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

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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

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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
  dont agree
• Leading candidate is two-photon exchange

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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
• tGM2 eGE2

• Polarization Transfer
• Scatter longitudinally polarized electrons from
  unpolarized protons
• The ratio GE/GM is proportional to pT/pL
• Does not give form factors directly

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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. C69022201, 2004 M.
Jones et al, Phys. Rev. Lett. 841398-1402,
2000 O. Gayou et al, Phys. Rev. Lett. 88092301,
2002
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Super-RosenbluthJLab 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. 94142301,
2005
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Two-Photon Exchange

• Both methods account for radiative corrections,
  but neither considers two-photon exchange
• Difficult to Calculate
• Rough qualitative agreement
• Different e dependence
• Scale not predicted

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Magnitude of the Discrepancy
Solid line fit to E01-001 Super-Rosenbluth Das
hed line taken from polarization transfer
ratio ? Quantify difference, look for nonlinearity
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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

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Rosenbluth 2007
102 Kinematics points Q2 0.40-5.76 GeV2 13
points at Q20.983 10 points at Q22.284
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Aerogel Calibration

• Aerogel distinguishes p 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

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Time of Flight Calibration

• Acceptance cuts
• Solid full delta-ß spectrum
• Small dashes - Aerogel cut to exclude pions
• Large dashes - Beta cut to exclude deuterons

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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

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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

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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

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Unpeeling

• Hydrogen elastics
• Compare to simulated elastics
• Background
• Dummy runs for endcap subtraction
• Simulated p0 photoproduction

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Unpeeling

• Hydrogen elastics
• Compare to simulated elastics
• Background
• Dummy runs for endcap subtraction
• Simulated pi-0 photoproduction

Low e setting (qp 12.5o)
High e setting
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Nonlinearity Tests

• Born approximation ? linear e 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. D505491,
1994
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Conclusion

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

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