Perturbative QCD and Tau Decay∗

Sufficiently inclusive observables in the decay of the tau lepton can be calculated using the methods of perturbative QCD. These include the asymmetry parameter Aτ that determines that angular distribution of the total hadron momentum in the decay of a polarized tau. It should be possible to measure Aτ accurately using existing data from LEP. Reliable estimates of theoretical errors are essential in order to determine whether a given observable is sufficiently inclusive to be calculated using perturbative methods. The theoretical uncertainties due to higher orders in αs can be estimated using recent calculations to all orders in the large–(33−2Nf ) limit. These estimates indicate that tau decay data can be used to determine αs(MZ) to a precision of 2% or better.