General analysis of two-photon exchange in elastic electron–4He scattering and e++e−→π++π−

Using a general parametrization of the spin structure of the matrix element for the elastic scattering and for the annihilation e++e−→π++π− 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 , in the light of this result.