| Using a general parametrization of the spin structure of the matrix element
for the elastic $e^-+^4\\!He$ 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 $^4\\!He$, in the light of this result. |
