Fully charmed resonance X(6900) and its beauty counterpart

Abstract

The fully heavy scalar tetraquarks T4Q=QQQ‾Q‾, (Q=c,b) are explored in the context of QCD sum rule method. We model T4Q 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 T4c and T4b prove that they can decay to hidden-flavor heavy mesons. The full width Γ4c of the T4c is evaluated by taking into account the decay channels T4c→J/ψJ/ψ, J/ψψ′, ηcηc, ηcηc(2S), ηcχc1(1P), and χc0χc0. The partial widths of these processes depend on strong couplings gi at vertices T4cJ/ψJ/ψ, T4cJ/ψψ′ etc., which are computed using the QCD three-point sum rule method. The decay T4b→ηbηb is used to find the width Γ4b of the T4b. The predictions for m and Γ4c=(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 T4c as a candidate to the resonance X(6900). The mass m′ and width Γ4b=(94±28)MeV of the exotic meson T4b can be used in future experimental investigations of these states.