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 q² (the square of the four-momentum transfer) from 0.3 to 5.2 F^-2. 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 q². The variation of the cross sections with q² has also been measured with high accuracy.The specific ionizations of 300-MeV positrons and electrons in hydrogen at one atmosphere have been compared in the course of the experiment, and found to be equal to within ±{}0.3%.