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

G. I. Gakh, E. Tomasi--Gustafsson

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 o…