Polarization Transfer in Wide-Angle Compton Scattering and Single-Pion Photoproduction from the Proton

TL;DR

This study measures polarization transfer in wide-angle Compton scattering and single-pion photoproduction from the proton, revealing discrepancies with theoretical predictions and suggesting unknown contributions to the scattering process.

Contribution

It provides the first polarization transfer measurement at 3.7 GeV and 70° scattering angle, challenging existing GPD-based models with significantly larger observed values.

Findings

Longitudinal transfer KLL measured at 0.645

Observed value is about three times larger than predictions

Results imply significant unknown contributions to the scattering amplitude

Abstract

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 \cma$= 70^\circ$. The longitudinal transfer \KLL, measured to be $0.645 \pm 0.059 \pm 0.048$, 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 $\sim$3~times larger than predicted by the generalized-parton-distribution-based calculations, which indicates a significant unknown contribution to the scattering amplitude.