Impressions of Parton Distribution Functions

TL;DR

This paper reviews recent advances in understanding parton distribution functions (DFs) through continuum and lattice QCD analyses, highlighting their consistency and implications for hadron structure, including pion DFs and proton spin.

Contribution

It presents new insights into the consistency of continuum and lattice methods for calculating DFs and discusses novel perspectives on gluon and quark orbital angular momentum.

Findings

Continuum and lattice analyses yield consistent pion DFs.

Understanding of proton mass and spin distributions varies with scale.

New perspectives on gluon and quark orbital angular momentum.

Abstract

Parton distribution functions (DFs) have long been recognised as key measures of hadron structure. Today, theoretical prediction of such DFs is becoming possible using diverse methods for continuum and lattice analyses of strong interaction (QCD) matrix elements. Recent developments include a demonstration that continuum and lattice analyses yield mutually consistent results for all pion DFs, with behaviour on the far valence domain of light-front momentum fraction that matches QCD expectations. Theory is also delivering an understanding of the distributions of proton mass and spin amongst its constituents, which varies, of course, with the resolving scale of the measuring probe. Aspects of the pion DF and proton spin developments are sketched herein, along with some novel perspectives on gluon and quark orbital angular momentum.