The Status of Exotic-quantum-number Mesons

TL;DR

This paper reviews the experimental and theoretical status of mesons with exotic quantum numbers, discussing evidence for specific states and their possible interpretations as hybrid mesons.

Contribution

It provides a comprehensive review of the experimental evidence and theoretical understanding of exotic-quantum-number mesons, highlighting the potential identification of the $ ext{π}_1(1600)$ as a hybrid meson.

Findings

Evidence exists for three isospin one exotic mesons.

The $ ext{π}_1(1600)$ may be the lightest hybrid meson.

Further observations are needed to confirm the nonet of exotic mesons.

Abstract

The search for mesons with non-quark-antiquark (exotic) quantum numbers has gone on for nearly thirty years. There currently is experimental evidence of three isospin one states, the $\pi_{1}(1400)$, the $\pi_{1}(1600)$ and the $\pi_{1}(2015)$. For all of these states, there are questions about the identification of these state, and even if some of them exist. In this article, we will review both the theoretical work and the experimental evidence associated with these exotic quantum number states. We find that the $\pi_{1}(1600)$ could be the lightest exotic quantum number hybrid meson, but observations of other members of the nonet would be useful.