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 $30^{\circ }<\hat{I}¸<40^{\circ }$. 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 $\ddot{\mathrm{I}}€\mathrm{N}$ inelastic intermediate states, suggests that there are other inelastic intermediate states such as $\ddot{\mathrm{I}}€\ddot{\mathrm{I}}€\mathrm{N}$, $\mathrm{K}\hat{\mathrm{I}}›$ and $\hat{\mathrm{I}}\cdot \mathrm{N}$. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.

6 pages, 3 figures