Determination of the Michel parameter in tau decay

Using the ARGUS detector at the DORIS II e+e− storage ring we have studied the electron and muon energy spectra in τ − → e − ν ν and τ − →μ − ν ν decays. The measured value of the Michel parameter, ρ=0.742±0.035±0.020, is in good agreement with a standard V-A coupling at the τ-ντ-W vertex. Pure V, A and V + A couplings are completely excluded by this measurement. Upper limits have been obtained for two-body tau decays, including final states with a Goldstone boson.