X(6200) as a compact tetraquark in the QCD string model

TL;DR

This paper proposes that the X(6200) observed near the double-J/psi threshold can be understood as a shallow bound state of a compact tetraquark within the QCD string model, combining quark and molecular dynamics.

Contribution

It demonstrates the compatibility of the QCD string model with a compact cc̄cc̄ tetraquark state near the double-J/psi threshold, integrating quark and molecular interactions.

Findings

X(6200) can be a shallow bound state of a compact tetraquark.

QCD string model supports the existence of a cc̄cc̄ tetraquark below the double-J/psi threshold.

Interplay of quark dynamics and gluon exchanges explains the observed state.

Abstract

Recently the LHCb Collaboration announced the first observation of nontrivial structures in the double-$J/\psi$ mass spectrum in the mass range 6.2-7.2 GeV, and a theoretical coupled-channel analysis of these data performed in Phys. Rev. Lett. 126, 132001 (2021) evidenced the existence of a new state $X(6200)$ close to the double-$J/\psi$ threshold. Although its molecular interpretation seems the most plausible assumption, the present data do not exclude an admixture of a compact component in its wave function, for which a fully-charmed compact tetraquark is the most natural candidate. It is argued in this work that the QCD string model is compatible with the existence of a compact $cc\bar{c}\bar{c}$ state bound by QCD forces just below the double-$J/\psi$ threshold. A nontrivial interplay of the quark dynamics associated with this compact state and the molecular dynamics provided by soft gluon exchanges between $J/\psi$ mesons is discussed and the physical $X(6200)$ is argued to be a shallow bound state, in agreement with the results of the aforementioned coupled-channel analysis of the LHCb data.