Beam normal spin asymmetry in elastic lepton-nucleon scattering
  We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions. 
M. Gorchtein, P.A.M. Guichon, M. Vanderhaeghen