GEp-III in Hall C

Andrew Puckett, MIT

On behalf of the Jefferson Lab Hall C GEp-III Collaboration

April 15, 2008

Introduction

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• Form Factors are fundamental properties of the nucleon• Characterize nucleon response to electromagnetic probe—quark structure and dynamics• Two ways to measure FF:

• Cross section (Rosenbluth separation)• Polarization Observables

• Beam-target asymmetry• Polarization transfer

• Rosenbluth and Polarization results incompatible at high momentum transfer

Rosenbluth Separation

Elastic scattering cross section in Born approximation:

Measure cross section at fixed momentum transfer and vary scattering angle:

Example Rosenbluth measurement—Qattan et al, PRL 94, 142301 (2005) (JLab Hall A).

Recoil Polarization• Transferred Polarization components related to FF• Electric-magnetic FF ratio proportional to ratio of transferred polarization components• Polarization only viable way to measure the electric FF at high momentum transfer

Example—Gayou et al. PRL 88, 092301 (2002), JLab Hall A

Physics Motivation• Measure proton FF to highest possible Q2—onset of pQCD scaling behavior?• Constrain parametrizations of GPDs—Dirac and Pauli FF are first moments of GPDs H and E• Understanding the nucleon spin—quark orbital angular momentum• Understand the disagreement between Rosenbluth and polarization results

• Knowledge of nucleon FF is a vital input in the interpretation of many experiments in nuclear and hadronic physics!

Experiments E04-108 and E04-019

New experiments carried out from

Oct. 2007-Jun. 2008 in Hall C at JLab—

New detectors BigCal and FPP

Kinematics

• Production kinematics—two new high Q2 points at 7.1 and 8.5 GeV2. Precession angle near optimal 270 degrees for higest Q2

• One high Q2 point overlapping Hall A data at 5.2 GeV2—spin precession angle chosen near 180 degrees to study systematics of spin transport• GEp-2g measures three different epsilon values at fixed Q2 with high precision, investigate TPEX contribution to elastic ep scattering

FPP

• Two polarimeters in series—improves efficiency

• Each CH2 Analyzer followed by pair of drift chambers

• Each chamber pair contains a total of six planes—2 X, 2 U, 2 V

• 2 cm between sense wires—size of drift cell

• Both polarimeters supported on one rigid frame

Polarimetry-FPP• HMS measures proton momentum and rotates target Pt, Pl into focal plane Pn, Pt components

which are measured by FPP—also defines incident track for FPP• FPP measures focal plane asymmetry• Spin transport matrix calculated event by event—dominant source of systematic uncertainty• Likelihood analysis determines target polarization from FP polarization• Beam polarization and analyzing power cancel in the FF ratio—no systematic uncertainty

Focal Plane Angular Asymmetries

Example helicity difference and helicity sum angular distributions from the lowest-epsilon point of GEp-2g: Ebeam=1.867 GeV, ε = .15

• After optimizing tracking and alignment in software, instrumental asymmetries are found to be small• False asymmetries are still a work in progress—much room for improvement

Polarimetry—Spin Precession

• Nominal bend angle of HMS is 25 degrees• Nominal spin precession angle:

• FPP measures Pn' and Pt' after

precession in HMS magnets• Want to measure target Pl and Pt

• In ideal dipole approx:• Pt' = Pt

• Pn' = Pl sin χ + Pn cos χ

• Pl' = Pl cos χ – Pn sin χGenerally speaking, sin χ = 0 is to be avoided since FPP is not sensitive to Pl'.

BigCal1. 1744 lead-glass bars 4x4x40 cm3,

coupled to PMTs via optical 'cookies'

2. ADC of each PMT, TDC of sums of 8

3. Overlapping sums of 64 with high threshold for trigger

4. 4” Al absorber in front, protects against rad. damage, degrades resolution

5. Position resolution ~5 mm

6. 135 msr acceptance at 4.35 m from target

7. Set at a distance to match acceptances of electron and proton arm

BigCal

Kinematic correlation crucial to suppress inelastic backgrounds, especially at high Q2

Summary—Preliminary Results

PRELIMINARY

• Initial analysis of the data shows strikingly good agreement with the Hall A polarization data.• The large error bar at Q2 = 5.2 GeV2 is a result of the unfavorable precession angle—deliberately chosen• New detectors work as advertised• Data taking for Q2=8.5 GeV2 is ongoing Will spend roughly last 2 weeks of allotted

time at 7.1 GeV2