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

Area of Science:

• Experimental nuclear physics
• Quantum electrodynamics
• Particle physics

Background:

• The beam-normal single-spin asymmetry is sensitive to the imaginary part of two-photon exchange.
• Precise measurements are needed to test theoretical models at forward scattering angles.

Purpose of the Study:

• To perform a high-precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering.
• To test theoretical predictions for two-photon exchange processes.

Main Methods:

• Scattering of transversely polarized electrons from unpolarized protons.
• Measurement of the beam-normal single-spin asymmetry at a mean scattering angle of 7.9° and 1.149 GeV.
• Achieved 2% precision in the measurement.

Main Results:

• The measured asymmetry is B_n = -5.194 ± 0.067 (stat) ± 0.082 (syst) ppm.
• This represents the most precise measurement to date.
• The result provides a stringent test for two-photon exchange models.

Conclusions:

• The precise measurement validates the sensitivity of beam-normal single-spin asymmetry to two-photon exchange.
• The findings support the reliability of theoretical models at far-forward scattering angles.
• This work advances the understanding of fundamental interactions in electron-proton scattering.