Too many $X's$, $Y's$ and $Z's$?

TL;DR

This paper examines how threshold effects influence the interpretation of new charmonium states called $X's$, $Y's$, and $Z's$, questioning the applicability of traditional quark models above open-flavor thresholds.

Contribution

It provides a comparative analysis of threshold impacts across different flavor sectors and discusses the constraints on meson-meson molecules within quark models.

Findings

Threshold effects significantly influence the observed spectrum.

The validity of quark models above open-flavor thresholds is challenged.

Channel restrictions limit meson-meson molecular interpretations.

Abstract

A large number of new states have been reported during the last few years in charmonium spectroscopy above the charmed meson production threshold. They have been called $X's$, $Y's$, and $Z's$. We reflect on the influence of thresholds on heavy meson spectroscopy comparing different flavor sectors and quantum numbers. The validity of a quark-model picture above open-flavor thresholds would severely restrict the number of channels that may lodge meson-meson molecules.