We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, A_⊥ᵉᵖ, at five beam energies from 315.1 to 1508.4 MeV and at a scattering angle of 30^∘<θ<40^∘. The covered Q² 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-Q² 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 πN inelastic intermediate states, suggests that there are other inelastic intermediate states such as ππN, KΛ, and ηN. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.
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Gou et al. (2020) studied this question.
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