1. Proton Form Factor ratio GEp/GMp with polarization method --on behalf of Jefferson lab GEp3 collaboration Wei Luo Lanzhou University, China April 14 2008 APS April Meeting 2008 St. Louis Missouri

2. Outline 1. Introduction 2. Experiment Setup 3. Current status Data analysis Detector issues 4. Summary

3. 1. Introduction JLab Experiment E04-108: GEp-III and E04-019: GEp-2γ These two experiments use the same polarization method to measure proton form factor ratio in ep elastic scattering process. Experiment G Ep -2 γ finished Oct. 2007-Jan. 2008. G Ep -III is running.

4. One photon exchange: What’s the explanation of the discrepancy between polarization and Rosenbluth data? 2γ? What will the proton f.f. be at higher Q2? ? ?

5. 2. Experiment setup Two New detectors in Jlab Hall C: Focal Pane Polarimeter( In HMS hut), E-M Calorimeter BigCal. Coincidence between HMS(proton) and BigCal(electron) beam electron proton electron HMS (FPP)

6. Kinematics table E beam (Gev) Q 2 (Gev 2 ) ε T p (Gev) Θ p (°) D BigCal (cm) θ e (°) E e’ (Gev) 1.8732.506 0.146 1.33214.495493.2105.16 0.541 2.8472.506 0.631 1.33230.9951200.044.90 1.515 3.6802.506 0.796 1.33236.1051102.330.60 2.348 4.0525.2000.3742.78117.94605.060.31.271 5.7148.5200.2394.54011.681435.068.61.174 5.7147.1000.4643.78417.853760.047.31.930 Red GEp—III Blue GEp—2γ Now here!

7. FPP 2 set of CH2 Analyzer + 3 layer drift chamber, active size: 166cm(v) x 134cm(h). Reconstruct 3 tracks: HMS track, FPP1 track, FPP2 track Two Analyzers improved efficiency

8. TF1-0 type lead glass, 12-stage Russian FEU-84 PMT, coupled by 1cm soft optical ‘cookie’. Protvino part: 1024 bars of 3.8x3.8x45cm 3 lead glass. Cover 1.479m 2 RCS part: 720 bars of 4.0x4.0x40cm 3 lead glass. Cover 1.152m 2 BigCal goal: identify elastic event, reduce high background rate BigCal Calorimeter 2.631m 2

9. 3. Current Status Data analysis: Elastic event identification: HMS spectrometer; HMS-BigCal correlation. Example: E beam =1.837GeV Q 2 =2.506 ε=0.146 Position correlation x-diff θp θp δpδp Y-diff Missing momentum momentum Gev/c

10. Asymmetry at Focal Plane is the azimuthal distribution with two beam helicity state; a 0,b 0,c 0,d 0 ……are the instrumental asymmetry. Sum of two distribution: Difference of two distribution:

11. Preliminary φφ Red: beam energy 2.847Gev Blue: beam energy 3.548Gev Elastic asymmetry Inelastic asymmetry Elastic peak Inelastic cut P miss Counts Inelastic dominated by π0 production and Compton scattering: Inelastic Focal Plane asymmetry Elastic peak Inelastic cut Counts P miss Counts

12. Detector issues: FPP The alignment of FPP to HMS track. Optimization of FPP DC can significantly reduce false asymmetry. BigCal Radiation damage made BigCal lead glass “dark” and the gain of signal drop, the energy and position resolution get worse. About 2 weeks low power UV light curing during the shutdown of accelerator (without take out phototubes). The lead glass recovered to about 70% of the beginning of the experiment.

13. Before experiment After experiment UV curing Note: Low power UV light curing, did not take out phototube from back side.

14. 4. Summary Detectors works well as designed G Ep -2 γ finished and data analysis is going on G Ep -III one kinematics data point is done. Focal plane asymmetry at beam energy: 2.847Gev shows difference between elastic and inelastic, resonance state near this energy region maybe the explanation of this discrepancy.