Unraveling the pattern of the $XYZ$ mesons

TL;DR

This paper proposes a standard quark-model mechanism explaining the origin of the enigmatic $XYZ$ mesons, emphasizing the role of meson-meson thresholds in charmonium and bottomonium spectra, and explores conditions for four-quark states.

Contribution

It introduces a plausible quark-model-based explanation for $XYZ$ mesons, highlighting the significance of meson-meson thresholds and four-quark configurations in heavy meson spectra.

Findings

Identifies conditions for four-quark bound states or resonances.

Highlights the importance of entangled meson-meson thresholds.

Provides a framework for understanding new heavy meson states.

Abstract

We present a plausible mechanism for the origin of the $XYZ$ mesons in the heavy meson spectra within a standard quark-model picture. We discuss the conditions required for the existence of four--quark bound states or resonances contributing to the heavy meson spectra, being either compact or molecular. We concentrate on charmonium and bottomonium spectra, where several new states, difficult to understand as simple quark-antiquark pairs, have been reported by different experimental collaborations. The pivotal role played by entangled meson-meson thresholds is emphasized.