Ethan Cline

Two-Photon Exchange Experiment

Abstract:

The two-photon exchange (TPE) contribution in elastic electron-proton scattering has been of significant interest to the nuclear physics community as a possible explanation for the proton form factor ratio discrepancy. Three experiments (VEPP-3, CLAS12, and OLYMPUS) have reported on direct measurements of TPE in recent years and show effects in agreement with phenomenological predictions, but in disagreement with theory, up to Q2 = 2.5 (GeV/c)2. At larger Q2, for which other theoretical approaches, like those based on GPDs, become feasible the effect remains untested. To measure the “hard” two-photon contribution, one needs access to both e+p and e−p scattering interactions at several Q2 and ε points in a kinematic region that only DESY can access. The proposed Two-Photon Exchange eXperiment (TPEX) will measure hard twophoton contribution to ep scattering in the momentum transfer range up to Q2 = 4.7 (GeV/c)2. The experiment uses electromagnetic calorimeter blocks at fixed angles to detect scattered electrons. In this talk I will provide an overview of the proton form factor ratio discrepancy and introduce TPEX.