A Measurement of Two-Photon Exchange in Unpolarized Elastic

Electron-Proton Scattering

James Johnson

Northwestern University & Argonne National Lab

For the Rosen07 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

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

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

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 discrepancyI. A. Qattan et. al, Phys. Rev. Lett. 94:142301, 2005

Magnitude of the Discrepancy

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

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

Rosenbluth 2007

102 Kinematics points

Q2 0.40-5.76 GeV2

13 points at Q2=0.983

10 points at Q2=2.284

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– 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– 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– xptar dependence– Non-gaussian tails in

SIMC

• Background– ‘Dummy’ runs for

endcap subtraction– Simulated pi-0

photoproduction

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

Conclusion

• Projected Uncertainties• More Q2 points

– Shifted range down

– Better separations at each Q2

• Analysis underway