Doubly-heavy tetraquark states and *

Abstract

Abstract Inspired by the recent observation of a very narrow state, called , by the LHCb collaboration, the possible bound states and low-lying resonance states of the doubly-heavy tetraquark states ( ) and ( ) are searched in the framework of a chiral quark model with an accurate few-body method, the Gaussian expansion method. The real scaling method is also applied to identify the genuine resonance states. In the calculation, the meson–meson structure, diquark–antidiquark structure, and their coupling are all considered. The numerical results show: (i) For and , only states are bound in different quark structures. The binding energy varies from a few MeV for the meson–meson structure to over 100 MeV for the diquark–antidiquark structure. For example, for , in the meson–meson structure, there exists a weakly bound molecule state around 3841.4 MeV, 1.8 MeV below the , which may be a good candidate for the observed state by LHCb; however in the diquark–antidiquark structure, a deeper bound state with mass 3700.9 MeV is obtained. When considering the structure mixing, the energy of system decreases to 3660.7 MeV and the shallow bound state disappears. (ii) Besides bound states, several resonance states for with are proposed.