Configuration mixing in strange tetraquarks $Z_{cs}$

TL;DR

This paper explores the possibility that observed $Z_{cs}$ states are mixed eigenstates, similar to axial-vector mesons, and discusses how their structure impacts the prediction of additional states in tetraquark models.

Contribution

It proposes a mixing framework for $Z_{cs}$ states and predicts additional tetraquark states, providing a testable distinction between compact tetraquark and molecular models.

Findings

$Z_{cs}$ states may be mixed eigenstates analogous to $K_1$ mesons.

Additional tetraquark states are predicted in the compact tetraquark model.

Experimental detection of these states can differentiate between models.

Abstract

The BESIII Collaboration has observed a candidate for a $c \bar c s \bar u$ tetraquark $Z_{cs}$ at $(3982.5^{+1.8}_{-2.6} \pm 2.1)$ MeV and width $(12.8^{+5.3}_{-4.4} \pm 3.0)$ MeV, while the LHCb Collaboration has observed a $Z_{cs}$ candidate in the $\jpsi K^-$ channel with mass of $(4003 \pm 6 ^{+4}_{-14})$ MeV and width $(131 \pm 15 \pm 26)$ MeV. In this note we examine the possibility that these two states are distinct eigenstates of a mixing process similar to that which gives rise to two axial-vector mesons labeled by the Particle Data Group $K_1(1270)$ and $K_1(1400)$. The main point is that on top of a $\bar c c$ pair, the $Z_{cs}$ states have the same light quark content as the $K_1$-s. In the compact tetraquark picture this implies several additional states, analogous to members of the $K_1$ nonet. These states have not yet been observed, nor are they required in the molecular approach. Thus experimental discovery or exclusion of these extra states will be a critical test for competing models of exotic mesons with hidden charm.