Measurement of the τ lifetime

The τ lifetime has been measured with the OPAL detector at LEP, from analyses using the impact parameters in decays to single charged tracks, and the decay lengths from τ decays to three charged tracks. The 1991 sample of approximately 12300 τ-pair events, of which 70% contain silicon microvertex detector information, has been combined with a re-analysis of the 5100 events recorded during 1990. The two statistically-independent determinations give: $$\begin{gathered} \tau (one - prong) = 296.4 \pm 7.1 (stat) \pm 3.8 (sys) fs, \hfill \\ \tau (three - prong) = 286.3 \pm 7.4 (stat) \pm 5.2 (sys) fs. \hfill \\ \end{gathered} $$ The weighted average of these results after combination of the uncorrelated systematic errors is: $$\tau _\tau = 291.9 \pm 5.1 (stat) \pm 3.1 (sys) fs.$$ .