Interpretation of structure in the di-J/ψ, spectrum

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 (c) over bar(c) over bar 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 - (c) over bar(c) over bar tetraquark, which we predict at 6.871 +/- 0.025 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 Xi(cc) (Xi) over bar (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-inv = 6400 MeV. Implications for bb (b) over bar(b) over bar tetraquarks are discussed.