Quarkonium mass splitting revisited: effects of closed mesonic channels

Modifications of the quarkonium mass spectrum induced by its virtual dissociation into a pair of heavy mesons are considered. Coupling between quark and mesonic channels results in noticeable corrections to spin-dependent mass splitting. In particular, accounting for the dissociation effect allows one to reproduce the observable hierarchy of mass splittings in the χc, χb and χ′b multiplets.