Deeply Virtual Exclusive Processes and Generalized Parton Distributions

The goal of the comprehensive program in deeply virtual exclusive scattering at Jefferson Lab is to create transverse spatial images of quarks and gluons as a function of their longitudinal momentum fraction in the proton, the neutron, and in nuclei. These functions are the generalized parton distributions (GPDs) of the target nucleus. Cross section measurements of the deeply virtual Compton scattering (DVCS) reaction ep → epγ in Hall A support the QCD factorization of the scattering amplitude for Q² >= 2 GeV². Quasi-free neutron-DVCS measurements on the deuteron indicate sensitivity to the quark angular momentum sum rule. Fully exclusive H(e,e'pγ) measurements have been made in a wide kinematic range in CLAS with polarized beam, and with both unpolarized and longitudinally polarized targets. Existing models are qualitatively consistent with the Jefferson Lab data, but there is a clear need for less constrained models. Deeply virtual vector meson production is studied in CLAS. The 12 GeV upgrade will be essential for for these channels. The ρ and ω channels reactions offer the prospect of flavor sensitivity to the quark GPDs, while the ϕ-production channel is dominated by the gluon distribution.