Deeply Virtual Exclusive Processes and Generalized Parton Distributions

TL;DR

This paper discusses the experimental efforts at Jefferson Laboratory to image the internal structure of protons, neutrons, and nuclei using Generalized Parton Distributions (GPDs) through various Deeply Virtual Exclusive Scattering processes, supported by recent measurements and future upgrades.

Contribution

It presents new experimental data on Deeply Virtual Compton Scattering and vector meson production, highlighting the need for improved models and the importance of the 12 GeV upgrade for flavor-sensitive measurements.

Findings

Support for QCD factorization at Q^2 > 2 GeV^2

Sensitivity to quark angular momentum sum rule

Qualitative agreement with existing GPD models

Abstract

The goal of the comprehensive program in Deeply Virtual Exclusive Scattering at Jefferson Laboratory 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 -> ep \gamma} in Hall A support the QCD factorization of the scattering amplitude for Q^2 > 2 GeV^2. Quasi-free neutron-DVCS measurements on the Deuteron indicate sensitivity to the quark angular momentum sum rule. Fully exclusive H(e,e' p\gamma) 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 JLab 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 these channels. The {\rho} and {\omega} channels reactions offer the prospect of flavor sensitivity to the quark GPDs, while the {\phi}-production channel is dominated by the gluon distribution.