1. GEp-III in Hall C Andrew Puckett, MIT On behalf of the Jefferson Lab Hall C GEp-III Collaboration April 15, 2008

2. Introduction e e' 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

3. 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).

4. 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

5. Physics Motivation Measure proton FF to highest possible Q 2 —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!

6. 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

7. Kinematics Production kinematics—two new high Q 2 points at 7.1 and 8.5 GeV 2. Precession angle near optimal 270 degrees for higest Q 2 One high Q 2 point overlapping Hall A data at 5.2 GeV 2 —spin precession angle chosen near 180 degrees to study systematics of spin transport GEp-2g measures three different epsilon values at fixed Q 2 with high precision, investigate TPEX contribution to elastic ep scattering

8. FPP Two polarimeters in series— improves efficiency Each CH 2 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

9. Polarimetry-FPP HMS measures proton momentum and rotates target P t, P l into focal plane P n, P t 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

10. Focal Plane Angular Asymmetries Example helicity difference and helicity sum angular distributions from the lowest-epsilon point of GEp-2g: E beam =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

11. Polarimetry—Spin Precession Nominal bend angle of HMS is 25 degrees Nominal spin precession angle: FPP measures P n ' and P t ' after precession in HMS magnets Want to measure target P l and P t In ideal dipole approx: P t ' = P t P n ' = P l sin χ + P n cos χ P l ' = P l cos χ – P n sin χ Generally speaking, sin χ = 0 is to be avoided since FPP is not sensitive to P l '.

12. BigCal 1.1744 lead-glass bars 4x4x40 cm 3, 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

13. BigCal Kinematic correlation crucial to suppress inelastic backgrounds, especially at high Q 2

14. Summary—Preliminary Results PRELIMINARY Initial analysis of the data shows strikingly good agreement with the Hall A polarization data. The large error bar at Q 2 = 5.2 GeV 2 is a result of the unfavorable precession angle—deliberately chosen New detectors work as advertised Data taking for Q 2 =8.5 GeV 2 is ongoing Will spend roughly last 2 weeks of allotted time at 7.1 GeV 2