ELEMENTARY PARTICLES AND FIELDS Theory Proton Form Factors and Two-Photon Exchange in Elastic Electron-Proton Scattering

Proton electromagnetic form factors are among the most important sources of information about the internal structure of the proton. Two different methods for measuring these form factors, the method proposed by Rosenbluth and the polarization-transfer method, yield contradictory results. It is assumed that this contradiction can be removed upon taking into account the hard part of the contribution of two-photon exchange to the cross section for elastic electron-proton scattering. This contribution can measured experimentally via a precision comparison of the cross sections for the elastic scattering of positrons and electrons on protons. Such a measurement, performed at the VEPP-3 storage ring in Novosibirsk at the beam energies of 1.6 and 1.0 GeV for positron (electron) scattering angles in the ranges of θe =1 5 ◦ −25 ◦ and 55 ◦ −75 ◦ in the first case and in the range of θe =6 5 ◦ −105 ◦ in the second case is described in the present article. Preliminary results of this experiment and their comparison with theoretical predictions are described.