Friday·August 18, 2017

[Submitted on 17 Aug 2017] (cross-list from hep-ph)

Beam normal spin asymmetry for the $ep\to e\Delta (1232)$ 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 $ep\to e\Delta (1232)$ 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 two-photon exchange process is on its mass shell, the asymmetry allows one to access for the first time the on-shell $\Delta \to \Delta$ as well as $N^{\ast }\to \Delta$ electromagnetic transitions. We present the general formalism to describe the $ep\to e\Delta$ beam normal spin asymmetry, and provide a numerical estimate of its value using the nucleon, $\Delta (1232)$, $S_{11}(1535)$, and $D_{13}(1520)$ intermediate states. We compare our results with the first data from the Qweak@JLab experiment and give predictions for the A4@MAMI experiment.