Hyperfine splitting of the S-levels of heavy quarkonium

We have found an expression for the hyperfine splitting of the S-levels of heavy quarkonium taking into account nonperturbative effects. The latter arise as a result of the interaction of the quarks with the large-scale fluctuations of the gluon field in the physical vacuum of QCD, which cannot be described by perturbation theory. We present an estimate of the splitting of the mas of the UPSILON resonance and the eta/sub b/:M(UPSILON)-M(eta/sub b/)< or approx. =30 MeV. We have also obtained estimates for the analogous splittings in t-bart-bar-quarkonium