Recalculation of Proton Compton Scattering in Perturbative QCD

Abstract

At very high energy and wide angles, Compton scattering on the proton (gamma- p --> gamma-p) is described by perturbative QCD. The perturbative QCD calculation has been performed several times previously, at leading twist and at leading order in alpha{sub s}, with mutually inconsistent results, even when the same light-cone distribution amplitudes have been employed. The authors have recalculated the helicity amplitudes for this process, using contour deformations to evaluate the singular integrals over the light-cone momentum fractions. They do not obtain complete agreement with any previous result. Their results are closest to those of the most recent previous computation, differing significantly for just one of the three independent helicity amplitudes, and only for backward scattering angles. They present results for the unpolarized cross section, and for three different polarization asymmetries. They compare the perturbative QCD predictions for these observables with those of the handbag and diquark models. In order to reduce uncertainties associated with alpha{sub s} and the three-quark wave function normalization, they have normalized the Compton cross section using the proton elastic form factor. The theoretical predictions for this ratio are about an order of magnitude below existing experimental data.