DNP 2013 – Newport News – 26/Oct/2013
Large acceptance magnetic spectrometer
for the 12 GeV2 GEp experiment
(at Jefferson Lab)
E. Cisbani
INFN Rome – Sanità Group and
Italian National Institute of Health
for the SBS collaboration
http://hallaweb.jlab.org/12GeV/SuperBigBite
Outlook
• Electromagnetic form factors at high Q2
• Experimental requirements
• Spectrometer details
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
1
Form Factors: discovery and formalism
• R.W. McAllister, R. Hofstadter Phys. Rev. 102 (1956) 851 “First measurement of
the proton electromagnetic radius”: RMS E/M radius of =(0.74 ± 0.24) 10-13 cm
Nucleon electromagnetic current operator has two “unknown”
functions (Dirac and Pauli FFs) that describe the internal
structure of the nucleon (one photon exchange approx.):
In terms of Sachs FFs:
Sachs FFs are FT of the charge and magnetization
distributions in the nucleon (in Breit frame)
Elastic Cross section (Rosenbluth):
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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Proton GE/GM – an «unexpected» discrepancy
d
 2
2
 GEp
 GMp
d

Rosenbluth Separation: assume
single photon approximation
Prior to JLab/2000, expectations were
that proton GE/GM fairly constant with Q2

GEp
GMp
 
Pt ( Ebeam  Ee )

tan e
Pl
2M p
2
Polarization transfer from the incident
electron to the scattered proton
At JLab, new class of experiments show
proton GE/GM decreasing linearly with Q2
Two Photon Exchange – favorite candidate
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
DA3: T. Averett
HA2: M. Kohl
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Proton GE/GM - Theoretical models
• Many theoretical models
– VMD (Iachello, Lomon, Bijker),
generally good description of all FF
– Relativistic CQM (Miller, Gross, ...)
spin dependent quark density
– Lattice QCD, start to give prediction
– Dyson-Schwinger, dressed quarks,
diquark correlation, ...
– pQCD-based: GE/GMconst Q2
– GPD-based: direct connection to quark
OAM, FF’s constraint GPD’s
Most of them agree with current data
but diverge at higher, unexplored, Q2
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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«Modern» Form Factor measurements at high Q2
Method:
Polarization Transfer:
𝒑 𝒆, 𝒆′𝒑
Target Perp. Polarization:
𝒑↑ 𝒆, 𝒆′ 𝒑
𝑝
Measure
𝐺𝐸
𝑃𝑡
𝜃
tan
∝
𝑝
𝑃𝑙
2
𝐺𝑀
(one photon approx.)
Pt, Pl : trans. and long. polarization of the recoil
proton
𝑝
𝐴=
𝑁+ −𝑁− 𝐺𝐸
~
𝑁+ +𝑁− 𝐺 𝑝
𝑀
N+ and N- : events with opposite transverse
target polarization
Many systematics effects (theory and exp.) cancel in ratio
Figure of Merit (stat.)
: acceptace
L: Luminosity
: elastic xsec ~ 𝐸 2 𝑄12
Pb: beam polarization
Ω L σ Pb2 PT2
Ω L σ Pb2 ϵ A2y
𝛀𝐋𝛜
~
𝐐𝟏𝟔
Ay: polarimeter analyzing power
ϵ: polarimeter efficiency
𝛀𝐋 𝟐
~ 𝟏𝟐 𝐏𝐓
𝐐
PT : Target polarization
At Q2~10 GeV2 expected: FoMpol_trans ~ 10  FoMtarg_pol (target polarization cannot tolerate large L)
Challenges at high Q2:
Maximize (coincidence) acceptance
Maximize luminosity
... keeping costs at «affordable» level
Mazimize polarization efficiency
Maximize beam polarization
(... having the needed beam energy)
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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Jefferson Lab - CEBAF after 2013
add Hall D
(and beam line)
6 GeV CEBAF (< 2013)
Max Current: 200 A
Max Energy: 0.8 - 5.7 GeV
Long. Polarization: 75-85%
Upgrade magnets
and power
supplies
CHL-2
12 GeV CEBAF
(>2013)
26/Oct/2013 (DNP2013)
Max Current: 90 A
Max Energy Hall A,B,C: 10.9 GeV
Max Energy Hall D: 12 GeV
Long. Polarization: 75-85% 6
E. Cisbani / SBS for GEp5 @ JLab12
Proton GE/GM at large Q2 by polarization transfer
(SBS)
Beam:
Current= 75 A,
Polarization= 85% long.
Energy= 6, 8 and 11 GeV
Target:
H2 Liquid
Length= 40 cm
Luminosity = 8 · 1038
Detectors:
GEp5 experiment in HallA
P-arm: SBS + Polarimeter
E-arm: BigCal + Coordinate
GOAL: Extend the measurement of the proton form factor ratio
GE/GM to the maximum Q2 that is possible with 11 GeV
beam with constraints:
Absolute error < 0.1 Beam time = 60 days
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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New SuperBigbite Spectrometer (SBS) in Hall A
Large luminosity
“Large” acceptance
Forward angles
Reconfigurable detectors
High photon up to 250 MHz/cm2
and electron 160 kHz/cm2
background
• Support event rate 10x higher
than with standard small
acceptance spectrometer
• GEM chambers to handle the high
rate of the background
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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Large Luminosity  Large Background
Hit
• Must be supported by the detectors
 GEM technology
• Must be handled by the trigger:
– spatial and time correlation between
electron and proton elastically scattered
– «high» energy threshold in segmented
CALO’s
Good tracking resolution needed
- momentum resolution: 1 %
- angular resolution:
1 mrad
- vertex reconstruction: 5 mm
Red: p0 photoproduction
Black: Elastics
Blue: Sum
For Emiss<0.35 GeV, remaining p0 background: 10%
Adequate proton polarization precession reconstruction
(next slide)
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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GEp5: Proton Polarimeter (PP)
Number of scattered protons:
Use azimuthal asymmetry of the proton
scattering off matter induced by spin-orbit
coupling
where  refers to electron beam helicity
A (a.u.)
Pypp
Pxpp
Track inTrack in
Track out
Track out
Polarimeter only measures components of
proton spin that are transverse to the proton’s
momentum direction
Maximize Pe
N=number of scattered proton, Pe beam polarization
Require: Dipole magnet to precess Pl at target to Pypp
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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SBS Dipole Magnet / 48D48 from BNL
Magnet Parameters
 Integral field strength 1.82 T-m
2.28 T-m with pole shims
 Yoke length 1.22 m
 Gap: 47 cm  121.9 cm
 Yoke Weight 85 tons
 6 1008 steel sectors, largest is 18.3 tons


