Understanding the proton's spin structure

TL;DR

This paper reviews the significant progress made over twenty years in understanding how the proton's spin is distributed among its quarks and gluons, highlighting discoveries about polarized gluons and quark orbital angular momentum.

Contribution

It provides a comprehensive overview of advancements in proton spin structure research, emphasizing new insights into gluon polarization and quark orbital angular momentum.

Findings

Increased understanding of gluon polarization contributions.

Evidence for quark orbital angular momentum.

Resolution of the proton spin crisis.

Abstract

We discuss the tremendous progress that has been towards an understanding of how the spin of the proton is distributed on its quark and gluon constituents. This is a problem that began in earnest twenty years ago with the discovery of the proton ``spin crisis'' by the European Muon Collaboration. The discoveries prompted by that original work have given us unprecedented insight into the amount of spin carried by polarized gluons and the orbital angular momentum of the quarks.