A Measurement of Two-Photon Exchange in Unpolarized Elastic Electron-Proton Scattering James Johnson Northwestern University & Argonne National Lab For the Rosen07 Collaboration I am the graduate student working on this particular experiment Three experiments in the collaboration

Outline The electromagnetic interactions of the proton are described by two form factors, GE (Q2) and GM(Q2) Two methods of extraction, but their results don’t agree Leading candidate is two-photon exchange Parameterize the proton charge and magnetic moment distributions – in nonrel. Limit, Fourier transforms

Prior Experiments Rosenbluth Scattering Measure electron-proton scattering Factor out Mott cross section, and get a function linear in the squares of the form factors τGM2 + εGE2 Polarization Transfer Scatter longitudinally polarized electrons from unpolarized protons The ratio GE/GM is proportional to pT/pL Does not give form factors directly Rosenbluth: angular dependant function Tau – proportional to momentum transfer, Q^2 / (4*Mp^2) Epsilon – angular dependant, virtual photon longitudinal polarization, (1 + 2*(1+tau) * tan^2 (theta_e/2))^-1 G_E/G_M = p_T/p_L * (E-E’)/(2*Mp) * tan(theta_e/2)

Disagreement Rosenbluth gives a ratio that stays flat The errors on GE increase with Q2 Polarization transfer shows a decreasing ratio Smaller errors at high Q2 Implies a difference between charge and magnetic distributions It was suggested that there may be a problem inherent in the Rosenbluth method to cause these errors J. Arrington, Phys. Rev. C69:022201, 2004 M. Jones et al, Phys. Rev. Lett. 84:1398-1402, 2000 O. Gayou et al, Phys. Rev. Lett. 88:092301, 2002

Precision Rosenbluth JLab E01-001 Detect scattered protons instead of electrons Same reaction, smaller angular dependant corrections Precision comparable to polarization transfer Agrees with electron Rosenbluth The disagreement is real High-precision measurement of the discrepancy I. A. Qattan et. al, Phys. Rev. Lett. 94:142301, 2005

Magnitude of the Discrepancy Shows the difference increasing as we increase Q^2 5-8% correction on reduced cross section Solid line – fit to E01-001 ‘Super-Rosenbluth’ Dashed line – taken from polarization transfer ratio

Two-Photon Exchange Both methods account for radiative corrections, but neither considers two-photon exchange Difficult to Calculate Rough qualitative agreement Different ε dependence Scale not predicted Delta is the percentage change in the cross section

Rosenbluth 2007 JLab E05-017 HMS in Hall C at Jefferson Lab 4cm liquid hydrogen target for elastics 4cm aluminum dummy for endcap subtraction May 8 – July 13, 2007 Add picture of HMS

Rosenbluth 2007 102 Kinematics points Q2 0.40-5.76 GeV2 13 points at Q2=0.983 10 points at Q2=2.284 Kinematics taken Each solid, colored line is a different beam energy, data is where the lines intersect our Q^2 values In particular, very low- and very high-epsilon points, where there is little previous data 16 Q^2 values 17 Ebeam values, range At each Q^2, several points for an L-T seperation

Time of Flight Calibration Acceptance cuts Solid – full delta-β spectrum Small dashes - Aerogel cut to exclude pions Large dashes - Beta cut to exclude deuterons

Time of Flight Calibration Six total calibrations Three momentum ranges Before/After discriminator replacement Solid line – uncalibrated Dashed line - calibrated

Aerogel Calibration Aerogel distinguishes π+ from heavier particles Fit the position of the 1-photoelectron peak Not possible on runs with low pion count due to interference from the pedestal

Analysis Steps Sum data & dummy runs at selected kinematic Simulate elastics, pion photoproduction, compton scattering Scale all to corrected charges Fit dummy + simulations to the data Extract ratio of simulation cross-section to actual cross-section

Charge Correction Included so far Not yet included Computer & Electronics livetimes, Scintillator ¾ efficiency, Prescale, VDC tracking efficiency, BCM Calibration, Target boiling Not yet included Particle Identification efficiency, Proton Absorbtion, Beam offset

Charge Correction Particle Identification Efficiency Proton Absorption Using delta-beta cut, need to find cut tails Proton Absorption Incomplete information on a few materials Beam Offset Surveyed using carbon target

Unpeeling Data/SIMC resolution mismatch Background xptar dependence Non-gaussian tails in SIMC Background ‘Dummy’ runs for endcap subtraction Simulated pi-0 photoproduction Solid – data Black dotted – sum of dummy, simulations Elastics – adding nongaussian tails Resolution – xptar correction

Nonlinearity Tests Rosenbluth expects linearity in ε, TPE would cause a deviation E01-001 and NE11 show quadratic terms consistent with zero Project P2 within ±0.020 for E05-017 NE-eleven is the best standard Rosenbluth, performed at SLAC NE11 @ 2.5 GeV^2 E01-001 @ 2.64 GeV^2 Projection is at Q^2 of 2.56 GeV2 NE11: L. Andivahis et al, Phys. Rev. D50:5491, 1994

Conclusion Projected Uncertainties More Q2 points Analysis underway Shifted range down Better separations at each Q2 Analysis underway From proposal