Hybrid Mesons

TL;DR

This review summarizes the current theoretical and experimental understanding of hybrid mesons, highlighting key experimental candidates, lattice results, and models, and discusses the evidence and challenges in confirming hybrid mesons.

Contribution

It provides a comprehensive comparison of experimental data, lattice calculations, and various theoretical models for hybrid mesons, clarifying the current status and future directions.

Findings

Evidence for a light isovector exotic meson exists.

Lattice gauge theory constrains hybrid meson models.

Confirmation of hybrid mesons requires discovery of iso-partners.

Abstract

A review of the theoretical and experimental status of hybrid hadrons is presented. The states $\pi_1(1400)$, $\pi_1(1600)$, and $\pi_1(2015)$ are thoroughly reviewed, along with experimental results from GAMS, VES, Obelix, COMPASS, KEK, CLEO, Crystal Barrel, CLAS, and BNL. Theoretical lattice results on the gluelump spectrum, adiabatic potentials, heavy and light hybrids, and transition matrix elements are discussed. These are compared with bag, string, flux tube, and constituent gluon models. Strong and electromagnetic decay models are described and compared to lattice gauge theory results. We conclude that while good evidence for the existence of a light isovector exotic meson exists, its confirmation as a hybrid meson awaits discovery of its iso-partners. We also conclude that lattice gauge theory rules out a number of hybrid models and provides a reference to judge the success of others.