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

The decay ${\tau }^{-}\to K^{-}\eta {\nu }_{\tau }$ has been observed with the CLEO II detector. The $\eta$ meson is reconstructed using two decay channels, $\eta \to \gamma \gamma$ and ${\pi }^{+}{\pi }^{-}{\pi }^0$. The measured branching fraction is $B({\tau }^{-}\to K^{-}\eta {\nu }_{\tau })=(2.6\pm 0.5\pm 0.4)\times {10}^{-4\mathrm{}}$, somewhat higher than theoretical estimates. An improved upper limit for the second-class-current decay ${\tau }^{-}\to {\pi }^{-}\eta {\nu }_{\tau }$ is set, $B({\tau }^{-}\to {\pi }^{-}\eta {\nu }_{\tau })<1.4\mathrm{}\times {10}^{-4\mathrm{}}$ at 95% C.L., consistent with theoretical expectations.