Physics of Gluon Helicity Contribution to Proton Spin

TL;DR

This paper links the gluon helicity contribution to proton spin with a gauge-invariant, frame-dependent quantity in QCD, enabling non-perturbative lattice calculations and validating light-cone gauge approaches.

Contribution

It establishes a gauge-invariant, frame-dependent definition of gluon helicity in QCD and connects it to measurable quantities in high-energy scattering.

Findings

Gluon helicity is the large momentum limit of a gauge-invariant gluon spin.

Provides a framework for non-perturbative lattice QCD calculations.

Justifies the use of light-cone gauge for physical observables.

Abstract

The total gluon helicity in a polarized proton, measurable in high-energy scattering, is shown to be the large momentum limit of a gauge-invariant but non-local, frame-dependent gluon spin $\vec{E}\times \vec{A}_\perp$ in QCD. This opens a door for a non-perturbative calculation of this quantity in lattice QCD, and also justifies using free-field expressions in the light-cone gauge as physical observables.