Spin and orbital angular momentum of the proton

TL;DR

This paper reviews the progress in understanding the spin and orbital angular momentum distribution within the proton, emphasizing the role of non-perturbative structures and recent experimental and lattice QCD results.

Contribution

It provides a comprehensive review of current experimental, theoretical, and lattice QCD insights into the proton's spin structure, highlighting the importance of non-perturbative effects.

Findings

Strong limits on polarized glue in the proton

More precise experimental determination of spin content

Relativistic, chirally symmetric models explain data

Abstract

Since the announcement of the proton spin crisis by the European Muon Collaboration there has been considerable progress in unravelling the distribution of spin and orbital angular momentum within the proton. We review the current status of the problem, showing that not only have strong upper limits have been placed on the amount of polarized glue in the proton but that the experimental determination of the spin content has become much more precise. It is now clear that the origin of the discrepancy between experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturbative structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon structure naturally explain the current data. The consequences of this explanation for the presence of orbital angular momentum on quarks and gluons is reviewed and comparison made with recent results from lattice QCD and experimental data.