A Measurement of Two-Photon Exchange in Unpolarized Elastic Electron-Proton Scattering

1. A Measurement of Two-Photon Exchange in Unpolarized Elastic Electron-Proton Scattering James Johnson Northwestern University & Argonne National Lab For the Rosen07 Collaboration

2. 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

3. 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 Prior Experiments

4. 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 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

5. Precision RosenbluthJLab 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

6. Magnitude of the Discrepancy Solid line – fit to E01-001 ‘Super-Rosenbluth’ Dashed line – taken from polarization transfer ratio

7. 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

8. Rosenbluth 2007JLab 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

9. Rosenbluth 2007 102 Kinematics points Q2 0.40-5.76 GeV2 13 points at Q2=0.983 10 points at Q2=2.284

10. 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

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

12. 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

13. 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

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

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

16. Unpeeling • Data/SIMC resolution mismatch • xptar dependence • Non-gaussian tails in SIMC • Background • ‘Dummy’ runs for endcap subtraction • Simulated pi-0 photoproduction

17. 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 NE11: L. Andivahis et al, Phys. Rev. D50:5491, 1994

18. Conclusion • Projected Uncertainties • More Q2 points • Shifted range down • Better separations at each Q2 • Analysis underway