Vector resonance X1(2900) and its structure

Abstract

The new vector resonance X1(2900) observed recently by LHCb in the D−K+ invariant mass distribution in the decay B+→D+D−K+ is studied to uncover internal structure of this state, and calculate its physical parameters. In the present paper, the resonance X1(2900) is modeled as an exotic vector state, JP=1−, built of the light diquark uTCγ5d and heavy antidiquark c‾γμγ5Cs‾T. The mass and current coupling of X1(2900) are computed using the QCD two-point sum rule approach by taking into account various vacuum condensates up to dimension 10. The width of the resonance X1(2900) is saturated by two decay channels X1→D−K+ and X1→D‾0K0. The strong couplings g1 and g2 corresponding to the vertices X1D−K+ and X1D‾0K0 are evaluated in the context of the QCD light-cone sum rule method and technical tools of the soft-meson approximation. Results for the mass of the resonance X1(2900)m=(2890±122)MeV, and for its full width Γfull=(93±13)MeV are smaller than their experimental values reported by the LHCb collaboration. Nevertheless, by taking into account theoretical and experimental errors of investigations, interpretation of the state X1(2900) as the vector tetraquark does not contradict to the LHCb data. We also point out that analysis of the invariant mass distribution D+K+ in the same decay B+→D+D−K+ may reveal doubly charged four-quark structures [uc][s‾d‾].