Precision Measurement of the Beam-Normal Single-Spin Asymmetry in Forward-Angle Elastic Electron-Proton Scattering

TL;DR

This paper presents a highly precise measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering, testing two-photon exchange models at small scattering angles.

Contribution

It provides the most accurate measurement to date of the asymmetry, offering critical data for validating two-photon exchange theories in forward-angle scattering.

Findings

Asymmetry measured as -5.194 ppm with 2.7% total uncertainty.

Results serve as a stringent test for two-photon exchange models.

Measurement at 7.9 degrees and 1.149 GeV energy.

Abstract

A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. We report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of theta_lab = 7.9 degrees and a mean energy of 1.149 GeV. The asymmetry result is B_n = -5.194 +- 0.067 (stat) +- 0.082 (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles (theta_lab -> 0) where they should be most reliable.