Nucleon Spin in QCD: Old Crisis and New Resolution

TL;DR

This paper addresses the gauge invariance issue in decomposing nucleon spin in QCD, proposing two expressions and advocating for the one where only binding gluons and quarks contribute, resolving a long-standing problem.

Contribution

It introduces two gauge-invariant expressions for nucleon spin using Abelian decomposition and argues for the physically correct one based on binding gluons and quarks.

Findings

Two gauge-invariant expressions for nucleon spin derived

The second expression, involving only binding gluons and quarks, is proposed as correct

Addresses and resolves the gauge invariance problem in nucleon spin decomposition

Abstract

We discuss the shortfalls of existing resolutions of the long-standing gauge invariance problem of the canonical decomposition of the nucleon spin to the spin and angular momentum of quarks and gluons. We provide two logically flawless expressions of nucleon spin which have different physical meanings, using the gauge independent Abelian decomposition. The first one is based on the assumption that all gluons (binding and valence gluons) contribute to the nucleon spin, but the second one is based on the assumption that only the binding gluons (and the quarks) contribute to it. We propose the second expression to be the physically correct one.