Fully charmed resonance X(6900) and its beauty counterpart

The fully heavy scalar tetraquarks T-4Q =QQQQ, (Q=c,b) are explored in the context of QCD sum rule method. We model T-4Q as diquark-antidiquark systems composed of pseudoscalar constituents, and calculate their masses m((')) and couplings f((')) within the two-point sum rule approach. Our results m=(6928 +/- 50)MeV and m '= (18858 +/- 50) MeV for masses of the tetraquarks T-4c and T-4b prove that they can decay to hidden-flavor heavy mesons. The full width Gamma(4c) of the T-4c is evaluated by taking into account the decay channels T-4c -> J/psi J/psi, J/psi psi ', eta(c)eta(c), eta(c)eta(c)(2S),eta X-c(c1)(1P), and Xc0Xc0. The partial widths of these processes depend on strong couplings g(i) at vertices T(4c)J/psi J/psi, T(4c)J/psi psi ' etc., which are computed using the QCD three-point sum rule method. The decay T-4b -> eta(b)eta(b )is used to find the width Gamma(4b) of the T-4b. The predictions for m and Gamma(4c)=(128 +/- 22)MeV are compared with parameters of the fully charmed resonances reported by the LHCb, ATLAS, and CMS Collaborations. Based on this analysis, we interpret the tetraquark T-4c as a candidate to the resonance X(6900). The mass m ' and width Gamma(4b)=(94 +/- 28)MeV of the exotic meson T-4b can be used in future experimental investigations of these states.