General analysis of two--photon exchange in elastic electron--$^4He$   scattering and $e^++e^-\to \pi^++\pi^-.$
  Using a general parametrization of the spin structure of the matrix element for the elastic $e^-+^4He$ scattering and for the annihilation $e^++e^-\to \pi^++\pi^-$ 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 reanalyze the existing experimental data on the differential cross section, for elastic electron scattering on $^4He$, in the light of this result. 
G. I. Gakh and E. Tomasi--Gustafsson