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

Le résumé fourni par la source

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 ${\ensuremath{\theta}}_{\mathrm{lab}}=7.9\ifmmode^\circ\else\textdegree\fi{}$ and a mean energy of 1.149 GeV. The asymmetry result is ${B}_{n}=\ensuremath{-}5.194\ifmmode\pm\else\textpm\fi{}0.067(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.082$ (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 (${\ensuremath{\theta}}_{\mathrm{lab}}\ensuremath{\rightarrow}0$) where they should be most reliable.

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