Wide Angle Compton Scattering.pdf

a r X i v : h e p - p h / 0 0 1 0 1 5 8 v 1 1 6 O c t 2 0 0 0 Wide Angle Compton Scattering Rainer Jakob Fachbereich Physik, Universita?t Wuppertal, D-42097 Wuppertal, Germany Abstract. We present the handbag contribution to Wide Angle Compton Scattering (WACS) at moderately large momentum transfer obtained with a proton distribution amplitude close to the asymptotic form. In comparison it is found to be significantly larger than results from the hard scattering (pQCD) approach. INTRODUCTION Compton scattering off nucleons provides us with valuable information on the nucleon structure. In general, the Compton process involves the relativistic propa- gation of an excited, composite system, a bound state, which is extremely difficult to describe. Fortunately, in special kinematic situations the description of the pro- cess becomes much simpler. In this contribution we focus on Compton scattering off protons with large momentum transfer and small (or zero) photon virtuality, i.e. Compton scattering at wide angles (WACS). Here the process receives its main contribution from the lowest Fock state of the nucleon, i.e. a three quark config- uration. The propagation of hard gluons and quarks is described perturbatively. We argue that in the region of large, but not yet asymptotically large, momen- tum transfer the process is dominated by the handbag contribution involving new Compton form factors defined from skewed parton distributions. We compare the handbag contribution calculated from the overlap of soft wave fuctions [1,2] with the results obtained in the hard scattering approach [3]. HARD VS. SOFT SCATTERING MECHANISM It is generally accepted that the hard scattering mechanism in the context of per- turbative QCD [4] provides the correct description for processes at asymptotically large momentum transfer. In this picture a hard scattering amplitude describes the redistribution of the transfered momentum among partons via hard gluon ex- change, and the non-perturb