Parton distributions and lattice QCD calculations: a community white paper

TL;DR

This white paper reviews recent progress in lattice QCD and global analysis methods for determining parton distribution functions, aiming to unify approaches and reduce uncertainties in understanding hadron structure.

Contribution

It provides an overview of lattice QCD and global analysis techniques for PDFs, benchmarks current calculations, and promotes collaboration between communities.

Findings

Benchmark numbers for lattice-QCD calculations of PDFs.

Potential to reduce uncertainties in global PDF analyses.

Framework for dialogue between lattice QCD and phenomenology communities.

Abstract

In the framework of quantum chromodynamics (QCD), parton distribution functions (PDFs) quantify how the momentum and spin of a hadron are divided among its quark and gluon constituents. Two main approaches exist to determine PDFs. The first approach, based on QCD factorization theorems, realizes a QCD analysis of a suitable set of hard-scattering measurements, often using a variety of hadronic observables. The second approach, based on first-principle operator definitions of PDFs, uses lattice QCD to compute directly some PDF-related quantities, such as their moments. Motivated by recent progress in both approaches, in this document we present an overview of lattice-QCD and global-analysis techniques used to determine unpolarized and polarized proton PDFs and their moments. We provide benchmark numbers to validate present and future lattice-QCD calculations and we illustrate how they could be used to reduce the PDF uncertainties in current unpolarized and polarized global analyses. This document represents a first step towards establishing a common language between the two communities, to foster dialogue and to further improve our knowledge of PDFs.