Physical Review Letters
Wide-angle exclusive Compton scattering and single-pion photoproduction from the proton have been investigated via measurement of the polarization transfer from a circularly polarized photon beam to the recoil proton. The wide-angle Compton scattering polarization transfer was analyzed at an incident photon energy of 3.7 GeV at a proton scattering angle of theta(p)(cm) cm = 70 degrees. The longitudinal transfer K-LL, measured to be 0.645 +/- 0.059 +/- 0.048, where the first error is statistical and the second is systematic, has the same sign as predicted for the reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton. However, the observed value is similar to 3 times larger than predicted by the generalized-parton-distribution-based calculations, which indicates a significant unknown contribution to the scattering amplitude.
Fanelli, C., Cisbani, E., Hamilton, D. J., Salmé, G., Wojtsekhowski, B., Ahmidouch, A., ... & Brash, E. J. (2015). Polarization transfer in wide-angle Compton scattering and single-pion photoproduction from the proton. Physical review letters, 115(15), 152001.