Transcript Proton Charge FF Measurement

Proton Charge Form Factor
Measurement
E. Cisbani
INFN Rome – Sanità Group
and
Italian National Institute of Health
13/Oct/2011
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Proton Form Factors at high Q2
As seen from G. Cates talk, Form Factors measurement at high
Q2 range are of paramount importance
Our 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
Approved Hall A experiment: E12-07-109 or GEp(5) :
Large Acceptance Proton Form Factor Ratio Measurement at
High Q2 Using Recoil Polarization Method
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Elastic Electron Scattering
• In 1 g approx. , form factors of elastic electron scattering
are a function of Q2 and the elastic cross section is:
At large Q2:
– t is large and GM2 terms dominate in cross section
– Extraction of GE/GM from Rosenbluth separation becomes
unreliable due to 2 g exchange
• Need to exploit interference between GE and GM by means
of double polarization methods:
● Polarized beam and target
● Polarization transfer from polarized beam to scattered proton
Relative FOM at Q2 up to 15 GeV2 is about 1 order of magnitude
larger for the polarization transfer method
(due to smaller achievable luminosity of a polarized target)
Use polarization transfer technique
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Generalized Configuration
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Polarization Transfer
Polarized
Electron
Beam
Unpolarized
proton target
Scattered
Electron
Pt
Scattered
proton
Transverse component of scattered proton spin
Longitudinal component of scattered proton spin
Pl
 Determine the form factor ratio and relative sign of GE to GM
 Intrinsic small systematic errors:
• Measuring ratio Pt/Pl
• No cross section measurement needed
• Fixed energy and angle
Require: Measurement of the recoil proton polarization
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Proton Polarimeter (PP)
Use azimuthal asymmetry of the proton
scattering off matter induced by spin-orbit
coupling
Number of scattered protons:
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
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From measured polarization ratio to Form Factor ratio
Back propagate Pppx, y ratio to the vertex to get Pt/Pl (under
geometric approximation):
Proton track
deflection
Proton spin precession in dispersive + non dispersive plane
gp  Q2 is big  good accuracy of Df is needed
Note: the analyzing power cancel out in ratio, but is important in overall statistics (as
well as beam polarization)
Consolidated experience with GEp(1), GEp(2) and GEp(3)
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Polarimeter Analyzing power
Rather well known; AY analyzing power decreases at larger p (or Q2)
Ay  1/p  1/Q2
For Q2 = 12 GeV2, p  7.3 GeV/c → 1/p  0.14 c/GeV  Ay  0.08
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General Requirements
• Select elastic events with multiple kinematic
correlation cuts
– to suppress the large inelastic background
• High Q2
– our goal
• Operation at high luminosity
• Provide large acceptance
• Provide efficient polarimeter
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to maximize statistics
and achieve adequate
accuracy
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Experiment’s Figure of Merit
For polarization transfer experiment with recoil
polarization measurement:
;
epp = Proton polarimeter efficiency
Pe = Beam polarization
Using DQ2/Q2=10% as baseline
(due to fast fall of statistics with Q2)
Maximize Luminosity (L) and polarimeter efficiency (epp)
Match electron and hadron acceptances
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Kinematics and projected data
qE
Qp
Days DmGE/GM
E
Q2
(GeV)
(GeV2)
(deg)
(GeV)
(deg)
(GeV)
6.6
8.8
5.0
8.0
25.3
25.9
3.94
4.54
29.0
22.8
3.48
5.12
1
10
0.023
0.032
11.0 12.0
28.2
4.60
17.4
7.27
30
0.074
Pe
Pp
Assumed:
Ibeam = 75 uA
Beam Polarization = 85%
Target Length = 40 cm
Proton Polar. Efficiency = 50%
Acceptance:
DWe = 130 msr (largest Q2)
Ee > 4.0 GeV
Ep > 3.5 GeV
Last point is the most demanding
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Polarimeter Requirements / Efficiency
Number of scattered protons
Double-polarimeter provides ~50% efficiency gain relative to singlepolarimeter of equivalent thickness
qpp (deg)
Require: Double polarimeter
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Polarimeter Requirements / Acceptance
Figure of Merit, FOMpp = epp·Ay2
FOM
Peaks
at 4 deg
qpp (deg)
Shape of FOM versus p*sinqpp is found to be universal.
