Measurement of the two-photon exchange contribution to the elastic $e^{\pm}p$ scattering cross sections at the VEPP-3 storage ring

TL;DR

This study precisely measured the ratio of positron-proton to electron-proton elastic scattering cross sections at VEPP-3 to quantify the two-photon exchange contribution, addressing discrepancies in proton form factor measurements.

Contribution

First direct experimental determination of the two-photon exchange effect in elastic lepton-proton scattering at VEPP-3, providing crucial data to resolve existing measurement discrepancies.

Findings

Evidence of a significant two-photon exchange effect

Results align with some theoretical predictions

Data helps clarify proton form factor measurement discrepancies

Abstract

The ratio of the elastic $e^+ p$ to $e^- p$ scattering cross sections has been measured precisely, allowing the determination of the two-photon exchange contribution to these processes. This neglected contribution is believed to be the cause of the discrepancy between the Rosenbluth and polarization transfer methods of measuring the proton electromagnetic form factors. The experiment was performed at the VEPP-3 storage ring at beam energies of 1.6 and 1.0 GeV and at lepton scattering angles between $15^\circ$ and $105^\circ$. The data obtained show evidence of a significant two-photon exchange effect. The results are compared with several theoretical predictions.