A Measurement of Two-Photon Exchange in Unpolarized Elastic Electron-Proton Scattering John Arrington and James Johnson Northwestern University & Argonne National Lab For the Rosen07/E Collaboration

Outline The electromagnetic interactions of the proton are described by two form factors, G E (Q 2 ) and G M (Q 2 ) Two methods of extraction, but their results don’t agree Leading candidate is two-photon exchange

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 τG M 2 + εG E 2 Polarization Transfer Scatter longitudinally polarized electrons from unpolarized protons The ratio G E /G M is proportional to p T /p L Does not give form factors directly

Disagreement Rosenbluth gives a ratio that stays flat –The errors on G E increase with Q 2 Polarization transfer shows a decreasing ratio –Smaller errors at high Q 2 –Implies a difference between charge and magnetic distributions J. Arrington, Phys. Rev. C69:022201, 2004 M. Jones et al, Phys. Rev. Lett. 84: , 2000 O. Gayou et al, Phys. Rev. Lett. 88:092301, 2002

“Super-Rosenbluth” JLab E E01-001Detect 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 –Tests radiative corrections I. A. Qattan et. al, Phys. Rev. Lett. 94:142301, 2005

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

Magnitude of the Discrepancy Solid line – fit to E ‘Super-Rosenbluth’ Dashed line – taken from polarization transfer ratio  Quantify difference, look for nonlinearity

Rosenbluth 2007 JLab E HMS in Hall C at Jefferson Lab 4cm liquid hydrogen target for elastics 4cm aluminum dummy for endcap subtraction May 8 – July 13, 2007

Rosenbluth Kinematics points Q GeV 2 13 points at Q 2 = points at Q 2 =2.284

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 Noisy ADC signals –Not needed for pion rejection at most (all?) settings –Mainly check TOF efficiency, pion contamination

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 No kinematic, aerogel cuts -cut on elastic peak supresses pions and deuterons No dummy subtraction -removes deuterons and tritons

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 –Computer & electronics deadtime –HSCIN (¾ scintillator) efficiency –default tracking efficiency (“HMS w/DC cuts”) –prescale factor Not yet included –Final BCM Calibration* –Target boiling* –Particle Identification efficiency* –Proton Absorption* –Beam offset * Should be  -independent

Unpeeling Hydrogen elastics –Compare to simulated elastics Background –‘Dummy’ runs for endcap subtraction –Simulated  0 photoproduction

Unpeeling Hydrogen elastics –Compare to simulated elastics Background –‘Dummy’ runs for endcap subtraction –Simulated pi-0 photoproduction Low  setting (  p = 12.5 o ) “High”  setting

Nonlinearity Tests Born approximation  linear ε dependence, TPE could cause a deviation E and NE11 show quadratic terms consistent with zero Project P 2 within ±0.020 for E Much better limits over wide Q 2 range NE11: L. Andivahis et al, Phys. Rev. D50:5491, 1994

Conclusion Projected uncertainties from proposal More Q 2 points –Shifted range down –Better linearity tests –Slightly smaller  range Analysis underway