Too many $X^{\prime }s$, $Y^{\prime }s$ and $Z^{\prime }s$?

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 $X^{\prime }s$, $Y^{\prime }s$, and $Z^{\prime }s$. 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.

Comments: 8 pages, 5 figures