Two-photon exchange and polarization physics in electron-proton   scattering
  We discuss effects of two-photon exchange (TPE) in various observables of the elastic ep-scattering. The imaginary part of the TPE amplitude manifests in target and beam normal spin asymmetries. The real part contributes to the cross-section. A model-independent phenomenological analysis of experimental data is performed. Its results are compared with theoretical calculations. 
A.P.Kobushkin and D.L.Borisyuk