(deg)
(mrs)
5
12
15
72
30
76
Beam
Magnetic field needed for:
• Momentum measurement
• Polarimetry
• Sweep off low energy charged particles
26/Oct/2013 (DNP2013)
Yoke modifications to allow beam pipe
passage at forward angle kinematics
E. Cisbani / SBS for GEp5 @ JLab12
Adapted from Robin Wines / JLab
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GEM Working principle
Recent Technology:
F. Sauli, Nucl. Instrum. Methods A386(1997)531
GEM foil: 50 m Kapton + few
m copper on both sides
with 70 m holes, 140 m pitch
Ionization
Multiplication
Multiplication
Strong electrostatic
field in GEM holes
Multiplication
Readout
Support high particle flux (≫ MHz/cm2)
Intrinsic resolution at 50 m level
Relatively unexpensive
SBS
Robust / Slow aging
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
Gain vs Particle Flux
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SBS - GEM Front Tracker
•
•
•
Six 150x40 cm2 chambers with small dead area (~10%)
Each chamber consists of 3 50x40 cm2 lightweight 3xGEM
modules with x/y strip readout (0.4 mm pitch)
Readout electronics based on high channel density APV25 ASIC
driven by VME64x modules
Use x/y charge correlation for false hit suppression
Large SNR
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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GEM Front Tracker MonteCarlo
Realistic MC and digitization
• Tracking efficiency 99%-85%
depending on background
• Track parameter resolutions at
acceptable values even at
largest background
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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CH2 Polarimeters with GEM tracking
•
•
Two Polarimeters in series to increase statistics by ~50%
Each polarimeters consists of CH2 analyzer (50 cm) and four
50x2000 cm2 GEM chambers
Each chamber is made of five 50x50 cm2 GEM modules
Similar design of GEM front tracker, optimized for focal
polarimetry (less demanding particle rate respect to main
tracker)
Number of scattered protons
•
•
qpp (deg)
N. Liyanage et al. / UVa
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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PJ6: B. Quinn
G. Franklin et al. / Carnegie Mellon
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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High Luminosity, impact on Trigger / DAQ
• Must efficienty select electron elastic scattering by
DJ4: A. Camsonne
angular correlation
• First level (L1) from electron arm
– Energy information (with cuts to reduce inelastic)
– Rate (from SLAC high energy data and RCS experiments):
Ethr/Emax %
• Hadron Arm:
Rate [kHz]
50
75
85
90
1400
203
60
38
– Energy information (with cuts to reduce inelastic)
– Rate: 1.5 MHz
• Second level (L2) from two-arm coincidence:
– in 30 ns gate: 9 kHz
– AND geometrical correlation: 2 kHz
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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Conclusions
Expected results on proton GE/GM
Kinematics and expected accuracy
E
(GeV)
Q2
(GeV2)
Days
DGE/GM
6.6
5.0
1
0.023
8.8
8.0
10
0.032
11.0
12.0
30
0.074
http://hallaweb.jlab.org/12GeV/SuperBigBite
 SBS, is a cost effective, new magnetic spectrometer; will use the recent GEM
technology to operate at high luminosity, providing “large” acceptance and
high reconstruction accuracy
 SBS will permit unprecedented measurements of the proton and neutron
Form Factors at high Q2 as well on SIDIS physics
NC9: A. Puckett
26/Oct/2013 (DNP2013)
E. Cisbani / SBS for GEp5 @ JLab12
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