Exploring Skewed Parton Distributions with Polarized Targets

I brie y review the physics of skewed parton distributions. Special emphasis is put on the relevance of target polarization, and on the di erent roles of small and of intermediate xB. 1 The physics of skewed parton distributions In recent years much progress has been made in the theory of skewed parton distributions (SPDs). Unifying the concepts of parton distributions and of hadronic form factors, they contain a wealth of information about how quarks and gluons make up hadrons. Advances in experimental technology raise hope to study the exclusive processes where these functions appear. While the usual parton distributions are matrix elements of quark or gluon operators for a given hadron state p, SPDs are obtained from the same operators sandwiched between two hadron states p and p with di erent momenta, corresponding to the nite momentum transfer the hadron undergoes in an exclusive process. A good example for this is deeply virtual Compton scattering (DVCS). This is the process p ! p (measured in electroproduction ep ! ep ) in the kinematical regime where the photon virtuality Q = q and the energy squared W 2 = (p+ q) are large, while the invariant momentum transfer t = (p p) to the proton is small. If Q is large enough, the transition amplitude factorizes [1] into a perturbatively calculable subprocess at the level of quarks and gluons and an SPD, which encodes the nonperturbative dynamics relating the quarks or gluons with the proton states (Fig. 1a). Work supported by Department of Energy contract DE{AC03{76SF00515 and by the Feodor Lynen Program of the Alexander von Humboldt Foundation. Based on talks given at the 2nd eRHIC Workhop, Yale, April 2000, and at the eRHIC Summer Meeting, Brookhaven, July 2000. Contributed to the eRHIC White Paper.