Proton Charge Form Factor Measurement E. Cisbani INFN Rome – Sanità Group and Italian National Institute of Health 1 13/Oct/2011 E. Cisbani / Proton FF
Jan 18, 2016
E. Cisbani / Proton FF 1
Proton Charge Form Factor Measurement
E. CisbaniINFN Rome – Sanità Group
andItalian National Institute of Health
13/Oct/2011
E. Cisbani / Proton FF 2
Proton Form Factors at high Q2
As seen from G. Cates talk, Form Factors measurement at high Q2 range are of paramount importance
Approved Hall A experiment: E12-07-109 or GEp(5) :Large Acceptance Proton Form Factor Ratio Measurement at
High Q2 Using Recoil Polarization Method
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
13/Oct/2011
E. Cisbani / Proton FF 3
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 GM
2 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 protonRelative 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) 13/Oct/2011
Use polarization transfer technique
E. Cisbani / Proton FF 4
Generalized Configuration
13/Oct/2011
E. Cisbani / Proton FF 5
Polarization Transfer
Require: Measurement of the recoil proton polarization
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
PolarizedElectronBeam
Unpolarized proton target
ScatteredElectron
Scatteredproton Pl
Pt
Transverse component of scattered proton spin
Longitudinal component of scattered proton spin
13/Oct/2011
E. Cisbani / Proton FF 6
Proton Polarimeter (PP) Use azimuthal asymmetry of the proton scattering off matter induced by spin-orbit coupling
Number of scattered protons:
Require: Dipole magnet to precess Pl at target to Pypp
Polarimeter only measures components of proton spin that are transverse to the proton’s momentum direction
Track in
Track out
Track in
Track out
Pypp Px
pp
N=number of scattered proton, Pe beam polarization
where refers to electron beam helicity
13/Oct/2011
Maximize Pe
A (a.u.)
E. Cisbani / Proton FF 7
From measured polarization ratio to Form Factor ratio
Back propagate Pppx, y ratio to the vertex to get Pt/Pl (under
geometric approximation):
Proton spin precession in dispersive + non dispersive plane
gp Q2 is big good accuracy of Df is needed
Consolidated experience with GEp(1), GEp(2) and GEp(3)
Note: the analyzing power cancel out in ratio, but is important in overall statistics (as well as beam polarization)
Proton trackdeflection
13/Oct/2011
E. Cisbani / Proton FF 8
Polarimeter Analyzing power
For Q2 = 12 GeV2, p 7.3 GeV/c → 1/p 0.14 c/GeV Ay 0.08
Rather well known; AY analyzing power decreases at larger p (or Q2)
Ay 1/p 1/Q2
13/Oct/2011
E. Cisbani / Proton FF 9
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
to maximize statisticsand achieve adequate accuracy
13/Oct/2011
E. Cisbani / Proton FF 10
Experiment’s Figure of MeritFor polarization transfer experiment with recoil polarization measurement:
epp = Proton polarimeter efficiencyPe = 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
13/Oct/2011
E. Cisbani / Proton FF 11
E(GeV)
Q2 (GeV2)
qE(deg)
Pe(GeV)
Qp(deg)
Pp(GeV)
Days DmGE/GM
6.6 5.0 25.3 3.94 29.0 3.48 1 0.0238.8 8.0 25.9 4.54 22.8 5.12 10 0.032
11.0 12.0 28.2 4.60 17.4 7.27 30 0.074
Kinematics and projected data
Assumed:Ibeam = 75 uABeam Polarization = 85%Target Length = 40 cmProton Polar. Efficiency = 50%Acceptance: DWe = 130 msr (largest Q2) Ee > 4.0 GeV Ep > 3.5 GeV
Last point is the most demanding
13/Oct/2011
E. Cisbani / Proton FF 12
Polarimeter Requirements / EfficiencyDouble-polarimeter provides ~50% efficiency gain relative to single-polarimeter of equivalent thickness
Require: Double polarimeter
qpp (deg)
Num
ber o
f sca
tter
ed p
roto
ns
13/Oct/2011
E. Cisbani / Proton FF 13
Polarimeter Requirements / AcceptanceFigure of Merit, FOMpp = epp·Ay
2
FOM
qpp (deg)
Peaks at 4 deg
Shape of FOM versus p*sinqpp is found to be universal.
