Too many Xʼs, Yʼs and Zʼs?

AbstractA 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 Xʼs, Yʼs, and Zʼs. 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