Deeply Virtual Compton Scattering
Xiangdong Ji
TL;DR
This work shows that deeply-virtual Compton scattering (DVCS) in perturbative QCD probes off-forward parton distributions (OFPDs), connecting their moments to nucleon form factors and spin content. It derives the leading-log evolution of OFPDs, yielding generalized Altarelli-Parisi equations that interpolate between standard AP evolution and Brodsky–Lepage-type behavior, including quark–gluon mixing in singlet sectors. The paper then analyzes DVCS cross sections and their interference with the Bethe–Heitler process across unpolarized and polarized observables, providing estimates for CEBAF and HERMES kinematics and highlighting the role of BH in small-$\Delta^2$ regimes. Overall, it argues for a factorization framework and demonstrates that DVCS offers a route to measure quark orbital angular momentum and the full spin decomposition of the nucleon via OFPDs, while noting experimental and theoretical challenges that require further study.
Abstract
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.