Measurement of the $\tau$-lepton Lifetime at Belle

We report that the lifetime of the $\tau$ lepton is measured using the process $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$, where both $\tau$ leptons decay to $3\pi {\nu }_{\tau }$. The result for the mean lifetime, based on $711{\mathrm{fb}}^{-1}$ of data collected with the Belle detector at the $Y\left(4S\right)$ resonance and $60\mathrm{MeV}$ below, is $\tau =(290.17\pm 0.53(\text{stat})\pm 0.33(\text{syst}\left)\right)\times {10}^{-15}s$. The first measurement of the lifetime difference between ${\tau }^{+}$ and ${\tau }^{-}$ is performed. The upper limit on the relative lifetime difference between positive and negative $\tau$ leptons is $\Delta \tau /\tau <7.0\times {10}^{-3}$ at 90% C.L.