Fully heavy tetraquark spectroscopy in relativistic quark model

Masses of the ground and excited states of tetraquarks, composed of charm $c$ and bottom $b$ quarks and antiquarks, are calculated in the relativistic quark model based on the quasipotential approach and quantum chromodynamics. Relativistic effects are consistently taken into account. A tetraquark is considered as a bound state of a diquark and an antidiquark. The finite size of the diquark is taken into account, using the form factors of the diquark-gluon interaction. It is shown that most of the investigated states of tetraquarks lie above the decay thresholds into a pair of quarkonia, as a result they can be observed as broad resonances. The narrow state in the $J/\psi$ meson pair production spectrum recently discovered by the LHCb collaboration at Large Hadron Collider corresponds to the excited state of a fully charmed tetraquark.