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

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 θ=55÷75° and θ=15÷25°) and 1 GeV (θ=65÷105°). We briefly describe the experimental method, paying special attention to the radiative corrections. Some preliminary results are presented.