Disconnected contributions to the spin of the nucleon

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 modifications to the QCD action. We find a statistically significant nonzero result for the disconnected quark spin contribution to the proton of about −5% at a pion mass of 470 MeV.