a Measurement of the Tauon Lifetime.

Using the ARGUS detector, located in the DORIS II storage ring, DESY, Hamburg, W. Germany, the lifetime of the tau<=pton has been measured using events of the type e^{+} e^{-}totau^{+ }tau^{-}. Events were selected where one tau decayed to a single charged particle plus neutrals and the second tau decayed to three charged particles plus neutrals. The data sample consisted of 166 pb ^{-1} of data, collected in the region of 10 GeV center of mass energy, and 5696 events passed the selection criteria. The decay point of the three charged track system was found using a parametric vertex fitting technique and was used to calculate the decay length of the tau. The lifetime was thus found to be tau_tau = (2.95 +/- 0.14 +/- 0.11) times 10 ^{-13}s, where the first error is statistical and the second systematic. A subset of the above data sample, which had no photons associated with the three charged particle system, was used to find an upper limit to the mass of nu_ tau. This was done by studying the neutrino's energy spectrum in the vicinity of E_nu = 0. By this technique, a 95% confidence limit to the neutrino's mass was found to be m_ {nu{_tau}} < 80 MeV/c^2. Both results are consistent with the standard model of elementary particle physics.