Heavy quarkonia and new hadrons with two heavy quarks

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.