Glueballs, a fulfilled promise of QCD?

TL;DR

This paper reviews the history and current evidence for glueballs in QCD, highlighting recent coupled-channel analysis results and decay data suggesting glueball production.

Contribution

It provides a comprehensive review of glueball searches, introduces new coupled-channel analysis evidence, and discusses decay data indicating glueball presence.

Findings

Evidence for scalar glueball from coupled-channel analysis

Hints of tensor glueball in experimental data

Scalar decay data suggest glueball production

Abstract

This is a contribution to the review "50 Years of Quantum Chromdynamics" edited by F. Gross and E. Klempt [arXiv:2212.11107], to be published in EPJC. The contribution remembers the early searches and explains how to find a glueball, based on its properties. The results of a coupled-channel analysis are presented that provides evidence for the scalar glueball and first hints for the tensor glueball. Data on radiative decays of $\psi(2S)$ and $\Upsilon(1S)$ show scalar intensity that is likely due to glueball production.