First Observation of the Decay ${\mathit{\tau }}^{-}\to {\mathit{K}}^{*-}\mathit{\eta }{\nu }_{\tau }$

The decay ${\tau }^{-}\to K^{*-}\eta {\nu }_{\tau }$ has been observed with the CLEO II detector. The $K^{*-}$ is reconstructed in two decay channels, $K^{*-}\to K_S{\pi }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}$ and $K^{*-}\to K^{-}{\pi }^0$. The $\eta$ is reconstructed from the decay $\eta \to \gamma \gamma$. The measured branching fraction is $B({\tau }^{-}\to K^{*-}\eta {\nu }_{\tau })=(2.9\pm 0.8\pm 0.4)\times {10}^{-4\mathrm{}}$. We also measure the inclusive branching fractions without requiring the $K^{*}$ resonance, $B({\tau }^{-}\to K_S{\pi }^{-}\eta {\nu }_{\tau })=(1.10\pm 0.35\pm 0.11)\times {10}^{-4\mathrm{}}$ and $B({\tau }^{-}\to K^{-}{\pi }^0\eta {\nu }_{\tau })=(1.77\pm 0.56\pm 0.71)\times {10}^{-4\mathrm{}}$. The results indicate that the $K^{*-}$ resonance dominates the $K_S{\pi }^{-}$ mass spectrum.