GPDs, form factors and Compton scattering

The basic theoretical ideas of the handbag factorization and its application to wide-angle scattering reactions are reviewed. With regard to the present experimental program carried out at JLab, wide-angle Compton scattering is discussed in some detail.Comment: 11 pages, 5 figures (using LATEX with ws-procs9x6.cls), talk presented at the workshop on Exclusive Processes, Jefferson Lab, Newport News, May 200

The gluon contents of the eta and eta' mesons

It is reported on a leading-twist analysis of the eta - photon and eta' - photon transition form factors. The analysis allows for an estimate of the lowest Gegenbauer coefficients of the quark and gluon distribution amplitudes.Comment: 5 pages, 5 figures (using LATEX with aipproc), talk presented at the X. International Conference on Hadron Spectroscopy Aschaffenburg, September 200

A modern view of wide-angle exclusive scattering

The basic theoretical ideas of the handbag mechanism for wide-angle exclusive scattering reactions are discussed and, with regard to the present experimental program carried out at JLab, its application to Compton scattering is reviewed in some detail. Results for other wide-angle reactions such as two-photon annihilations into pairs of hadrons or virtual Compton scattering are presented as well.Comment: 10 pages, 6 figures (using LATEX with ws-procs9x6.cls), talk presented at the Nstar2002 workshop on the physics of excited nucleons, Pittsburgh, October 200

Deeply virtual vector meson electroproduction at small Bjorken-x

It is reported on an analysis of vector meson electroproduction at small Bjorken-x within the handbag approach. Using a model for the generalized parton distribution and calculating the partonic subprocess, electroproduction off gluons, within the modified perturbative approach, cross sections and spin density matrix elements are evaluated. The numerical results agree fairly well with recent HERA data.Comment: 4 pages, 4 figures, contribution to DIS05, Madison, US

The GPD H-tilde and spin correlations in wide-angle Compton scattering

Wide-angle Compton scattering (WACS) is discussed within the handbag approach in which the amplitudes are given by products of hard subprocess amplitudes and form factors, specific to Compton scattering, which represent 1/x-moments of generalized parton distributions (GPDs). The quality of our present knowledge of these form factors and of the underlying GPDs is examined. As will be discussed in some detail the form factor R_A and the underlying GPD H-tilde are poorly known. It is argued that future data on the spin correlations A_{LL} and/or K_{LL} will allow for an extraction of R_A which can be used to constrain the large -t behavior of H-tilde.Comment: 27 pages, 10 figure

The transition of virtual photons into pseudoscalar mesons

The possibility to constrain the meson distribution amplitude from the transitions of virtual photons to pseudoscalar mesons is investigated. It is shown that for a large range in the two photon virtualities the transition form factors are essentially independent of the distribution amplitudes. This in turn entails parameter-free predictions of QCD.Comment: 4 pages, 1 figure (using LATEX with ws-p8-50x6-00.cls), talk presented at the International Conference on the Structure and Interactions of the Photon (PHOTON2001), Ascona, September 200

Spin and hard processes

It is argued that spin is a fundamental aspect of gauge theories at short distances. As a consequence there are characteristic helicity asymmetries in hard inclusive and exclusive reactions of which a few are discussed.Comment: 8 pages, 5 figures (using LATEX with ws-procs9x6.cls), talk presented at the 2nd international symposium on the Gerasimov-Drell-Hearn sum rule and the spin structure of the nucleon, Genova, July 200