Normal single-spin asymmetries in electron-proton scattering: Two-photon exchange with intermediate-state resonances
Jaseer Ahmed, P. G. Blunden, Wally Melnitchouk
Abstract
We calculate the beam (${B}_{n}$) and target (${A}_{n}$) normal single-spin asymmetries in electron--proton elastic scattering from two-photon exchange amplitudes with resonance intermediate states of spin-parity $1/{2}^{\ifmmode\pm\else\textpm\fi{}}$ and $3/{2}^{\ifmmode\pm\else\textpm\fi{}}$ and mass $W\ensuremath{\lesssim}1.8$ GeV. The latest CLAS exclusive meson electroproduction data are used as input for the transition amplitudes from the proton to the excited resonance states. For ${B}_{n}$, the spin 3/2 resonances dominate by an order of magnitude over the spin 1/2 states. In general we observe cancellations between the negative contributions of the $\mathrm{\ensuremath{\Delta}}(1232)$ and $N(1520)$ across both beam energy and scattering angle, and the positive contributions of the $\mathrm{\ensuremath{\Delta}}(1700)$ and $N(1720)$, leading to a rather large overall uncertainty band in the total ${B}_{n}$. At forward angles and beam energies ${E}_{\text{lab}}<1$ GeV, where the $\mathrm{\ensuremath{\Delta}}(1232)$ dominates, the calculated ${B}_{n}$ tend to overshoot the A4 and SAMPLE data. The calculated ${B}_{n}$ compare well with the measured values from the A4 and ${Q}_{\text{weak}}$ experiments with ${E}_{\text{lab}}>1$ GeV.