Quarkonium Mass Splitting Revisited: Effects of Closed Mesonic Channels
  Modifications of the mass spectrum the quarkonium induced by its virtual dissociation into a pair of heavy mesons is considered. Coupling between quark and mesonic channels results in noticeable corrections to spin-dependent mass splitting. In particular, the observable hierarchy of mass splittings in the $\chi_c, \chi_b$ and $\chi'_b$ multiplets is reproduced. 
M.Shmatikov (Russian Research Center "Kurchatov Institute")