Proton Isovector Helicity Distribution on the Lattice at Physical Pion Mass

TL;DR

This paper reports a high-precision lattice-QCD calculation of the proton's isovector helicity distribution at physical pion mass, matching lattice results to experimental data and revealing flavor asymmetry in polarized antiquarks.

Contribution

It introduces a systematic approach using large-momentum effective theory to extract the helicity distribution from lattice QCD at physical pion mass with unprecedented accuracy.

Findings

Distribution agrees with experimental analyses

Finds $ riangle ar{u}(x) > riangle ar{d}(x)$ in the proton

Achieves high precision through extensive simulations

Abstract

We present a state-of-the-art calculation of the isovector quark helicity Bjorken-$x$ distribution in the proton using lattice-QCD ensembles at the physical pion mass. We compute quasi-distributions at proton momenta $P_z \in \{2.2, 2.6, 3.0\}$~GeV on the lattice, and match them systematically to the physical parton distribution using large-momentum effective theory (LaMET). We reach an unprecedented precision through high statistics in simulations, large-momentum proton matrix elements, and control of excited-state contamination. The resulting distribution with combined statistical and systematic errors is in agreement with the latest phenomenological analysis of the spin-dependent experimental data; in particular, $\Delta \bar{u}(x)>\Delta \bar{d}(x)$.