Skewed Parton Distributions in Real and Virtual Compton Scattering

The handbag contribution to Compton scattering factorises in parton-photon subpro-cess amplitudes and new form factors representing 1/x-moments of skewed parton distributions. A detailed phenomenological study for polarised and unpolarised real and virtual Compton scattering is presented. 1. The interest in the interplay between hard inclusive and exclusive reactions has recently been revived by theoretical work on deeply virtual Compton scattering (DVCS) and skewed parton distributions (SPDs) [1]. These SPDs are hybrid objects, which combine properties of form factors and of ordinary parton distributions. In fact, reduction formulas reveal the close connection of these quantities. It has also been shown recently [2, 3] that at moderately large momentum transfer real and virtual Compton scattering off protons approximately factorises into a hard parton-photon subprocess and a soft proton matrix element described by new form factors specific to Compton scattering. These new form factors, as the ordinary electromagnetic ones, represent moments of SPDs and can be modelled by overlaps of light-cone wave functions [2, 3], which provide the link between exclusive and inclusive reactions. In this overlap representation , which implies Feynman's end-point mechanism, the SPDs are given as products of ordinary parton distributions and exponentials of t (1 − x)/x, provided one makes a simplifying assumption about the wave functions. Here t is the squared momentum transfer experienced by 1