Canonical and kinetic decompositions of the proton spin

TL;DR

This paper reviews the current debates on how the proton's spin is decomposed into its constituent parts, highlighting issues of gauge invariance, uniqueness, and measurability, and concludes some controversies are undecidable.

Contribution

It provides a concise overview of the proton spin decomposition debates, clarifying which aspects are fundamentally undecidable.

Findings

Highlights unresolved issues in gauge invariance and measurability

Identifies controversies that are fundamentally undecidable

Summarizes current perspectives on proton spin decomposition

Abstract

We propose a short summary of the present situation concerning the proton spin decomposition. We briefly discuss some of the main controversies about the issues of gauge invariance, uniqueness and measurability. As a conclusion, we argue that part of the controversies is actually undecidable.