Extraction of two-photon contributions to the proton form factors
  Significant discrepancies have been observed between proton form factors as measured by Rosenbluth separation and polarization transfer techniques. There are indications that this difference may be caused by corrections to the one photon exchange approximation that are not taken into account in standard radiative correction procedures. In this paper, we constrain the two-photon amplitudes by combining data from Rosenbluth, polarization transfer, and positron-proton scattering measurements. This allows a rough extraction of these two-photon effects in elastic electron-proton scattering, and provides an improved extraction of the proton electromagnetic form factors. 
J. Arrington