Y(4260) and Y(4360) as mixed hadrocharmonium

Abstract

Recent BESIII data indicate a significant rate of the process $e^+e^- \to h_c \pi^+ \pi^-$ at the Y(4260) and Y(4360) resonances, implying a substantial breaking of the heavy quark spin symmetry. We consider these resonances within the picture of hadrocharmonium, i.e. of (relatively) compact charmonium embedded in a light quark mesonic excitation. We suggest that the resonances Y(4260) and Y(4360) are a mixture, with mixing close to maximal, of two states of hadrochamonium, one containing a spin-triplet $c \bar c$ pair and the other containing a spin-singlet heavy quark pair. We argue that this model is in a reasonable agreement with the available data and produces distinctive and verifiable predictions for the energy dependence of the production rate in $e^+e^-$ annihilation of the final states $J/\psi \pi \pi$, $\psi' \pi \pi$ and $h_c \pi \pi$, including the pattern of interference between the two resonances.