QCD predictions for the decay of theτlepton

The semileptonic decay rate of a heavy lepton is shown to be rigorously calculable by use of perturbative QCD. Even the {τ} lepton is heavy enough for the perturbative approximation to be accurate. The prediction for the ratio R={Γ}({τ}{→}v_τ+hadrons)/{Γ}({τ}{→}v_τ+e^-+vₑ) is 3.29, with an uncertainty of about 1% due to the uncertainty in the {Λ} parameter of QCD. Nonperturbative corrections to this prediction are estimated to be on the order of 1%. More importantly, these corrections are shown to be negative because of a fortuitous cancellation of the leading contribution from the gluon condensate. The resulting prediction is significantly smaller than the present experimental result R=3.65±{}0.13.