Abstract
We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, , at five beam energies from 315.1 to 1508.4 MeV and at a scattering angle of . The covered values are 0.032, 0.057, 0.082, 0.218, 0.613 . The measurement clearly indicates significant inelastic contributions to the two-photon-exchange (TPE) amplitude in the low- 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 inelastic intermediate states, suggests that there are other inelastic intermediate states such as , , and . Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.
- Received 14 February 2020
- Revised 28 February 2020
- Accepted 28 February 2020
DOI:https://doi.org/10.1103/PhysRevLett.124.122003
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