$Z_b$ and $Z_c$ Exotic States as Coupled Channel Cusps

Abstract

It is demonstrated that the candidate tetraquark states $Z_b(10610)$, $Z_b(10650)$, $Z_c(3900)$, and $Z_c(4025)$ are coupled channel cusp effects. The model explains in a natural way the masses and quantum numbers of the putative states and the near equality of the widths of the $Z_b$ states. It is argued that the $Z_c(3900)$ and $Z_c(4025)$ should be visible in $\bar B^0 \to J/\psi \pi^0\pi^0$ or $B^- \to J/\psi \pi^- \pi^0$, but should {\it not} appear in $\bar B^0 \to J/\psi \pi^+\pi^-$, in agreement with recent LHCb results. Additional tests for cusp effects are suggested.