Measurement of the two-photon exchange contribution to the elastic   $e^{\pm}p$ scattering cross sections at the VEPP-3 storage ring
  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. 
I. A. Rachek, J. Arrington, V. F. Dmitriev, V. V. Gauzshtein, R. E.   Gerasimov, A. V. Gramolin, R. J. Holt, V. V. Kaminskiy...
nucl-ex hep-ex hep-ph