A Future Two-photon Exchange Experiment at DESY

The discrepancy between unpolarised and polarised measurements of the proton form factor ratio μ_p G^p_E / G^p_M is still not resolved. Recent experiments: VEPP-3, CLAS, and OLYMPUS; measured the two-photon exchange contribution but only at Q² < 2.3 (GeV/c)² where the discrepancy is not so evident and two-photon exchange contributions are small, ∼1%. Two-photon exchange is still considered the leading explanation for the discrepancy but experiments are needed that probe higher Q² where larger two-photon exchange contributions can be expected. A possibility exists in the near future at DESY that would reach Q² ∼ 4.6 (GeV/c)². Higher Q² measurements could also be possible. The proposed experiment and Monte Carlo simulations to investigate the event rates and background conditions are presented.