The gluon structure of hadrons and nuclei from lattice QCD

Phiala Shanahan,M Della Morte,C Pena Ruano,E Gámiz Sánchez,P Fritzsch

doi:10.1051/epjconf/201817501015

Abstract

I discuss recent lattice QCD studies of the gluon structure of hadrons and light nuclei. After very briefly highlighting new determinations of the gluon contributions to the nucleon’s momentum and spin, presented by several collaborations over the last year, I describe first calculations of gluon generalised form factors. The generalised transversity gluon distributions are of particular interest since they are purely gluonic; they do not mix with quark distributions at leading twist. In light nuclei they moreover provide a clean signature of non-nucleonic gluon degrees of freedom, and I present the first evidence for such effects, based on lattice QCD calculations. The planned Electron-Ion Collider, designed to access gluon structure quantities, will have the capability to test this prediction, and measure a range of gluon observables including generalised gluon distributions and transverse momentum dependent gluon distributions, within the next decade.

Highlights

A detailed picture of the quark structure of nucleons has emerged over the last half-century, through increasingly precise measurements of quantities such as the nucleon electromagnetic form factors and quark momentum distribution functions, as well as many spin-dependent aspects of nucleon structure [1, 2]
Gluon structure quantities are much harder to measure experimentally than their quark analogues, as the gluon does not couple to electromagnetic probes and other approaches must be taken, for example using the Drell-Yan process. This lack of information has inspired the proposal and planning of an Electron-Ion Collider (EIC), which was the highest priority for new construction in the 2015 NSAC long range plan of the US nuclear physics community [8]
There is the exciting possibility for fully-controlled lattice QCD calculations of these gluon observables to serve as QCD predictions ahead of first data-taking at the EIC

Summary

Introduction

A detailed picture of the quark structure of nucleons has emerged over the last half-century, through increasingly precise measurements of quantities such as the nucleon electromagnetic form factors and quark momentum distribution functions, as well as many spin-dependent aspects of nucleon structure [1, 2]. Complementary experiments at Jefferson National Accelerator Facility [11] and at the LHC [12] aim to study various gluon distributions In this light, lattice QCD calculations of gluon structure quantities have taken on new importance, and there has been significant progress on this front over the last year [5, 13,14,15,16], expanding and building on pioneering lattice QCD studies of the unpolarised gluonic structure of the pion. On a decade-long timescale, such calculations will guide the design requirements of the EIC by revealing the magnitude of observables that are key targets of the experimental program These include the gluon generalised parton distributions (GPDs) and transverse-momentum distributions (TMDs) of the nucleon, as well as nuclear physics quantities such as gluon analogues of the EMC effect and ‘exotic glue’ observables which encode non-nucleonic gluon structure in nuclei. How is the gluon structure of a nucleon modified in a nucleus i.e., what are the magnitudes of gluon analogues of the EMC effect?

Gluon structure of the hadrons

Nucleon momentum fraction

Nucleon spin

Gluon structure of nuclei

Conclusion