A Direct Measurement of Hard Two-Photon Exchange with Electrons and Positrons at CLAS12

One of the most surprising discoveries made at Jefferson Lab is the discrepancy in determinations of the proton's form factor ratio μ_pG^p_E/G^p_M between unpolarized and polarized measurements.  The leading hypothesis for this discrepancy is a non-negligible contribution from hard two-photon exchange (TPE).  Theoretical calculations of TPE are highly model-dependent, and existing experimental data are not able to conclusively confirm or refute this hypothesis.  A better understanding of multi-photon exchange over a wide phase space is needed.  We propose a comprehensive measurement of TPE at JLab using the CLAS12 detector.  Specifically, the measurement would use the proposed positron injector upgrade at JLab to measure the ratio of positron-proton to electron-proton elastic scattering cross sections over a wide range of momentum transfer and scattering angle.  This measurement would be a definitive test of whether or not hard TPE is the root cause of the proton form factor discrepancy, and provide critical benchmarks for theoretical calculations of TPE.  This talk will summarize the technique, kinematic coverage, and impact of the proposed measurement.