Disconnected contributions to the spin of the nucleon

TL;DR

This paper introduces a Feynman-Hellmann method to compute disconnected quark spin contributions to the proton, addressing computational challenges and providing new insights into the proton's spin structure.

Contribution

It demonstrates a novel application of the Feynman-Hellmann approach to calculate disconnected contributions in lattice QCD, which are traditionally difficult to evaluate.

Findings

Disconnected quark spin contribution is approximately -5% at 470 MeV pion mass.

The Feynman-Hellmann method effectively measures small energy shifts in lattice QCD.

The approach offers a promising way to include gluonic and strange-quark contributions.

Abstract

The spin decomposition of the proton is a long-standing topic of much interest in hadronic physics. Lattice QCD has had much success in calculating the connected contributions to the quark spin. However, complete calculations, which necessarily involve gluonic and strange-quark contributions, still present some challenges. These "disconnected" contributions typically involve small signals hidden against large statistical backgrounds and rely on computationally intensive stochastic techniques. In this work we demonstrate how a Feynman-Hellmann approach may be used to calculate such quantities, by measuring shifts in the proton energy arising from artificial modifications to the QCD action. We find a statistically significant non-zero result for the disconnected quark spin contribution to the proton of about -5% at a pion mass of 470 MeV.