THE TRANSITION TO PERTURBATIVE QCD IN COMPTON-SCATTERING

We investigate the transition to perturbative QCD in Compton processes , by concentrating on the specific reaction pi gamma --> pi gamma at m oderate energy scales. New sum rules for each of the helicities involv ed in the scattering and for the cross section ate given, together wit h a detailed stability analysis. Our results are then compared with th ose derived from higher-power factorization, with a Sudakov suppressio n for quark elastic scattering. An overlap of the predictions obtained by the two approaches is observed at momentum transfer as low as 4 Ge V2 and at a photon scattering angle around 40 degrees. Our work shows that factorization theorems and sum rule methods complement each other in the description of Compton scattering at moderate energy scales, a nd give information on the transition to perturbative QCD in these pro cesses, The sum rule formalism is further applied to the crossed react ion gamma gamma --> pi(+)pi(-), and the predictions are in good agreem ent with the experimental data.