# Heavy quarkonia and new hadrons with two heavy quarks

## TL;DR

This paper provides a comprehensive overview of heavy quarkonia and exotic hadrons with two heavy quarks, discussing their properties, discovery history, and the challenges they pose to understanding non-perturbative QCD.

## Contribution

It offers a pedagogical review of heavy quarkonium states and exotic hadrons, emphasizing recent discoveries and the complex nature of the so-called "XYZ" states.

## Key findings

- Review of foundational discoveries like $J/\psi$ and $\Upsilon$
- Analysis of properties and spectroscopy of heavy quarkonia
- Discussion of exotic states challenging conventional models

## Abstract

We give a pedagogical introduction to heavy quarkonia -- bound states of a heavy quark and its antiquark (e.g., charmonium $c\bar{c}$, bottomonium $b\bar{b}$) -- as well as to the exotic hadrons containing two heavy quarks that have been discovered since 2003. The review covers the foundational discoveries ($J/\psi$ and $\Upsilon$), basic properties, spectroscopy interpreted via potential models, production mechanisms at colliders, and decay modes. A significant focus is placed on the so-called ``$XYZ$" states -- particles like the $X(3872)$, $Y(4260/4230)$, $Z_c(3900)$, and $P_c$ -- whose properties defy conventional quark model expectations. These states, considered candidates for hybrids, multi-quark states, hadronic molecules, or hadroquarkonia, provide unprecedented probes of non-perturbative QCD and challenge our understanding of quark confinement and hadron formation. The chapter summarizes the current experimental landscape and highlights key open questions driving future research in hadron spectroscopy.