TOP-QUARK CONTRIBUTION TO HADRONIC DECAYS OF THE Z-BOSON AT O(ALPHA(2)(S)) IN QCD

We evaluate the effect of a virtual top quark on the coefficient of al pha(s)(2) in the decay rates Gamma(Z --> hadrons) and Gamma(Z --> b (b ) over bar). We treat the dependence on the top quark mass exactly ins tead of using a large mass expansion. The present work completes the e valuation of the alpha(s)(2) contributions to these quantities. The ca lculation uses both the modified minimal subtraction and Collins-Wilcz ek-Zee renormalization prescriptions. The results can be applied to th e hadronic decays of the tau lepton.