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 to 1508.4 MeV and at a scattering angle of $30°<\theta <40°$. The covered $Q^2$ values are 0.032, 0.057, 0.082, 0.218, 0.613 $(\mathrm{GeV}/c{)}^2$. 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 $\pi N$ inelastic intermediate states, suggests that there are other inelastic intermediate states such as $\pi \pi N$, $K\mathrm{\Lambda }$, and $\eta N$. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.

中文翻译：

---

宽能量范围内电子-质子弹性散射中束流横向单自旋不对称性的双光子交换研究

我们报告了电子质子弹性散射中束横向单自旋不对称性的新测量， ${\mathrm{一种}}_{\perp }^{Ëp}$，在315.1至1508.4 MeV的五个束能量和散射角为 $30°<\theta <40°$。被盖${问}^2$ 值是0.032、0.057、0.082、0.218、0.613 $（\mathrm{GeV}/C{）}^2$。测量结果清楚地表明，在低能级中，两个光子交换（TPE）振幅具有显着的非弹性贡献。${问}^2$运动区域。没有理论计算能够重现我们的结果。与基于统一性的计算进行比较，该计算仅考虑弹性和$\pi ñ$ 非弹性中间状态，表明还有其他非弹性中间状态，例如 $\pi \pi ñ$， $ķ\mathrm{\Lambda }$和 $\eta ñ$。我们的高精度数据涵盖了广泛的能量范围，为研究这些中间状态提供了基准。