On the nature of X(6900) and other structures in the LHCb di-j/ψ spectrum

Abstract

The LHCb di-J/ψ spectrum is studied within the framework of effective field theory with four coupled channels {J/ψJ/ψ, J/ψψ(2S), J/ψψ(3770), ψ(2S)ψ(2S)}, in order to unveil possible underlying fully-charmed tetraquark states. The partial-wave analysis is performed properly, and the unitarity of the scattering amplitudes is restored via Bethe-Salpeter equation under on-shell approximation. Four states are found in the energy region [6.2 GeV, 7.6 GeV]. For the partial wave with quantum numbers 0 ++, a bound state X(6200) and a narrow resonance X(7200) can be dynamically generated, while for the 2 ++ partial wave, two different resonant states, one named X(6900) with a narrow width and the other X(6680) with a broad width, can be found. Our results of the mass and width of X(6900) agree well with the experimental ones given by the LHCb collaboration. Furthermore, our findings shed first light on the determination of the J P C quantum numbers of these states