Beam normal spin asymmetry in elastic lepton–nucleon scattering Original Research Article

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 single spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal single spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions.