Scattering of High-Energy Positrons from Protons

Positron and electron beams from the Stanford Mark III linear accelerator have been used to investigate the importance of two-photon exchanges in electron scattering from the proton. The scattering cross sections for positrons and electrons have been compared at 200 and 300 MeV for $q^2$ (the square of the four-momentum transfer) from 0.3 to 5.2 ${\mathrm{F}}^{-2\mathrm{}}$. The results show no evidence of anomalous two-photon effects and verify the correctness of the first Born approximation form factor analysis to within a few percent or better for this range of $q^2$. The variation of the cross sections with $q^2$ has also been measured with high accuracy.