Measurement of the two-photon exchange contribution in elastic $ep$ scattering at VEPP-3

TL;DR

This paper reports on a precision experiment measuring the ratio of elastic positron-proton to electron-proton scattering cross sections at VEPP-3 to quantify the two-photon exchange effect, with preliminary results indicating progress in understanding this contribution.

Contribution

The study provides new high-precision measurements of the two-photon exchange contribution in elastic ep scattering at specific energies and angles, improving upon previous data.

Findings

Initial measurements of the ratio R at different energies and angles.

Preliminary results suggest a measurable two-photon exchange effect.

Method includes detailed radiative correction analysis.

Abstract

We report on the status of the Novosibirsk experiment on a precision measurement of the ratio $R$ of the elastic $e^+ p$ and $e^- p$ scattering cross sections. Such measurements determine the two-photon exchange effect in elastic electron-proton scattering. The experiment is conducted at the VEPP-3 storage ring using a hydrogen internal gas target. The ratio $R$ is measured with a beam energy of 1.6 GeV (electron/positron scattering angles are $\theta = 55 \div 75^{\circ}$ and $\theta = 15 \div 25^{\circ}$) and 1 GeV ($\theta = 65 \div 105^{\circ}$). We briefly describe the experimental method, paying special attention to the radiative corrections. Some preliminary results are presented.