A comparison of elastic electron-proton and positron-protonscattering has been made at eight different values of 4-momentumtransfer squared, ranging from 0.20 to 5.0 (GeV/c)2. The object ofthe comparison is to determine whether the ratio (R) of the radiativelycorrected positron-proton cross section to the electron-proton crosssection differs measurably from 1.
An R ≠ 1 result can be interpreted as being due to the two-photonexchange contribution, since the interference between the first orderscattering amplitude and the two-photon exchange amplitude occurs withopposite signs for electrons and positrons.
The data were obtained using positron and electron beams from theStanford Linear Accelerator at two energies. Comparisons were made at4 GeV for scattering angles of 12.5, 20.0, 27.5, and 35.0 degrees, andat 10 GeV for 2.6, 5.0, 12.5, and 15.0 degrees. The incident beam waspassed through a liquid hydrogen target and the scattered electrons detectedin either the SLAC 8-GeV/c or 20-GeV/c magnetic spectrometers.
The accuracy to which R was determined ranged from ±1.6% for themeasurement at 4 GeV, 12.5° to ±10.8% at 4 GeV, 35°. The measurements,after radiative corrections, are consistent with R=1 within one standarderror. Limits for the size of the two-photon amplitude and theconsequences of these limits on theoretical two-photon estimates, aregiven.
