Abstract The target normal single-spin asymmetry in electron nucleon scattering is studied in the framework of the 1/Nc expansion of QCD, which allows for a rigorous description in the energy range that includes the Δ resonance and below the second baryon resonance region. The asymmetry is driven by the absorptive part of the two-photon exchange component of the scattering amplitude, being therefore the most unambiguous two-photon exchange effect. Such amplitude is shown to be described up to the next to leading order in the 1/Nc expansion only in terms of the charge and magnetic form factors of the nucleons, a consequence of the approximate SU(4) spin flavor symmetry valid in the large Nc limit for baryons. A discussion is provided of the 1/Nc expansion framework along with the results for the asymmetries in elastic (e−N↑→e−N), inelastic (e−N↑→e−Δ), and inclusive scattering.
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Precision Measurement of the Beam-Normal Single-Spin Asymmetry in Forward-Angle Elastic Electron-Proton Scattering
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 θlab=7.9° and a mean energy of 1.149 GeV. The asymmetry result is Bn=−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 (θlab→0) where they should be most reliable.
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- PAR ID:
- 10276390
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- Physical review letters
- Volume:
- 125
- Issue:
- 11
- ISSN:
- 1079-7114
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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