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 Pz∈{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, Δuˉ(x)>Δdˉ(x).
@article{arxiv.1807.07431,
title = {Proton Isovector Helicity Distribution on the Lattice at Physical Pion Mass},
author = {Huey-Wen Lin and Jiunn-Wei Chen and Xiangdong Ji and Luchang Jin and Ruizi Li and Yu-Sheng Liu and Yi-Bo Yang and Jian-Hui Zhang and Yong Zhao},
journal= {arXiv preprint arXiv:1807.07431},
year = {2019}
}