1. G E p -2γ experiment and the new JLab Hall-C Focal Plane Polarimeter Mehdi Meziane The College of William & Mary - APS Meeting April 14, 2008 - On behalf of the JLab-Gep Collaboration

2. OUTLINE INTRODUCTION TWO PHOTONS EXCHANGE (TPEX) THE NEW FOCAL PLANE POLARIMETER IN HALL-C EXPECTED UNCERTAINTIES SUMMARY

3. Two methods, two different results ROSENBLUTH AND POLARIZATION TRANSFER MEASURMENTS OF OF THE PROTON Recoil polarization and Rosenbluth ratios are clearly different in the Born approximation. Difference increase systematically with Q 2. Something beyond the Born approximation Two photons exchange Either cross section or recoil polarization measurements to extract the proton form factor

4. THE GEP-2γ (04-019) EXPERIMENT AT JLAB HALL-C In the Born approximation we don’t have such dependence We carried out the experiment last year:  Q 2 = 2.5 GeV 2 for 3 values of ε : 0.15, 0.63 and 0.78  Requires <0.01 statistics for a ratio of 0.7  Systematics cancelled out because Q 2 and p p fixed We look for a kinematical dependence of to detect a possible two photons exchange effect in the scattering

5. T matrix: with complex and, In the Born approximation: are form factors (real) and The virtual photon polarization is: TWO PHOTONS EXCHANGE (TPEX)

6. Polarization components: The reduced cross section is: with In and the TPEX contribution enters aswhile inonly as For a non-zero contribution to the cross section at small  the absolute value of the TPEX contribution will change by a factor 1/  for TWO PHOTONS EXCHANGE (TPEX)

7. BASIC PRINCIPLES OF THE EXPERIMENT Measure the polarization of the recoil proton in the scattering In the Born approximation: Two non-zero polarization components:and Measurement of the ratio By measuring the asymmetry distribution f after (re)scattering of the recoil proton in the analyser:

8. Analyser 1 Analyser 2 HMS DC 1 HMS DC 2 FPP DC 1,2 FPP DC 3,4 Focal Plane Polarimeter (FPP) in Hall-C

9. 2 Chambers with 3 planes each per polarimeter u x v Consists in 2 polarimeters in series to increase the efficiency 12 planes of detection Active area 134*166 (cm 2 ) Measure coordinates u, x, v 1164 instrumented wires Need 3 tracking procedures (2 polarimeters and HMS) Gas mixture Argon50%/Ethane50% Operating HV 2350V (field and cathode wires) Focal Plane Polarimeter (FPP) in Hall-C

10. CH 2 Analysers 145cm *111cm *60cm Layered, split (L/R), 1 ton each Open like a sliding door for straight through tracking Independently supported on interleaved frame Focal Plane Polarimeter (FPP) in Hall-C

11. EXPECTED UNCERTAINTIES Fixed Q2, fixed proton momentum Same analysing power and HMS settings (spin transport) for the 3 kinematics We measured the transferred polarization component ratio P t /P l and relative P l separately in ep elastic for a fixed Q 2 of 2.5GeV 2 at three different kinematics (epsilons) by changing the beam energy. Statistical errors only Statistics < 0.01 for a ratio of 0.7

12. LONGITUDINAL POLARIZATION TRANSFERT The ε dependence of the form factor ratio mainly comes from P t As Q 2 is constant Relative polarization have small systematic uncertainties ≈ 1% (Möller measurements) Statistical (expected) and systematic errors A.Afanasev et al. P.G. Blundel et al.

13. SUMMARY Absolute value of TPEX contribution changes by a factor 1/  for P t A new detector: the HALL C focal plan polarimeter The form factor ratio ε dependence comes from P t Statistics < 0.01 for a ratio of 0.7