Glue spin and helicity in proton from lattice QCD

TL;DR

This paper presents the first lattice QCD calculation of the glue spin in the proton, showing its value at the physical pion mass and its mild dependence on proton momentum, contributing to understanding proton spin structure.

Contribution

It introduces a novel lattice QCD approach to compute the glue spin in the nucleon, including physical quark masses and multiple lattice spacings, with results consistent across parameters.

Findings

Glue spin in proton is approximately 0.25 in MS scheme at 10 GeV^2.

Results are insensitive to lattice spacing and quark mass variations.

Proton momentum dependence of glue spin is very mild.

Abstract

We report the first lattice QCD calculation of the glue spin in the nucleon. The lattice calculation is carried out with valence overlap fermions on 2+1 flavor DWF gauge configurations on four lattice spacings and four volumes including an ensemble with physical values for the quark masses. The glue spin $S_{G}$ in the Coulomb gauge in the $\overline{\text{MS}}$ scheme is obtained with the 1-loop perturbative matching. We find the results fairly insensitive to lattice spacing and quark masses. We also find that the proton momentum dependence of $S_G$ in the range $0\leq |\vec{p}| < 1.5$ GeV is very mild, and we determine it in the large momentum limit to be $S_{G}=0.251(47)(16)$ at the physical pion mass in the $\overline{\text{MS}}$ scheme at $\mu^2=10$ GeV$^2$. If the matching procedure in large momentum effective theory is neglected, $S_G$ is equal to the glue helicity measured in high-energy scattering experiments.