Resolution of a conflict between Laser and Elementary Particle Physics

TL;DR

This paper resolves a longstanding controversy in particle physics regarding the gauge-invariance of photon and gluon angular momentum decomposition, reconciling it with laser physics measurements of angular momentum.

Contribution

It provides a theoretical reconciliation between the gauge-invariant angular momentum decomposition in particle physics and the gauge-dependent measurements in laser physics.

Findings

Clarifies the gauge dependence of gluon spin in nucleons.

Reconciles the gauge-invariant and gauge-dependent perspectives on angular momentum.

Bridges the conceptual gap between particle physics and laser physics measurements.

Abstract

The claim some years ago, contrary to all textbooks, that the angular momentum of a photon (and gluon) can be split in a gauge-invariant way into an orbital and spin term, sparked a major controversy in the Particle Physics community. A further cause of upset was the realization that the gluon polarization in a nucleon, a supposedly physically meaningful quantity, corresponds only to the gauge-variant gluon spin derived from Noether's theorem, evaluated in a particular gauge. On the contrary, Laser Physicists have, for decades, been happily measuring physical quantities which correspond to orbital and spin angular momentum evaluated in a particular gauge. This paper reconciles the two points of view.