Charge asymmetry for electron (positron)-proton elastic scattering
  Charge asymmetry in electron (positron) scattering arises from the interference of the Born amplitude and the box-type amplitude corresponding to two virtual photons exchange. It can be extracted from electron proton and positron proton scattering experiments, in the same kinematical conditions. Considering the virtual photon Compton scattering tensor, which contributes to the box-type amplitude, we separate proton and inelastic contributions in the intermediate state and parametrize the proton form-factors as the sum of a pure QED term and a strong interaction term. Arguments, based on analyticity, are given in favor of cancellation of contributions from proton strong interaction form factors and of inelastic intermediate states in the box type amplitudes. In frame of this model, and assuming a dipole character of form-factors, numerical estimations are given for moderately high energies. 
E. A. Kuraev, V. V. Bytev