Heavy quarkonia: the beauty and the beasts

Abstract

New enhancements in the charmonium and bottomonium spectra observed since 2003 are very briefly reviewed. Special attention is paid to $\chi_{c1}(3872)$ (formerly $X(3872)$) owing to its remarkable proximity to the $\bar{D}^{\star0}\!D^0$ threshold, which allows modelling as a quasibound axial-vector $c\bar{c}$ state with a large $\bar{D}^{\star0}\!D^0$ admixture. In contrast, the interpretation of many other charmonium-like and bottomonium-like states is still very controversial and some may not even correspond to genuine resonances. Accordingly, several entries in the PDG tables have been wildly changing over the years. Three representative states are reviewed here as non-resonant enhancements due to threshold effects, viz. $\psi(4260)$, $\psi(4660)$, and $\Upsilon(10580)$.