GEp-2γ experiment (E04-019) UPDATE - HALL C USER Meeting January 30, 2009 - On behalf of the JLab-Gep Collaboration Mehdi Meziane The College of William & Mary

OUTLINE INTRODUCTION THE GEp-2γ (04-019) EXPERIMENT AT JLAB HALL-C TWO-PHOTON EXCHANGE (TPEX) THEORETICAL PREDICTIONS ELASTIC EVENTS SELECTION CONCLUSION

ROSENBLUTH AND POLARIZATION TRANSFER MEASURMENTS OF OF THE PROTON Either cross section or recoil polarization measurements to extract the proton form factor Recoil polarization and Rosenbluth ratios are clearly different in the Born approximation. Difference increase systematically with Q2. Two methods, two different results Something beyond the Born approximation Two-photon exchange

THE GEP-2γ (04-019) EXPERIMENT AT JLAB HALL-C We look for a kinematical ε dependence of to detect a possible two-photon exchange effect in the scattering In the Born approximation we don’t have such dependence We carried out the experiment last year: Q2 = 2.5 GeV2 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 Q2 and pp fixed Measure separatly the two observables Pt/Pl and relative Pl

TWO-PHOTON EXCHANGE (TPEX) T matrix: with complex and , q1 q2 N q1 q2 N In the Born approximation: are form factors (real) respectively and The virtual photon polarization is:

TWO-PHOTON EXCHANGE (TPEX) Transverse polarization component Longitudinal polarization component Reduced cross section Born Approx. Beyond Born Approx. with

THEORETICAL PREDICTIONS: HADRONIC CALCULATIONS Only nucleon intermediate states are taken into account P.Blunden et al., Phys.Rev.Lett.91: 142304 (2003) The inclusion of Δ reduces the TPEX correction (Figure) P.Blunden et al., Phys.Rev.C72: 034612 (2005) Higher nucleon resonances has smaller effect S.Kondratyuk et al., Phys.Rev. C75:038201(2007) Structure function method: 2γ effects small, higher orders change Rosenbluth slope (Figure) Yu. Bystricky, E.A.Kuraev, E. Tomasi-Gustafsson Phys. Rev. C75, 015207 (2007) Proton off-shell form factors make almost no difference D.Borisuyk, A.Kobushkin arXiv:0804.4128

THEORETICAL PREDICTIONS GPD CALCULATIONS TPEX contributions are calculated assuming factorization of the soft nucleon-quark part, and the hard electron-quark interaction, where the TPEX takes place via the box diagram. Absolute correction to FF ratio Ge/Gm: slow Q2 variation, strong effects at low ε valid for high Q2 or high ε A.Afanasev et al., Phys.Rev.D72:013008 (2005) – GPD models: Gauss on Fig., smaller effect with Regge, or non-zero quark mass

THEORETICAL PREDICTIONS hadronic (elastic) : dominated by correction to GM GPD (includes inelastic): dominated by Y2g and correction to GE Both theories describe Rosenbluth data but have opposite predictions for GE/GM

GEP-2γ at JEFFERSON LAB HALL C Key idea: fixed Q2 same spin transport same analysing power. e e’ p precision limited only by statistics (~ 1%), unlike Rosenbluth, very small p.t.p systematics: Ay , h cancel out in the Pt/Pl ratio Q2 fixed, Pp fixed, spin precession fixed Ee, GeV pp Ee’ θp, deg θe ε range <Q2> 1.867 2.068 0.527 14.13 106 .130-.160 2.49 2.839 1.507 30.76 45.3 .611-.647 3.549 2.207 35.39 32.9 .765-.786 80μA beam current 85% pol. 20cm LH target

DETECTORS Focal Plane Polarimeter FPP Analyser 2 FPP DC 1,2 Analyser 1 HMS DC 1 HMS DC 2 FPP DC 1,2 FPP DC 3,4 Focal Plane Polarimeter FPP 2 analyzers in serie 2 sets of drift chamber pairs Electromagnetic Calorimeter BigCal 1744 channels

KINEMATIC FORMULA Proton momentum calculated from the electron angle: with Proton momentum calculated from the proton angle: HMS proton momentum: and p0 = 2.0676 GeV/c

ELASTIC EVENTS SELECTION Green: (θe - θp) and (φe- φp) cut pθp pθe (PHMS-pθe)/p0 Black: all events pθp pθe (PHMS-pθe)/p0 Q2 = 2.49 GeV2 ε = 0.149 (smallest) Ebeam = 1.867 GeV p0 = 2.0676 GeV/c Blue: normalized background pθp pθe (PHMS-pθe)/p0 Red: background pθp pθe (PHMS-pθe)/p0 σ = 0.28% σ = 0.22% σ = 0.18% Bgd=3.2% Bgd=3.3% Different shapes of the elastic peaks Ratio elastic/inelastic changes with the kinematic variables Upper estimate of the background of 3.2% with ±3σ cuts Need a Monte-Carlo simulation to fit the background shape

PRELIMINARY RESULTS The 2 theoretical curves have been normalized by a different coefficient to fit the data. No evidence of TPEX effect at a percent level PRELIMINARY Prove the validity of the Born approximation for data obtained by the polarization technique at a percent level.

CONCLUSION Fixed Q2 and pp Measure separatly two observables: relative Pl and the ratio Pt/Pl Same analyzing power Same spin transport Upper estimate of the background of 3.2% Need a Monte-Carlo simulation to fit the background shape No evidence of effects beyond the Born approximation Prove the validity of the polarization method for the proton form factor measurements