Two-photon-exchange and γZ-Exchange corrections to parity-violating elastic electron-proton scattering

Leading electroweak corrections play an important role in precision measurements of the strange form factors. We calculate the two-photon-exchange (TPE) and gamma Z-exchange corrections to the parity-violating asymmetry of the elastic electron-proton scattering in a simple hadronic model including the finite size of the proton. We find both can reach a few percent and are comparable in size with the current experimental measurements of strange-quark effects in the proton neutral weak current. The effect of gamma Z exchange is in general larger than that of TPE, especially at low momentum transfer Q(2)<= 1 GeV(2). Their combined effects on the values of G(E)(s)+G(M)(s) extracted in recent experiments can be as large as -40% in certain kinematics.