Experimental limit on the decay ${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$$K^{\mathrm{-}}$$K^0$

We set an upper limit on the branching fraction B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$ $K^{\mathrm{-}}$ $K^0$)<0.26% at the 95% confidence level. The data sample was obtained with the TPC/Two-Gamma detector facility at the SLAC $e^{+}$ $e^{\mathrm{-}}$ storage ring PEP. The process ${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$ $K^{\mathrm{-}}$ $K^0$ is related via SU(3) to the second-class-current decay ${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$ ${\pi }^{\mathrm{-}}$η. Our limit is nearly 20 times smaller than the recently reported branching fraction B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$ ${\pi }^{\mathrm{-}}$η) of (5.1±1.0±1.2)%, whereas SU(3) symmetry predicts the ratio of πη to KK¯ production to be at most 5:1. We also measure the branching fraction B(${\tau }^{-}$→${\nu }_{\tau }$ $K^{*-}$)=(1.5±0.4*0.4%.