Generalized Parton Distributions (GPDs)


A model-dependent extraction of up- and down-quark contributions (orbital angular momentum plus spin) to the spin of the proton (Hall A E03-106).

Example of DVCS asymmetry data obtained in the e1-DVCS experiment.  Left: kinematic coverage and binning in the (xBQ2) space

An important goal of Jefferson Lab is to provide a detailed, three-dimensional picture of the nucleon in terms of its quark and gluon constituents, and to understand how this complex structure leads to its well known properties such as mass, spin and magnetic moment. A promising theoretical framework for this task is provided by generalized parton distributions (GPDs), which are hybrids of the usual form factors and parton distributions, but in addition include correlations between states of different longitudinal and transverse momenta.

The simplest processes that allow extraction of GPDs from data and provide the cornerstone of their exploration are deeply virtual Compton scattering (DVCS) and deeply virtual meson production (DVMP).  In these processes, the scattering takes place from a single quark in the nucleon, producing a highly energetic photon or meson in the final state.

alt text

DVCS cross section results for one of twelve kinematics bins measured in Hall A E00-110. See C. Munoz Camacho et al. for a full description.

The first comprehensive study of DVCS was performed in the e1-DVCS experiment in Hall B, the first part of which ran in 2005.  In this experiment the electron beam was alternately polarized parallel and anti-parallel to the beam direction, enabling measurements of beam spin asymmetries (BSA) in more than 600 kinematic intervals.

In Hall A, accurate measurements of the helicity-dependent DVCS cross sections e p → e p γ and e n →e n γ have been obtained in two dedicated experiments (E00-110: DVCS on the proton and E03-106: DVCS on the deuteron). The Q2 dependence of the proton cross section indicates scattering on individual quarks, which allows the DVCS data to be interpreted in terms of GPDs. The feasibility of accessing experimentally the quark orbital angular momentum, which may constitute the missing link in the nucleon spin puzzle, was also demonstrated in E03-106.

R. DeMasi et al., Phys. Rev. C 77 (2008) 042201
F-X. Girod et al, Phys. Rev. Lett. 100 (2008) 162002
C. Munoz Camacho et al., Phys. Rev. Lett. 97 (2006) 262002
M. Mazouz et al., Phys. Rev. Lett. 99 (2007) 242501
M. Guidal, M.V. Polyakov, A.V. Radyushkin, and M. Vanderhaeghen, Phys. Rev. D 72 (2005) 054013