G E p -2 γ experiment (E04-019) UPDATE

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

and complex with , TWO-PHOTON EXCHANGE (TPEX) T matrix: q1 q2 q1 q2 N N In the Born approximation: form factors (real) respectively are 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.KobushkinarXiv: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. precision limited only by statistics (~ 1%), unlike Rosenbluth, very small p.t.psystematics: Ay , h cancel out in the Pt/Pl ratio Q2 fixed, Pp fixed, spin precession fixed p e’ 80μA beam current 85% pol. 20cm LH target e

FPP DC 3,4 Analyser 2 FPP DC 1,2 Analyser 1 HMS DC 2 HMS DC 1 DETECTORS 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

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

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

G E p -2 γ experiment (E04-019) UPDATE