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

TL;DR

This paper develops a general framework to analyze two-photon exchange effects in elastic electron-helium-4 scattering and electron-positron to pion pair production, revealing their impact on observable angular dependencies.

Contribution

It introduces a general parametrization of the spin structure for these processes, highlighting the effects of two-photon exchange on observables and reanalyzing existing experimental data.

Findings

Two-photon exchange induces specific angular dependence in differential cross sections.

The framework applies to both elastic scattering and annihilation processes.

Reanalysis of experimental data supports the significance of two-photon effects.

Abstract

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.