Heavy quarkonia: the beauty and the beasts

TL;DR

This paper reviews recent experimental enhancements in charmonium and bottomonium spectra, focusing on the controversial nature of many states and the specific case of $ ext{X}(3872)$ as a quasibound state with threshold effects.

Contribution

It provides a concise overview of new spectral enhancements and discusses the interpretation challenges of various charmonium-like and bottomonium-like states, emphasizing threshold effects.

Findings

$ ext{X}(3872)$ likely a quasibound state with large $ar{D}^{ ext{star}0}D^0$ admixture

Several states are non-resonant threshold enhancements rather than genuine resonances

The interpretation of many charmonium-like and bottomonium-like states remains controversial

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 $\Upsilon(10580)$.