Polarization effects for the reaction $e^++e^-\to \bar p+p $ in presence of two--photon exchange - Lecture Notes

The aim of this work is to give a pedagogical derivation of polarization observables for the annihilation reaction $e^+ + e^- \to N+ \bar N$. The reaction mechanism is one photon + two photon exchange, the last is described by axial parametrization. After deriving the general expressions for the cross section of a binary process, the matrix element is written in terms of three complex amplitudes. The method to derive polarization observables is detailed and all expressions are given in terms of generalized form factors. The strategy for determining physical form factors in annihilation reactions in presence of two photon exchange is suggested, on the basis of model independent properties of the relevant observables.