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 ${\theta }_{\mathrm{lab}}=7.9°$ and a mean energy of 1.149 GeV. The asymmetry result is $B_n=-5.194\pm 0.067(\mathrm{stat})\pm 0.082$ (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 (${\theta }_{\mathrm{lab}}\to 0$) where they should be most reliable.

• Received 23 June 2020
• Revised 7 August 2020
• Accepted 11 August 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.112502