Deeply virtual Compton scattering

We study in QCD the physics of deeply virtual Compton scattering (DVCS)—the virtual Compton process in the large $s$ and small $t$ kinematic region. We show that DVCS can probe a new type of off-forward parton distributions. We derive an Altarelli-Parisi-type of evolution equations for these distributions. We also derive their sum rules in terms of nucleon form factors of the twist-two quark and gluon operators. In particular, we find that the second sum rule is related to fractions of the nucleon spin carried separately by quarks and gluons. We estimate the cross section for DVCS and compare it with the accompanying Bethe-Heitler process at CEBAF and HERMES kinematics.