Helicity-Flip Off-Foward Parton Distributions of the Nucleon

TL;DR

The paper studies twist-two helicity-flip off-forward parton distributions in the nucleon, identifying four independent distributions (two quark, two gluon) and deriving their leading-logarithmic evolution, which keeps quark and gluon sectors decoupled. It provides a perturbative calculation of the photon helicity-flip Compton amplitude in terms of gluon operators and develops a DVCS cross section analysis that reveals angular signatures of gluon helicity-flip distributions. The work thereby offers a clean gluon probe of nucleon structure in off-forward kinematics and connects operator-level descriptions to measurable observables such as angular distributions in DVCS.

Abstract

We identify quark and gluon helicity-flip distributions defined between nucleon states of unequal momenta. The evolution of these distributions with change of renormalization scale is calculated in the leading-logarithmic approximation. The helicity-flip gluon distributions do not mix with any quark distribution and are thus a unique signature of gluons in the nucleon. Their contribution to the generalized virtual Compton process is obtained both in the form of a factorization theorem and an operator product expansion. In deeply virtual Compton scattering, they can be probed through distinct angular dependence of the cross section.

Figures (3)

• Figure 1: Feynman diagrams for evolution of quark helicity-flip parton distributions.
• Figure 2: Representative Feynman diagrams for evolution of gluon helicity-flip parton distributions.
• Figure 3: Diagrams for photon helicity-flip Compton scattering.