Experimental Probes of Two-Photon Exchange

The observed discrepancy in measurements of the proton electric to magnetic form factor ratio between the Rosenbluth and recoil polarization method has been interpreted as due to previously neglected effects of two-photon exchange (TPE). Calculations involving TPE in turn lead to changes in the angular dependence of elastic cross sections and double polarization observables. Evidence for TPE can be directly and most stringently tested by comparison of positron-proton and electron-proton elastic cross sections. Further, the imaginary part of the TPE amplitude gives rise to transverse single-spin asymmetries for polarized beam or target. The experimental efforts to address TPE will be reviewed.