Octet Spin Fractions and the Proton Spin Problem

TL;DR

This paper investigates the distribution of spin among quarks in the proton and other baryons using lattice QCD, aiming to understand whether the small quark spin fraction in the proton is a general feature or specific to its structure.

Contribution

It introduces a novel lattice QCD approach to compare quark spin fractions across the baryon octet, providing insights into the proton spin problem beyond traditional methods.

Findings

Quark spin fractions in other baryons show similar suppression as in the proton.

Results align well with a model including confinement, relativity, gluon exchange, and meson cloud effects.

The approach offers a new perspective on the universality of the proton spin puzzle.

Abstract

The relatively small fraction of the spin of the proton carried by its quarks presents a major challenge to our understanding of the strong interaction. Traditional efforts to explore this problem have involved new and imaginative experiments and QCD based studies of the nucleon. We propose a new approach to the problem which exploits recent advances in lattice QCD. In particular, we extract values for the spin carried by the quarks in other members of the baryon octet in order to see whether the suppression observed for the proton is a general property or depends significantly on the baryon structure. We compare these results with the values for the spin fractions calculated within a model that includes the effects of confinement, relativity, gluon exchange currents and the meson cloud required by chiral symmetry, finding a very satisfactory level of agreement given the precision currently attainable.