Beam normal spin asymmetry for the process
Carl E. Carlson (William and Mary)🇺🇸 · Barbara Pasquini (University of Pavia)🇮🇹 · Vladyslav Pauk (Jefferson Laboratory)🇺🇸 · Marc Vanderhaeghen (Johannes Gutenberg University)🇩🇪
We calculate the single spin asymmetry for the process, for an electron beam polarized normal to the scattering plane. Such single spin asymmetries vanish in the one-photon exchange approximation, and are directly proportional to the absorptive part of a two-photon exchange amplitude. As the intermediate state in such two-photon exchange process is on its mass shell, the asymmetry allows one to access for the first time the on-shell as well as electromagnetic transitions. We present the general formalism to describe the beam normal spin asymmetry, and provide a numerical estimate of its value using the nucleon, , , and intermediate states. We compare our results with the first data from the Qweak@JLab experiment and give predictions for the A4@MAMI experiment.