Transverse Beam Spin Asymmetries at Backward Angles in Elastic Electron-Proton and Quasi-elastic Electron-Deuteron Scattering

TL;DR

This study measures transverse beam spin asymmetries in elastic electron-proton and quasi-elastic electron-deuteron scattering at backward angles, providing data that supports theoretical models involving two-photon exchange processes.

Contribution

First measurement of beam-normal single-spin asymmetries in quasi-elastic electron-deuteron scattering at backward angles, expanding understanding of two-photon exchange effects.

Findings

Results for proton asymmetries agree with models including inelastic piN states.

Deuteron asymmetry estimates align with theoretical predictions.

Data supports the role of two-photon exchange in scattering asymmetries.

Abstract

We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasi-elastic scattering on the deuteron, at backward angles (lab scattering angle of 108 degrees) for Q2 = 0.22 GeV^2/c^2 and 0.63 GeV^2/c^2 at beam energies of 362 MeV and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (piN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasi-static deuterium approximation, and is also in agreement with theory.