Partonic calculation of the two-photon exchange contribution to elastic   electron-proton scattering at large momentum transfer
  We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. We relate the process on the nucleon to the generalized parton distributions which also enter in other wide angle scattering processes. We find that when taking the polarization transfer determinations of the form factors as input, adding in the 2 photon correction, does reproduce the Rosenbluth data. 
Y.C. Chen, A. Afanasev, S.J. Brodsky, C.E. Carlson, and M.   Vanderhaeghen