Hidden-flavour four-quark states in the charm and bottom region

Abstract: We discuss the spectrum and the internal composition of ground and excited four-quark states in the charm and bottom energy region. To this end we extend previous calculations within the framework of the relativistic four-body Faddeev-Yakubovsky equation to include quantum numbers with $J^{P C} = 0^{++} , 0^{-+} , 1^{--} , 1^{+-}$ and $1^{+ +}$ and study their internal composition in terms of heavy-light meson pairs, hadroquarkonia and diquark-antidiquark clusters. We observe similar patterns in the charm and bottom energy region with different compositions of the four-quark states depending on $J^{P C}$ quantum numbers. Most notably, we find that all states with $C \cdot P = +1$ are dominated by heavy-light meson contributions, whereas for axialvector states with $J^{P C} = 1^{+-}$ including the $Z_c (3900)$ we find a much more complicated picture depending on the flavour content. We systematically compare our results for the spectrum with existing experimental results and provide predictions for future analyses.