Two-photon exchange from intermediate state resonances in elastic electron-proton scattering

We use a recently developed dispersive approach to compute the two-photon exchange (TPE) correction to elastic electron-proton scattering, including contributions from hadronic J^P=1/2^± and 3/2^± resonant intermediate states below 1.8 GeV. For the transition amplitudes from the proton ground state to the resonant excited states we employ new exclusive meson electroproduction data from CLAS at Q^2 ≲ 5 GeV^2, and we explore the effects of both fixed and dynamic widths for the resonances. Among the resonant states, the N(1520) 3/2^- becomes dominant for Q^2 ≳ 2 GeV^2, with a sign opposite to the comparably sized Δ(1232) 3/2^+ contribution, leading to an overall increase in the size of the TPE correction to the cross section relative to the nucleon only contribution at higher Q^2 values. The results are in good overall agreement with recent e^+ p to e^- p cross section ratio and polarization transfer measurements, and provide compelling evidence for a resolution of the electric to magnetic form factor ratio discrepancy.