Requirement:
Polarimeter must cover qpp up to 10 deg
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Elastic process selection / p0 Background Suppression
Dominant background expected from p0 photo-production (as in previous GEP
experiments) (eH,p0gp)
Maximal exploitation of two-body kinematic correlations
Red: p0 photoproduction
Black: Elastics
Blue: Sum
Proton arm:
- momentum resolution: 1 %
- angular resolution:
1 mrad
- vertex reconstruction: 5 mm
For Emiss<0.35 GeV,
remaining background: 10%
(Background is going ~ quadratically respect
to angular resolution)
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Energy difference between electron calorimeter
and the one expected from hadron arm (in
elastic kinematics)
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High Luminosity, impact on Trigger / DAQ
• Must efficienty select electron elastic scattering by
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 %
Rate [kHz]
50
75
85
90
1400
203
60
38
• Hadron Arm:
– 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
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Refer to A. Camson
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Requirements for Instrumentation in GEp/GMp
Electron spectrometer requirements in
Proton Charge Form Factor Measurement
Proton arm requirements in
Proton Charge Form Factor Measurement
Electron-nucleon
luminosity
1039 Hz/cm2
Electron-nucleon
luminosity
1039 Hz/cm2
Calorimeter rate*
200 kHz
Front tracker rate
500 kHz/cm2
Angular acceptance
150 msr
Calorimeter rate**
1.5 MHz
Momentum range
4-5 GeV
Angular acceptance
40 msr
Energy resolution
10%
Momentum range
3-8 GeV
Central angle (range)
Angular resolution
Time resolution
* for threshold 0.75 Eelectronelastic
** for threshold 0.5 Eprotonelastic
25-30 degrees
1 mrad
2 ns
Momentum resolution
Central angle (range)
17-30 degrees
Angular resolution
1 mrad
Vertex reconstruction
5 mm
Time resolution
Proton spin rotation
Accuracy of spin rotation
In non-dispersive plane
Proton polarimeter
Polarimeter acceptance
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1%
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1 ns
90 +/- 30 degrees
0.1 mrad
analyzer 50 cm x 2
10 degrees
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Backup slides
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Physics Process Investigated
Elastic scattering of longitudinally polarized electrons on proton
Cross section up to 2g exchange approximation:
Rosenbluth at 1g approx.
Varying angle and beam
energy,
keeping Q2 constant – that i
vary epsilon, tau is constant
Not negligible at high Q2
• Rosenbluth separation does not provide “simple” relation on form factors;
• Its interpretation not fully understood
• Systematics can be large
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Beam and Target
• Beam
– Highest energy (11 GeV) to measure up to the
largest Q2 (3 energy values, one for each Q2 point)
– Intensity as large as possible (75 uA) to maximize
luminosity
– polarization as large as possible (85% expected
from JLab beam)
• Target
– Hydrogen, as thick as possible (keep liquid) to
maximize luminosity (compatible with
spectrometers acceptances and resolution)
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Systematic from p0 background
f is ratio of elastic to total events
Q2 = 12
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Proton Polarimeter (PP)
Use azimuthal asymmetry of the proton
scattering off matter induced by spin-orbit
coupling
Number of scattered protons:
where  refers to electron beam helicity
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
Extract Pppx/Pppy from trigonometric analysis
Maximize Pe
N=number of scattered proton, Pe beam polarization
Require Dipole magnet to precess Pl at target to Pypp
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Polarimeter Requirements / Efficiency
Q2 = 2.5 GeV2
Ay
p*sinqfpp
Double-polarimeter provides ~50%
efficiency gain relative to singlepolarimeter of equivalent thickness
Number of scattered protons
Comparison of Ay at Q2 = 2.5 for:
• Single outgoing track
72% of all tracks
• Multiple outgoing tracks
28% of all tracks
•Similar fractions at higher Q2
qpp (deg)
(GeV)
Require:
• Double polarimeter
• Tracker has to identify multiple tracks in small angular range
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Polarimeter Requirements / Acceptance
Figure of Merit, FOM = N*Ay2 where N = number of scattered protons
FOM
Integral
of
FOM
FOM saturates
at 10 deg
Peaks
at 4 deg
qpp (deg)
qpp (deg)
Shape of FOM versus p*sinqpp is found to be universal.
Require: Maximum qpp = 16 and 7.5 deg for Q2 = 5 and 12
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