Regarding the distribution of glue in the pion

TL;DR

This paper discusses the distribution functions of partons, especially glue, in the pion and compares them with the proton to understand the emergence of hadron mass, highlighting recent theoretical progress and experimental prospects.

Contribution

It provides a clearer theoretical understanding of the pion's parton distribution functions, especially the glue component, using continuum and lattice predictions, and emphasizes upcoming experimental tests.

Findings

Agreement between continuum and lattice predictions for pion DFs

Glue distribution plays a key role in the pion's structure

Experimental facilities will enable tests of these theoretical predictions

Abstract

Understanding why the scale of emergent hadron mass is obvious in the proton but hidden in the pion may rest on mapping the distribution functions (DFs) of all partons within the pion and comparing them with those in the proton; and since glue provides binding in quantum chromodynamics, the glue DF could play a special role. Producing reliable predictions for the proton's DFs is difficult because the proton is a three valence-body bound-state problem. As sketched herein, the situation for the pion, a two valence-body problem, is much better, with continuum and lattice predictions for the valence-quark and glue DFs in agreement. This beginning of theory alignment is timely because experimental facilities now either in operation or planning promise to realise the longstanding goal of providing pion targets, thereby enabling precision experimental tests of rigorous theory predictions concerning Nature's most fundamental Nambu-Goldstone bosons.