Two-photon exchange contribution to elastic electron–proton scattering: measurements at the VEPP-3 storage ring

A striking discrepancy between the results for the ratio of electric and magnetic form factors of the proton obtained by two approaches: the Rosenbluth extraction from the unpolarized cross section and the double-polarization measurements has led to a serious re-examination of the role played by two-photon exchange diagrams. The evaluation of such diagrams could not be done in model-independent way, which means an experimental verification is required. One of the direct ways to do this is to measure the ratio of positron-proton and electron-proton elastic scattering cross-sections for identical kinematics. Such a measurement has been performed at the VEPP-3 storage ring at Novosibirsk. There were two runs at beam energies of 1.6 and 1.0 GeV at several ranges of scattering angle. The measurements covered the transferred momentum Q(2) up to 1.5 GeV2 and the virtual photon polarization parameter is an element of down to 0.2. The VEPP-3 experimental approach is described and the results of the measurements are presented.