Experimental constraints on non-linearities induced by two-photon   effects in elastic and inelastic Rosenbluth separations
  The effects of two-photon exchange corrections, suggested to explain the difference between measurements of the proton elastic electromagnetic form factors using the polarization transfer and Rosenbluth techniques, have been studied in elastic and inelastic scattering data. Such corrections could introduce epsilon-dependent non-linearities in inelastic Rosenbluth separations, where epsilon is the virtual photon polarization parameter. It is concluded that such non-linear effects are consistent with zero for elastic, resonance, and deep-inelastic scattering for all Q^2 and W^2 values measured. 
V. Tvaskis, J. Arrington, M. E. Christy, R. Ent, C. E. Keppel, Y.   Liang, G. Vittorini