General analysis of two--photon exchange in elastic electron--${}^{4}He$ scattering and $e^{+}+e^{-}\to {\pi }^{+}+{\pi }^{-}.$

Abstract

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.

Comments: 15 pages 1 figure