for
Different cuts give very different raw correlation functions.
But corrected correlation function is robust.
2 GeV results
10 cm exit separation cut
2 cm exit separation cut
28
Data points consistent - fits are sensitive
29
4 GeV results stable (and reasonable) as well
10 cm exit separation cut
2 cm exit separation cut
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8 GeV results not stable or reasonable
(under study)
10 cm exit separation cut
2 cm exit separation cut
31
Summary
• E895 can measure low-Q correlations well
• Difficulties of close pairs (splitters/mergers) largely addressed through
pairwise cuts
• Detailed simulations generate corrections that track with cuts
– These corrections are significant and important
• Different quality cuts
  very different measured correlation functions
  very different measured corrections
  NOT different corrected correlation functions
• Must figure out what is going on at high energy
• Multi-dimensional HBT and phase space cuts come next
(present analysis on < 5% of data)
32