Positron-Proton Scattering

The importance of two-photon exchange in elastic electron-proton scattering was investigated by measuring the ratio of positron-proton to electron-proton scattering. Four-momentum transfers as large as 0.756 ${(\mathrm{B}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ (19.5 ${\mathrm{F}}^{-2\mathrm{}}$) were used. The data indicate that two-photon effects are (4.0±1.5)% larger than those predicted by the radiative corrections at the highest momentum transfers attained in these experiments. The two-photon corrections predicted using a static charge distribution fit the data well at lower momentum transfers and forward angles, but appear to be small at higher momentum transfers and backward angles.