General analysis of polarization phenomena in $ e^++e^-\to N+\bar N$ for   axial parametrization of two--photon exchange
  General properties of the differential cross section and of the polarization observables are derived for the $e^++e^-\to N+\bar N$ reaction, in presence of two--photon exchange. Polarization effects are investigated for longitudinally polarized electron and polarized antinucleon and/or polarized nucleon in the final state. The single--spin asymmetry induced by the transverse polarization of the electron beam is also discussed. 
G. I. Gakh and E. Tomasi-Gustafsson