A continuum-extrapolated lattice calculation gives a^{0+}_{eta_c eta_c} = -0.104(09) fm and a^{2+}_{J/psi J/psi} = -0.165(16) fm, indicating repulsive interactions.
Deciphering the recently discovered tetraquark candidates around 6.9 GeV
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abstract
Recently a novel hadronic state of mass 6.9 GeV, that decays mainly to a pair of charmonia, was observed in LHCb. The data also reveals a broader structure centered around 6490 MeV and suggests another unconfirmed resonance centered at around 7240 MeV, very near to the threshold of two doubly charmed $\Xi_{cc}$ baryons. We argue in this note that these exotic hadrons are genuine tetraquarks and not molecules of charmonia. It is conjectured that they are V-baryonium tetraquarks, namely, have an inner structure of a baryonic vertex with a $cc$ diquark attached to it, which is connected by a string to an anti-baryonic vertex with a $\bar c \bar c$ anti-diquark. We examine these states as the analogs of the states $\Psi(4360)$ and $Y(4630)$/$\Psi(4660)$ which are charmonium-like tetraquarks. One way to test these claims is by searching for a significant decay of the state at 7.2 GeV into $\Xi_{cc}\overline\Xi_{cc}$. Such a decay would be the analog of the decay of the state $Y(4630)$ into to $\Lambda_c\overline\Lambda_c$. We further argue that there should be trajectories of both orbital and radial excited states of the $X(6900)$. We predict their masses. It is possible that a few of these states have already been seen by LHCb.
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Lattice calculation of the $\eta_c\eta_c$ and $J/\psi J/\psi$ s-wave scattering length
A continuum-extrapolated lattice calculation gives a^{0+}_{eta_c eta_c} = -0.104(09) fm and a^{2+}_{J/psi J/psi} = -0.165(16) fm, indicating repulsive interactions.