Beam normal spin asymmetry for the $e p \to e \Delta(1232)$ process

TL;DR

This paper calculates the beam normal spin asymmetry in the $e p o e \Delta(1232)$ process, providing a new way to probe electromagnetic transitions via two-photon exchange effects and comparing results with experimental data.

Contribution

It introduces a formalism for the beam normal spin asymmetry in $e p o e \Delta$ and estimates its value considering various intermediate states, linking theory with experimental measurements.

Findings

The asymmetry is sensitive to on-shell $\Delta o \Delta$ and $N^* o \Delta$ transitions.

Numerical estimates align with initial experimental data from Qweak@JLab.

Predictions are made for upcoming A4@MAMI measurements.

Abstract

We calculate the single spin asymmetry for the $e p \to e \Delta(1232)$ 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 $e p \to e \Delta$ 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.