Upper Bound on theτ-Neutrino Mass from the Previously Unobserved Decay Modeτ→KKπντ

We report a measurement of the branching ratios for two decay modes of the $\tau$ lepton: $R_B({\tau }^{\pm }\to K^{\pm }{K}^{\mp }{\pi }^{\pm }{\nu }_{\tau })=\left({0.22}_{-0.11\mathrm{}}^{+0.17\mathrm{}}\right)\%$ and $R_B({\tau }^{\pm }\to K^{\pm }{\pi }^{\mp }{\pi }^{\pm }({\pi }^0\left){\nu }_{\tau }\right)=\left({0.22}_{-0.13\mathrm{}}^{+0.16\mathrm{}}\right)\%$. The separation of these channels is accomplished in the DELCO detector by use of a gas Čerenkov counter to identify fast kaons. The invariant-mass distribution of the $K^{\pm }{K}^{\mp }{\pi }^{\pm }$ system has been used to set an upper bound on the $\tau$ neutrino mass of 157 MeV at 95% confidence limit.