Speaker
Description
Two-photon exchange (TPE), a small contribution to electron scattering amplitudes, can occasionally, if not properly accounted for, have a big impact. For example, neglected TPE corrections may be the cause of the significant discrepancy between polarization-transfer and Rosenbluth-separation determinations of the proton's form factors at large momentum transfer. Though theoretical calculations of TPE corrections are necessarily model-dependent, TPE can be isolated experimentally through differences between electron scattering and positron scattering in a number of observables. The effort to quantify TPE is one of the primary motivations for adding the capability of accelerating positrons at Jefferson Lab. While the flagship measurements would compare electron-proton and positron-proton cross section ratios with the goal of resolving the proton form factor discrepancy, other measurements, such as target-normal single-spin asymmetries, polarization transfer, and nuclear-elastic scattering would provide new and independent constraints of TPE models. In this talk, I review the previous experimental and theoretical efforts and present how the future positron program at Jefferson Lab can significantly advance our understanding of TPE.
