Observation ofμ3πEvents ine+e−Annihilation

We have observed events of the type ${\mu }^{\pm }+{\left(3\mathrm{}\pi \right)}^{\mp }$ in $e^{+}{e}^{-}$ interactions at center-of-mass energies above 6 GeV. The properties of the events are consistent with their coming from the heavy-lepton decays $\tau \to \mu \nu \overline{\nu }$ and $\overline{\tau }\to \overline{\nu }3\pi$ or $\overline{\tau }\to \overline{\nu }4\pi$. The three-charged-pion invariant-mass distribution shows a significant peak at a mass 1.1 GeV/${\mathit{c}}^2$. We find the branching ratio into three charged and any number of neutral pions $B({\tau }^{+}\to \overline{\nu }{\pi }^{+}{\pi }^{+}{\pi }^{-}n{\pi }^0)=0.18\mathrm{}\pm 0.065$.