Transverse Beam Spin Asymmetries in Forward-Angle Elastic Electron-Proton Scattering

TL;DR

This paper reports measurements of transverse beam spin asymmetries in elastic electron-proton scattering at specific momentum transfers, revealing discrepancies with elastic-only models and highlighting the importance of inelastic contributions for understanding two-photon exchange effects.

Contribution

First measurement of beam-normal single-spin asymmetry at these energies, showing the significance of inelastic intermediate states in theoretical models.

Findings

Results disagree with elastic-only calculations.

Data agree with models including inelastic hadronic states.

Highlights the role of inelastic contributions in two-photon exchange amplitudes.

Abstract

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely-polarized 3 GeV electrons from unpolarized protons at Q^2 = 0.15, 0.25 (GeV/c)^2. The results are inconsistent with calculations solely using the elastic nucleon intermediate state, and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2-gamma exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.