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 × 10 5 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) × 10 −3 , B(τ− → K − π + π − ν τ ) = (2.14 ± 0.37 ± 0.29) × 10 −3 , B(τ − → K − K + π − π 0 ν τ ) = (0.75 ± 0.29 ± 0.15) × 10 −3 , and B(τ − → K − π + π − π 0 ν τ ) = (0.61 ± 0.39 ± 0.18) × 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 × 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.