Some comments on the handbag approach to wide-angle exclusive scattering.pdf

a r X i v : h e p - p h / 0 4 1 0 1 4 7 v 1 1 1 O c t 2 0 0 4 adp header will be provided by the publisher Some comments on the handbag approach to wide-angle exclu- sive scattering P. Kroll ?1 1 Fachbereich Physik, Universita?t Wuppertal, D-42097 Wuppertal, Germany PACS 12.38Bx,13.40Gp,13.60Fz The handbag mechanism for wide-angle exclusive scattering reactions is discussed and compared to other theoretical approaches. The role of power laws in observables is critically examined. Applications of the handbag mechanism to Compton scattering and meson photoproduction are presented. The soft physics input to these processes are specific form factors which represent 1/x moments of generalized parton dis- tributions at zero skewness. A recent analysis of the nucleon form factors provides these GPDs and, hence, the new form factors. 1 Introduction Recently a new approach to wide-angle Compton scattering off protons has been proposed [1, 2] where, for Mandelstam variables s,?t,?u that are large as compared to the square of a typical hadronic scale Λ (being of the order of 1 GeV2), the process amplitudes factorize into a hard parton-level subprocess, Compton scattering off quarks, and in soft form factors which represent 1/x moments of generalized parton distributions (GPDs) and encode the soft physics (see Fig. 1). Subsequently it has been realized that this so-called handbag mechanism also applies to a number of other wide-angle reactions such as meson photo- and electroproduction [3] or two-photon annihilations into pairs of mesons [4] or baryons [4, 5]. It should be noted that the handbag mechanism bears resemblance to the treatment of inelastic Compton scattering advocated for by Bjorken and Paschos [6] long time ago. There are competing mechanisms which also contribute to wide-angle scattering besides the handbag which is characterized by one active parton, i.e. one parton from each hadron participates in the hard subprocess (e.g. γq → γq in Compton scattering) wh