Speaker
Description
We present lattice QCD calculations of the proton isovector quark helicity parton distribution function (PDF) within the Large Momentum Effective Theory (LaMET) framework, together with the first determination of the twist-3 moment $\tilde{d}_2$ from the operator product expansion of nonlocal matrix elements. The helicity PDF is calculated using both gauge-invariant (GI) and Coulomb-gauge (CG) quasi-PDF constructions. The GI quasi-PDF is computed on a HotQCD ensemble with lattice spacing $a=0.076$ fm, pion mass $m_\pi=140$ MeV, and proton momentum $P_z=1.53$ GeV, while the CG quasi-PDF is simulated on a second ensemble with $a=0.06$ fm, $m_\pi=300$ MeV, and $P_z=2.43, 3.04$ GeV. The resulting helicity PDFs agree within uncertainties for $0.25\lesssim x\lesssim0.5$. At larger $x$, the CG results fall more rapidly and are in better agreement with recent global analyses, likely because the larger $P_z$ suppresses LaMET power corrections. We also obtain $\tilde{d}_2^{u-d}(2,\mathrm{GeV})=0.0024(46)$ in the $\overline{\mathrm{MS}}$ scheme at next-to-leading order, indicating a small genuine twist-three contribution.