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

Understand

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.