Model independent study of massive lepton elastic scattering on the   proton, beyond the Born approximation
  Model independent expressions for all polarization observables in $\mu+p\to \mu+p$ elastic scattering are obtained, taking into account the lepton mass and including the two-photon exchange contribution. The spin structure of the matrix element is parametrized in terms of six independent complex amplitudes, functions of two independent kinematical variables. General statements about the influence of the two--photon--exchange terms on the differential cross section and on polarization observables are given. Polarization effects have been investigated for the case of a longitudinally polarized lepton beam and polarized nucleon in the final state. 
G. I. Gakh, M. Konchatnyi, A. Dbeyssi, E. Tomasi-Gustafsson
nucl-th hep-ph