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 electronproton 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.