Measurement of τ decays involving η mesons

The decay ${\mathrm{\tau }}^{\mathrm{-}}$→${\nu }_{\mathrm{\tau }}$ ${\mathrm{\pi }}^{\mathrm{-}}$ ${\mathrm{\pi }}^0$η has been observed for the first time using the CLEO-II detector at the Cornell Electron Storage Ring. The measured branching ratio (0.17±0.02±0.02)%, agrees with the CVC (conserved vector current) prediction based on ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$→${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$η data. Upper limits on the branching ratios for other τ decays to final states including η mesons are improved by an order of magnitude compared to previous measurements.