Three-prong τ decays with charged kaons

Final states with charged kaons in three-prong τ decays are studied by exploiting the particle identification from the dE/dx measurement. The results are based on a sample of about 1.6 x 10⁵ detected τ pairs collected with the ALEPH detector between 1991 and 1995 around the Z peak. The following branching ratios have been measured: B(τ^- → K^-K⁺π^-ν_τ) = (1.63 ± 0.21 ± 0.17) x 10^-3, B(τ^- → K^-π⁺π^-ν_τ)=(2.14 ± 0.37 ± 0.29) x 10^-3, B (τ^- → K^-K⁺π^-π⁰ν_τ) = (0.75 ± 0.29 ± 0.15) x 10^-3, and B(τ^- → K^-π⁺π^-π⁰ν _τ) = (0.61 ± 0.39 ± 0.18) x 10^-3. The first two measurements are more precise than the current world averages, while the last two channels are investigated for the first time. The 95% C.L. upper limit on the branching ratio for the decay τ^- → K^-K⁺K^-ν_τ is 0.19 x 10^-3. A study of intermediate states occurring in the K^-K⁺π^-ν_τ and K^-π⁺π^-ν_τ decays is also presented.