Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA.
Department of Computational Mathematics, Michigan State University, East Lansing, Michigan 48824, USA.
Phys Rev Lett. 2018 Dec 14;121(24):242003. doi: 10.1103/PhysRevLett.121.242003.
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 quasidistributions at proton momenta P_{z}∈{2.2,2.6,3.0} GeV on the lattice and match them systematically to the physical parton distribution using the large-momentum effective theory. 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, Δuover ¯>Δdover ¯.