Requirement:Polarimeter must cover qpp up to 10 deg
13/Oct/2011
E. Cisbani / Proton FF 14
Maximal exploitation of two-body kinematic correlations
Elastic process selection / p0 Background SuppressionDominant background expected from p0 photo-production (as in previous GEP experiments) (eH,p0gp)
For Emiss<0.35 GeV,remaining background: 10%
(Background is going ~ quadratically respect to angular resolution)
Red: p0 photoproductionBlack: Elastics
Blue: Sum
Proton arm: - momentum resolution: 1 % - angular resolution: 1 mrad - vertex reconstruction: 5 mm
Energy difference between electron calorimeter and the one expected from hadron arm (in
elastic kinematics)
13/Oct/2011
E. Cisbani / Proton FF 15
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):
• 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
Ethr/Emax % 50 75 85 90Rate [kHz] 1400 203 60 38
13/Oct/2011
Refer to A. Camson
E. Cisbani / Proton FF 16
Electron spectrometer requirements in Proton Charge Form Factor Measurement
Electron-nucleon luminosity
1039 Hz/cm2
Calorimeter rate* 200 kHzAngular acceptance 150 msr
Momentum range 4-5 GeV
Energy resolution 10%
Central angle (range) 25-30 degrees
Angular resolution 1 mrad
Time resolution 2 ns
Proton arm requirements inProton Charge Form Factor Measurement
Electron-nucleon luminosity
1039 Hz/cm2
Front tracker rate 500 kHz/cm2
Calorimeter rate** 1.5 MHzAngular acceptance 40 msr
Momentum range 3-8 GeV
Momentum resolution 1%
Central angle (range) 17-30 degrees
Angular resolution 1 mrad
Vertex reconstruction 5 mm
Time resolution 1 nsProton spin rotation 90 +/- 30 degreesAccuracy of spin rotationIn non-dispersive plane
0.1 mrad
Proton polarimeter analyzer 50 cm x 2Polarimeter acceptance 10 degrees
* for threshold 0.75 Eelectronelastic
** for threshold 0.5 Eprotonelastic
13/Oct/2011
Requirements for Instrumentation in GEp/GM
p
E. Cisbani / Proton FF 17
Backup slides
13/Oct/2011
E. Cisbani / Proton FF 18
Elastic scattering of longitudinally polarized electrons on proton
Physics Process Investigated
Cross section up to 2g exchange approximation:
Rosenbluth at 1g approx.
Not negligible at high Q2
• Rosenbluth separation does not provide “simple” relation on form factors;• Its interpretation not fully understood• Systematics can be large
Varying angle and beam energy, keeping Q2 constant – that is vary epsilon, tau is constant
13/Oct/2011
E. Cisbani / Proton FF 19
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)
13/Oct/2011
E. Cisbani / Proton FF 2013/Oct/2011
E. Cisbani / Proton FF 2113/Oct/2011
E. Cisbani / Proton FF 2213/Oct/2011
Systematic from p0 background
23
f is ratio of elastic to total events
Q2 = 12
13/Oct/2011 E. Cisbani / Proton FF
E. Cisbani / Proton FF 24
Proton Polarimeter (PP) Use azimuthal asymmetry of the proton scattering off matter induced by spin-orbit coupling
Number of scattered protons:
Extract Pppx/Ppp
y from trigonometric analysis
Require Dipole magnet to precess Pl at target to Pypp
Polarimeter only measures components of proton spin that are transverse to the proton’s momentum direction
Track in
Track out
Track in
Track out
Pypp Px
pp
N=number of scattered proton, Pe beam polarization
where refers to electron beam helicity
13/Oct/2011
Maximize Pe
E. Cisbani / Proton FF 25
Polarimeter Requirements / EfficiencyDouble-polarimeter provides ~50% efficiency gain relative to single-polarimeter of equivalent thickness
Require:• Double polarimeter• Tracker has to identify multiple tracks in small angular range
Q2 = 2.5 GeV2
p*sinqfpp
Ay
(GeV) qpp (deg)N
umbe
r of s
catt
ered
pro
tons
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
13/Oct/2011
E. Cisbani / Proton FF 26
Polarimeter Requirements / AcceptanceFigure of Merit, FOM = N*Ay
2 where N = number of scattered protons
FOM
qpp (deg)
Peaks at 4 deg
qpp (deg)
Integral ofFOM
FOM saturates at 10 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
13/Oct/2011