We perform a coupled-channels study of the $bb\bar c\bar c$ and $bc\bar b\bar c$ tetraquark systems in a molecular approach using a constituent quark model which has been widely used to satisfactorily describe a broad range of properties of heavy quark hadron systems, either conventional or exotic. Within a molecular framework, the interaction in the heavy quark sector is governed by gluon exchange or confinement forces that are inherently color-dependent. While the $B_c B_c$ system contains two identical quarks, enabling stronger interactions via exchange diagrams, the forces in the $B_c \bar{ B}_c$ and $(c\bar{ c})-(b\bar{ b})$ systems are expected to be significantly weaker. Consequently, the theoretical and experimental analysis of $B_c^{ (*)} B_c^{ (*)}$, $B_c^{ (*)} \bar{ B}_c^{ (*)}$, and charmonium-bottomonium bound structures could play a crucial role in clarifying the dominant mechanisms responsible for the formation of fully-heavy tetraquarks. For the $bb\bar c\bar c$ tetraquark sector, we find several resonance states with different spin-parity quantum numbers. These resonances are characterized by their proximity, but not too close, to the $B_c^{ (\ast)}B_c^{ (\ast)}$ thresholds and their large total decay widths, indicating strong decay channels. In contrast, our analysis of the $bc\bar b\bar c$ tetraquark sector reveals no bound states, virtual states, or resonances; suggesting that tetraquark states of the $(c\bar c)-(b\bar b)$ or $B_c^{ (\ast)}\bar B_c^{ (\ast)}$ molecular type are unlikely to be formed, within our model assumptions.
Categories: hep-ph