Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

Abstract

We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, $A_{\perp }^{ep}$, at five beam energies from 315.1 MeV to 1508.4 MeV and at a scattering angle of $30^{\circ }<\theta <40^{\circ }$. The covered $Q^2$ values are 0.032, 0.057, 0.082, 0.218, 0.613 (GeV/c)${}^2$. The measurement clearly indicates significant inelastic contributions to the two-photon-exchange (TPE) amplitude in the low-$Q^2$ kinematic region. No theoretical calculation is able to reproduce our result. Comparison with a calculation based on unitarity, which only takes into account elastic and $\pi \mathrm{N}$ inelastic intermediate states, suggests that there are other inelastic intermediate states such as $\pi \pi \mathrm{N}$, $\mathrm{K\Lambda }$ and $\eta \mathrm{N}$. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.

arXiv comment6 pages, 3 figures

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Cited by in corpus (7)

• The proton charge radius
• The Present and Future of QCD
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• Beam-normal single-spin asymmetry in elastic scattering of electrons from a spin-0 nucleus
• Precision Measurement of the Beam-Normal Single-Spin Asymmetry in Forward-Angle Elastic Electron-Proton Scattering
• Measurement of the Beam-Normal Single-Spin Asymmetry for Elastic Electron Scattering from ${}^{12}$C and ${}^{27}$Al
• Exact Relations for Two-Photon-Exchange Effect in Elastic $ep$ Scattering by Dispersion Relation and Hadronic Model