Lattice Calculation of Parton Distribution Function from LaMET at Physical Pion Mass with Large Nucleon Momentum

TL;DR

This paper reports a lattice-QCD calculation of the unpolarized isovector parton distribution function at physical pion mass using LaMET, with improved statistics, higher nucleon momentum, and refined systematic controls, aligning well with phenomenological PDFs.

Contribution

The study advances lattice-QCD PDF calculations by increasing nucleon momentum, improving systematic controls, and applying a new operator to reduce mixing, providing more precise results at physical pion mass.

Findings

Final PDF agrees with phenomenological analysis

Enhanced control over excited-state contamination

Higher nucleon momentum improves accuracy

Abstract

We present a lattice-QCD calculation of the unpolarized isovector parton distribution function (PDF) using ensembles at the physical pion mass with large proton boost momenta $P_z \in \{2.2,2.6,3.0\}$~GeV within the framework of large-momentum effective theory (LaMET). In contrast to our previous physical-pion PDF result, we increase the statistics significantly, double the boost momentum, increase the investment in excited-state contamination systematics, and switch to $\gamma_t$ operator to avoid mixing with scalar matrix elements. We use four source-sink separations in our analysis to control the systematics associated with excited-state contamination. The one-loop LaMET matching corresponding to the new operator is calculated and applied to our lattice data. We detail the systematics that affect PDF calculations, providing guidelines to improve the precision of future lattice PDF calculations. We find our final parton distribution to be in reasonable agreement with the PDF provided by the latest phenomenological analysis.