$Z_c(4100)$ and $Z_c(4200)$ as hadrocharmonium

Abstract

It is suggested that the recently reported charged charmoniumlike resonance $Z_c(4100)$ and the previously observed $Z_c(4200)$ are two states of hadrocharmonium, related by the charm quark spin symmetry in the same way as the lowest charmonium states $\eta_c$ and $J/\psi$. Namely, the $Z_c(4100)$ is (dominantly) the $\eta_c$ embedded in an light-quark excitation with quantum numbers of a pion, while the $Z_c(4200)$ is a similar four quark state containing $J/\psi$ instead of $\eta_c$. It is argued that this picture suggests certain relations between the properties of the two exotic resonances and a distinctive pattern of their decays that can be tested experimentally. It is also pointed out that due to the presence of a spatially compact $c \bar c$ charmonium the discussed exotic $Z_c$ states may be produced as resonances in the $s$ channel in $\bar p p$ annihilation.