Interpretation of structure in the di-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>J</mml:mi><mml:mo stretchy="false">/</mml:mo><mml:mi>ψ</mml:mi></mml:math> spectrum
Marek Karliner, Jonathan L. Rosner
Abstract
Structure in the di-$J/\ensuremath{\psi}$ 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 $cc\ensuremath{-}\overline{c}\overline{c}$ tetraquark, which we predict at $6.871\ifmmode\pm\else\textpm\fi{}0.025\text{ }\text{ }\mathrm{GeV}$. The dip around 6.75 GeV is ascribed to the opening of the $S$-wave di-${\ensuremath{\chi}}_{c0}$ channel, while the dip around 7.2 GeV could be correlated with the opening of the di-${\ensuremath{\eta}}_{c}(2S)$ or ${\mathrm{\ensuremath{\Xi}}}_{cc}{\overline{\mathrm{\ensuremath{\Xi}}}}_{cc}$ channel. The low-mass part of the di-$J/\ensuremath{\psi}$ structure appears to require a broad resonance consistent with a predicted ${2}^{++}(1S)$ state with invariant mass around ${M}_{\mathrm{inv}}=6400\text{ }\text{ }\mathrm{MeV}$. Implications for $bb\overline{b}\overline{b}$ tetraquarks are discussed.