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

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 Dong et al. (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 cccc 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.

Automated summary

The LHCb Collaboration recently observed nontrivial structures in the double-J/psi mass spectrum in the mass range of 6.2-7.2 GeV, and identified a new state X(6200) close to the double-J/psi threshold. The QCD string model suggests the existence of a compact cccc state below the double-J/psi threshold, with an interplay between quark dynamics and molecular dynamics. The physical X(6200) is argued to be a shallow bound state, consistent with the results of the coupled-channel analysis of the LHCb data.