Recalculation of proton Compton scattering in perturbative QCD - art. no. 114021

At very high energy and wide angles, Compton scattering on the proton (gamm ap-gammap) is described by perturbative QCD. The perturbative QCD calculati on has been performed several times previously, at leading twist and at rea ding order in alpha (s), with mutually inconsistent results, even when the same light-cone distribution amplitudes have been employed. We have recalcu lated the helicity amplitudes for this process, using contour deformations to evaluate the singular integrals over the light-cone momentum fractions. We do not obtain complete agreement with any previous result. Our results a re closest to those of the most recent previous computation, differing sign ificantly for just one of the three independent helicity amplitudes, and on ly for backward scattering angles. We present results for the unpolarized c ross section, and for three different polarization asymmetries. We compare the perturbative QCD predictions for these observables with those of the ha ndbag and diquark models. In order to reduce uncertainties associated with alpha (s) and the three-quark wave function normalization, we have normaliz ed the Compton cross section using the proton elastic form factor. The theo retical predictions for this ratio are about an order of magnitude below ex isting experimental data.