A Comparison of Electron-Proton and Positron-Proton Elastic Scattering at Four-Momentum Transfers up to 5.0 (GeV/c)²

Citation

Mar, Jerry (1969) A Comparison of Electron-Proton and Positron-Proton Elastic Scattering at Four-Momentum Transfers up to 5.0 (GeV/c)². Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/K4KS-FQ69. https://resolver.caltech.edu/CaltechTHESIS:03292017-144558554

Abstract

A comparison of elastic electron-proton and positron-proton scattering has been made at eight different values of 4-momentum transfer squared, ranging from 0.20 to 5.0 (GeV/c)². The object of the comparison is to determine whether the ratio (R) of the radiatively corrected positron-proton cross section to the electron-proton cross section differs measurably from 1.

An R ≠ 1 result can be interpreted as being due to the two-photon exchange contribution, since the interference between the first order scattering amplitude and the two-photon exchange amplitude occurs with opposite signs for electrons and positrons.

The data were obtained using positron and electron beams from the Stanford Linear Accelerator at two energies. Comparisons were made at 4 GeV for scattering angles of 12.5, 20.0, 27.5, and 35.0 degrees, and at 10 GeV for 2.6, 5.0, 12.5, and 15.0 degrees. The incident beam was passed through a liquid hydrogen target and the scattered electrons detected in either the SLAC 8-GeV/c or 20-GeV/c magnetic spectrometers.

The accuracy to which R was determined ranged from ±1.6% for the measurement at 4 GeV, 12.5° to ±10.8% at 4 GeV, 35°. The measurements, after radiative corrections, are consistent with R=1 within one standard error. Limits for the size of the two-photon amplitude and the consequences of these limits on theoretical two-photon estimates, are given.

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