Abstract
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 $\bar c \bar c$ 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 $cc$-$\bar c \bar c$ tetraquark, which we predict at $6.871\pm 0.025$ GeV. The dip around 6.75 GeV is ascribed to the opening of the \Swave\ di-$\chi_{c0}$ channel, while the dip around 7.2 GeV could be correlated with the opening of the di-$\eta_c(2S)$ or $\Xi_{cc} \bar \Xi_{cc}$ 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 $M_{\rm inv} = 6400$ MeV. Implications for $bb \bar b \bar b$ tetraquarks are discussed.
Highlights
The picture of hadrons as bound states of colored quarks described the observed mesons as qqand baryons as qqq states and could accommodate more complicated color-singlet combinations such as qqqq ̄ or q4q
Since 2003, experimental evidence has accumulated for such combinations, but it has not been clear whether they are genuine bound states with equal roles for all constituents or loosely bound “molecules” of two mesons or a meson and a baryon, with quarks mainly belonging to one hadron or the other
If the major resonant di-J=ψ activity is in the JPC 1⁄4 0þþ channel, a pair of χc0ð3415Þ charmonia can be produced in an S wave as soon as Minvðdi − J=ψÞ exceeds 6829 MeV
Summary
The picture of hadrons as bound states of colored quarks described the observed mesons as qqand baryons as qqq states and could accommodate more complicated color-singlet combinations such as qqqq ̄ (tetraquarks) or q4q (pentaquarks). The LHCb Collaboration at CERN has presented evidence for structure in the spectrum of a pair of J=ψ mesons, Minvðdi − J=ψÞ [3], interpreted as a narrow structure around 6.9 GeV and a broad structure just above twice the J=ψ mass. A dip in Minvðdi − J=ψÞ around 6.75 GeV suggests interference with nonresonant behavior in a channel with the same JPC. Such behavior is difficult to regard from a molecular standpoint but is compatible with a picture of a compact ccccstate. If the major resonant di-J=ψ activity is in the JPC 1⁄4 0þþ channel, a pair of χc0ð3415Þ charmonia can be produced in an S wave as soon as Minvðdi − J=ψÞ exceeds 6829 MeV.
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