Beam normal spin asymmetry for the ep →e Δ (1232) process

We calculate the single spin asymmetry for the ep→eΔ(1232) 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 a two-photon exchange process is on its mass shell, the asymmetry allows one to access for the first time the on-shell Δ→Δ as well as N∗→Δ electromagnetic transitions. We present the general formalism to describe the ep→eΔ beam normal spin asymmetry, and we provide a numerical estimate of its value using the nucleon, Δ(1232), S11(1535), and D13(1520) intermediate states. We compare our results with the first data from the Qweak@JLab experiment and give predictions for the A4@MAMI experiment.