General analysis of two-photon exchange in elastic electron–4He scattering and image

Using a general parametrization of the spin structure of the matrix element for the elastic image scattering and for the annihilation image reactions in terms of two complex amplitudes, we derive general properties of the observables in presence of two-photon exchange. We show that this mechanism induces a specific dependence of the differential cross section on the angle of the emitted particle. We re-analyze the existing experimental data on the differential cross section, for elastic electron scattering on image, in the light of this result.