A precise measurement of the tau lifetime

The tau lepton lifetime has been measured with the ${ {\rm e}^+ {\rm e}^-\rightarrow \tau^+\tau^-}$ events collected by the DELPHI detector at LEP in the years 1991-1995. Three different methods have been exploited, using both one-prong and three-prong $\tau$ decay channels. Two measurements have been made using events in which both taus decay to a single charged particle. Combining these measurements gave $\tau_\tau \mbox{(1 prong)} = 291.8\pm 2.3_{\mathrm{stat}} \pm 1.5_{\mathrm{sys}}\,\mathrm{fs}$. A third measurement using taus which decayed to three charged particles yielded $\tau_\tau \mbox{(3 prong)} = 288.6\pm 2.4_{\mathrm{stat}} \pm 1.3_{\mathrm{sys}}\,\mathrm{fs}. $ These were combined with previous DELPHI results to measure the tau lifetime, using the full LEP1 data sample, to be $\tau_\tau = 290.9\pm 1.4_{\mathrm{stat}} \pm 1.0_{\mathrm{sys}}\,\mathrm{fs}$ .