Two-Photon-Exchange andγZ-Exchange Corrections to Parity-Violating Elastic Electron-Proton Scattering

Leading electroweak corrections play an important role in precisionmeasurements of the strange form factors. We calculate the two-photon-exchange(TPE) and $\gamma Z$-exchange corrections to the parity-violating asymmetry ofthe elastic electron-proton scattering in a simple hadronic model including thefinite size of the proton. We find both can reach a few percent and arecomparable in size with the current experimental measurements of strange-quarkeffects in the proton neutral weak current. The effect of $\gamma Z$-exchangeis in general larger than that of TPE, especially at low momentum transfer$Q^2\le 1 GeV^2$. Their combined effects on the values of $G^s_E+\beta G^s_M$extracted in recent experiments can be as large as -40% in certain kinematics.Comment: 4 pages, 3 Fig