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We report on a new measurement of the beam transverse single spin asymmetryin electron-proton elastic scattering, $A^{ep}_{\perp}$, at five beam energiesfrom 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 inelasticcontributions 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 accountelastic and $\mathrm{\pi N}$ inelastic intermediate states, suggests that thereare other inelastic intermediate states such as $\mathrm{\pi \pi N}$,$\mathrm{K \Lambda}$ and $\mathrm{\eta N}$. Covering a wide energy range, ournew high-precision data provide a benchmark to study those intermediate states.

physical review letters

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