Physical Review Letters
American Physical Society
Journal Article URL
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.
Qweak Collaboration; Armstrong, David; Deconinck, Wouter; Bartlett, K.; Cornejo, J. C.; Carlini, R. D.; Dowd, James F.; Finn, J. M.; Gray, Valerie M.; Grimm, K.; Hoskins, J. R.; Leckey, J. P.; Lee, Jeong Han; Magee, Joshua A.; and Owen, Victoria F., Precision Measurement of the Beam-Normal Single-Spin Asymmetry in Forward-Angle Elastic Electron-Proton Scattering (2020). Physical Review Letters, 125(11), 112502.