Proton Spin Structure from Lattice QCD

TL;DR

This paper reports a lattice QCD calculation of the proton's spin structure, including both connected and disconnected contributions, providing results that align reasonably well with experimental data.

Contribution

First lattice QCD calculation of the proton's flavor singlet axial-vector current including disconnected contributions using a wall source method.

Findings

Calculated total quark spin contribution: +0.18(10)

Disconnected contributions to Δu and Δd are approximately -0.119(44)

Results are consistent with experimental measurements

Abstract

A lattice QCD calculation of the proton matrix element of the flavor singlet axial-vector current is reported. Both the connected and disconnected contributions are calculated, for the latter employing the variant method of wall source without gauge fixing. From simulations in quenched QCD with the Wilson quark action on a $16^3\times 20$ lattice at $\beta=5.7$ (the lattice spacing $a\approx 0.14$fm), we find $\Delta\Sigma=\Delta u+\Delta d+\Delta s=+0.638(54)-0.347(46)-0.109(30) = +0.18(10)$ with the disconnected contribution to $\Delta u$ and $\Delta d$ equal to $-0.119(44)$, which is reasonably consistent with the experiment.