JRA5-GPD-ACT: Generalized Parton Distributions

The concept of Generalized Parton Distributions (GPDs) allows to study the structure of nucleons in terms of quarksand gluons (partons) at a previously unexpected level. GPDs give information, for instance, on the correlation between the transverse spatial distributions and longitudinal momentum distributions of the partons, thus providing a threedimensional mapping of the nucleon. They are related also to the orbital angular momentum contribution of partons to the nucleon's spin. It is the objective of this WP to access GPDs experimentally through hard exclusive reactions such as the lepto-production of a photon (“Deep Virtual Compton Scattering”, DVCS) or of a meson (“Deep Virtual Meson Production”, DVMP) or in photo-production of a lepton pair (“Timelike Compton Scattering”, TCS). These exclusive reactions have been measured in certain kinematical regions and one particular objective is the analysis of the numerous data already collected these past years at COMPASS and TJNAF and not published. Thus, one of our objectives is the analysis of DVCS with a recoil detector and DVMP at COMPASS, and of TCS and DVMP at TJNAF@6 GeV. Other objectives are the preparation, data taking and analysis of new experiments for TJNAF@12GeV (DVCS on the neutron and on light nuclei, TCS, Double DVCS, 2-hadron (pi-pi, pi-rho,etc…) or 2-photon production), as well as producing projections for experiments to propose for EIC. The final objective includes building models of GPDs (standard twist-2, but also twist-3 and transversity GPDs), using also results on moments of GPDs to be obtained by lattice QCD calculations. Besides, improved studies, including higher order and higher twist corrections, are planned of several processes listed above, providing complementary access to GPDs. Both experimental and theoretical efforts will be combined in extraction of GPD information by fits to the data.