Proton Charge Form Factor Measurement E. Cisbani INFN Rome – Sanità Group and Italian National...

E. Cisbani / Proton FF 1

Proton Charge Form Factor Measurement

E. CisbaniINFN Rome – Sanità Group

andItalian National Institute of Health

13/Oct/2011


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


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


Generalized Configuration

13/Oct/2011


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


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.)


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


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


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


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(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


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


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


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


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


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


Backup slides

13/Oct/2011


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


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

Systematic from p0 background

23

f is ratio of elastic to total events

Q2 = 12

13/Oct/2011 E. Cisbani / Proton FF


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


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


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

Proton Charge Form Factor Measurement E. Cisbani INFN Rome – Sanità Group and Italian National...

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