Gluon Spin, Canonical Momentum, and Gauge Symmetry

TL;DR

This paper examines the gauge invariance issues of gluon spin and orbital angular momentum in QCD, emphasizing the importance of gauge choices, physical observability, and the interpretation of these quantities in bound states and experiments.

Contribution

It clarifies the gauge symmetry constraints on gluon spin and orbital angular momentum, and discusses their physical relevance and measurability in QCD and optics.

Findings

Gluon helicity is gauge-invariant, but gluon spin and orbital angular momentum are not.

Gauge choices like the light-cone gauge influence the interpretation of gluon spin.

The paper discusses the experimental prospects of measuring gluon contributions to proton spin.

Abstract

It is well known that in gauge theories, the spin (and orbital angular momentum) of gauge particles is not gauge invariant, although the helicity is; neither are the canonical momentum and canonical angular momentum of charged particles. However, the simple appeal of these concepts has motivated repeated attempts to resurrect them as physical descriptions of gauge systems. In particular, measurability of the gluon-spin-contribution to the proton helicity in polarized proton scattering has generated many theoretical efforts in generalizing it and others as gauge-invariant quantities. In this work, we analyze the constraints of gauge symmetry, the significance of gluon spin in the light-cone gauge, and what is possible and natural in QCD parton physics, emphasizing experimental observability and physical interpretation in the structure of bound states. We also comment on the measurability of the orbital angular momentum of the Laguerre-Gaussian laser modes in optics.