Non-singlet quark helicity PDFs of the nucleon from pseudo-distributions

TL;DR

This paper presents a lattice QCD approach to determine non-singlet helicity quark PDFs of the nucleon using pseudo-distributions, addressing systematic errors and achieving results consistent with global analyses.

Contribution

It introduces a novel lattice QCD method combining pseudo-distributions and spatial smearing to extract helicity PDFs with systematic error control.

Findings

Agreement with global analyses of helicity PDFs

Small non-singlet anti-quark helicity PDF across momentum fractions

Effective modeling of systematic uncertainties

Abstract

The non-singlet helicity quark parton distribution functions (PDFs) of the nucleon are determined from lattice QCD, by jointly leveraging pseudo-distributions and the distillation spatial smearing paradigm. A Lorentz decomposition of appropriately isolated space-like matrix elements reveals pseudo-distributions that contain information on the leading-twist helicity PDFs, as well as an invariant amplitude that induces an additional $z^2$ contamination of the leading-twist signal. An analysis of the short-distance behavior of the space-like matrix elements using matching coefficients computed to next-to-leading order (NLO) exposes the desired PDF up to this additional $z^2$ contamination. Due to the non-conservation of the axial current, we elect to isolate the helicity PDFs normalized by the nucleon axial charge at the same scale $\mu^2$. The leading-twist helicity PDFs as well as several sources of systematic error, including higher-twist effects, discretization errors, and the aforementioned $z^2$ contaminating amplitude are jointly determined by characterizing the computed pseudo-distribution in a basis of Jacobi polynomials. The Akaike Information Criterion is exploited to effectively average over distinct model parameterizations and cuts on the pseudo-distribution. Encouraging agreement is observed with recent global analyses of each non-singlet quark helicity PDF, notably a rather small non-singlet anti-quark helicity PDF for all quark momentum fractions.