Hard exclusive reactions and generalized parton distributions

Abstract

The recently developed formalism of Generalized Parton Distributions (GPDs) allows connecting the experimental information of hard exclusive reactions to the spin contribution and even to the angular momentum contribution of quarks in the nucleon. By selecting different quantum numbers of the final state in exclusive productions, different GPDs can be addressed separately. The HERMES experiment at the HERA ring at DESY (Hamburg) made pioneering contributions and first constraints to Generalized Parton Distributions (GPDs), using hard exclusive vector meson production (EVMP) and Deeply Virtual Compton Scattering (DVCS). Using a novel recoil detector, HERMES managed to measure DVCS and EVMP free of any significant background. Selected results are highlighted and discussed in this paper.