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
nucl-ex
/ Authors
B. Gou, J. Arvieux, K. Aulenbacher, D. Balaguer Ríos, S. Baunack, D. Becker, L. Capozza, W. Deconinck, J. Diefenbach, R. Frascaria
and 19 more authors
M. Gorchtein, B. Gläser, D. von Harrach, Y. Imai, E. M. Kabuß, R. Kothe, S. Kowalski, R. Kunne, F. E. Maas, H. Merkel, M. C. Mora Espí, M. Morlet, U. Müller, S. Ong, E. Schilling, C. Weinrich, J. van de Wiele, M. Zambrana, I. Zimmermann
/ Abstract
We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, $A^{ep}_{\perp}$, at five beam energies from 315.1 MeV to 1508.4 MeV and at a scattering angle of $30^{\circ} < θ< 40^{\circ}$. 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 $\mathrm{πN}$ inelastic intermediate states, suggests that there are other inelastic intermediate states such as $\mathrm{ππN}$, $\mathrm{K Λ}$ and $\mathrm{ηN}$. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.