The pole counting rule and XYZ states

TL;DR

The paper discusses the pole counting rule as a model-independent tool to distinguish between confining states and hadronic molecules, applying it to various XYZ particles and suggesting most are molecular states except for some like X(3872).

Contribution

It applies the pole counting rule to analyze XYZ states, providing new insights into their nature and proposing most are hadronic molecules.

Findings

X(6900) could be a confining or molecular state due to limited data.

X_1(2900) is likely a $ar{D}_1 K$ molecule with $J^P=1^-$.

Most XYZ particles with exotic quantum numbers are interpreted as hadronic molecules.

Abstract

The pole counting rule is a powerful and model-independent method to distinguish a confining state from a hadronic molecule. It has been applied to the explorations of $X(6900)$, $X_1(2900)$ as well as $Z_c(3900)$, $X(3872)$, $X(4660)$, etc.. For $X(6900)$, both a confining state and a molecular state are not excluded, because lacking of enough data. For $X_1(2900)$, the analysis shows that it should be a $\bar{D}_1 K$ molecule, with $J^P=1^-$ and an iso-singlet interpretation is much more favorable. Finally, it is noted that almost all XYZ particles with exotic quantum numbers can be interpreted as hadronic molecules. The $X(3872)$ is, however, more like a charmonium.