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

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/E05-017 Collaboration. Outline.

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

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  1. 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/E05-017 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. “Super-Rosenbluth”JLab E01-001 • 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

  6. 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

  7. Magnitude of the Discrepancy Solid line – fit to E01-001 ‘Super-Rosenbluth’ Dashed line – taken from polarization transfer ratio  Quantify difference, look for nonlinearity

  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. 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

  11. 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

  12. 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

  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 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 e-independent

  15. Unpeeling • Hydrogen elastics • Compare to simulated elastics • Background • ‘Dummy’ runs for endcap subtraction • Simulated p0 photoproduction

  16. Unpeeling • Hydrogen elastics • Compare to simulated elastics • Background • ‘Dummy’ runs for endcap subtraction • Simulated pi-0 photoproduction “High” e setting Low e setting (qp = 12.5o)

  17. Nonlinearity Tests • Born approximation  linear ε dependence, TPE could cause a deviation • E01-001 and NE11 show quadratic terms consistent with zero • Project P2 within ±0.020 for E05-017 • Much better limits over wide Q2 range NE11: L. Andivahis et al, Phys. Rev. D50:5491, 1994

  18. Conclusion • Projected uncertainties from proposal • More Q2 points • Shifted range down • Better linearity tests • Slightly smaller e range • Analysis underway

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