Structure in the di-$J/\psi$ mass spectrum observed by the LHCb experiment around 6.9 and 7.2 GeV is interpreted in terms of $J^{PC}=0^{++}$ and $2^{++}$ resonances between a $cc$ diquark and a $\overline{c}\overline{c}$ antidiquark, using a recently confirmed string-junction picture to calculate tetraquark masses. The main peak around 6.9 GeV is likely dominated by the $0^{++}(2S)$ state, a radial excitation of the $cc-\overline{c}\overline{c}$ tetraquark, which we predict at $6.871\pm 0.025\mathrm{GeV}$. The dip around 6.75 GeV is ascribed to the opening of the $S$-wave di-${\chi }_{c0}$ channel, while the dip around 7.2 GeV could be correlated with the opening of the di-${\eta }_c(2S)$ or ${\mathrm{\Xi }}_{cc}{\overline{\mathrm{\Xi }}}_{cc}$ channel. The low-mass part of the di-$J/\psi$ structure appears to require a broad resonance consistent with a predicted $2^{++}(1S)$ state with invariant mass around $M_{\mathrm{inv}}=6400\mathrm{MeV}$. Implications for $bb\overline{b}\overline{b}$ tetraquarks are discussed.

• Received 30 October 2020
• Accepted 30 November 2020

DOI:https://doi.org/10.1103/PhysRevD.102.114039

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Published by the American Physical Society