TPEX@DESY - A Two-Photon Exchange Experiment at DESY

The striking discrepancy in the proton form factor ratio, μ_pG^p_E/G^p_M, measured using unpolarized and polarized techniques is still not resolved. The proposed explanation is that hard two-photon exchange (TPE) is responsible. Hard TPE is difficult to calculate without significant model dependence, and has generally not been included in radiative corrections. Furthermore, three recent experiments (VEPP-3, CLAS and OLYMPUS) found only a small contribution but were limited to relatively low Q² < 2.5 (GeV/c)² where the discrepancy is not pronounced. A new test beam experiment, TPEX@DESY, would use a proposed extracted beam at the DESY test beam facility together with a liquid hydrogen target and high precision lead tungstate calorimeters to measure hard TPE at higher beam energies. This would permit measurements in a Q² range up to 4.7 (GeV/c)², where the discrepancy in the proton form factor ratio is significant and where the expected hard TPE contribution is predicted to be large. An overview of the proposed experiment as well as results from the recent tests of a 5x5 prototype lead tungstate calorimeter at the DESY test beam facility will be presented and discussed.