Search for evidence of two photon contribution in elastic electron   proton data
  We reanalyze the most recent data on elastic electron proton scattering. We look for a deviation from linearity of the Rosenbluth fit to the differential cross section, which would be the signature of the presence of two photon exchange. The two photon contribution is parametrized by a one parameter formula, based on symmetry arguments. The present data do not show evidence for such deviation. 
E. Tomasi-Gustafsson and G. I. Gakh