GPDs - JLab12

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Physics program > Nucleon Structure

Generalised (or unintegrated) parton distributions (GPDs) allow for a unified description of hadron structure as they encompass standard parton distributions and nucleon form factors as forward limits and integrals, respectively, and go beyond projections of nucleon structure by providing also information about the correlation of the different degrees of freedom. These new functions offer opportunities to study a uniquely new aspect of nucleon structure: the localisation of partons in the plane transverse to the motion of the nucleon and correlated with their longitudinal momentum. As such they provide a nucleon tomography (analogous to CT scans in medical imaging). The ability to describe longitudinal momentum distributions at a fixed transverse localisation is a prerequisite for studying the so-called Ji-relation, which links a certain combination of GPDs to the total angular momentum of a parton in the nucleon. From this quantity, the still hunted orbital angular momentum of quarks in the nucleon could possibly be extracted, a question of crucial importance for an understanding nucleon structure. Multi-dimensional parton distributions are of important relevance for the understanding of hadron and nuclear structure within QCD. For example, the knowledge about the initial spatial distribution of quarks and gluons in the nucleon is essential for the interpretation of relativistic heavy-ion collision data and the quest for the quark-gluon plasma.

GPDs are studied in hard exclusive reactions, where the nucleon stays intact and the final state is fully observed. The most prominent processes are deeply virtual Compton scattering and deeply virtual meson production. While the former can be most cleanly interpreted in terms of GPDs, the latter provides also flavour dependent information. Both processes will be studied intensively Jefferson Lab at 12 GeV.periments.

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