Dispersive approach to two-photon exchange in elastic electron-proton scattering

We examine the two-photon exchange corrections to elastic electron-proton scattering within a dispersive approach, including contributions from both nucleon and Delta intermediate states. The dispersive analysis avoids off-shell uncertainties inherent in traditional approaches based on direct evaluation of loop diagrams and guarantees the correct unitary behavior in the high-energy limit. Using empirical information on the electromagnetic nucleon elastic and N Delta transition form factors, we compute the two-photon exchange corrections both algebraically and numerically. Results are compared with recent measurements of e(+) p to e(-) p cross section ratios from the CLAS, VEPP-3, and OLYMPUS experiments, as well as with polarization transfer observables.