Polarization effects in the reaction $\bar p+p\to e^++e^-$ in presence   of two-photon exchange
  Polarization observables for the reaction $\bar p+p\to e^++e^-$ are given in terms of three independent complex amplitudes, in presence of two photon exchange. General expressions for the differential cross section and the polarization observables are given and model independent properties are derived. Polarization effects depending on the polarization of the antiproton beam, the target and of the electron in the final state, have been calculated. 
G. I. Gakh and E. Tomasi--Gustafsson