Determination of the Michel parameters ξ and δ in leptonic τ decays

Using the ARGUS detector at the e⁺e^- storage ring DORIS II, we have determined the Michel parameters ξ and δ of τ^∓ → l^∓ν ν - decays. With an integrated luminosity of 445 pb^-1 around s= 10GeV, we have produced approximately 415000 τ-pairs. From this data sample, 3262 events were selected with e⁺e^- → τ⁺τ^- → (l^±ν ν -) (π^∓π⁺π^-ν). The semihadronic decay was used as analyser of the τ-spin and made possible - owing to the spin correlations - the determination of the parity-violating Michel parameters ξ and δ in the decay τ^∓ → l^∓ν ν -. Simultaneously to the determination of the Michel parameters, a measurement of the τ-neutrino helicity h_ντ in the decay τ^∓ → π^∓π⁺π^-ν was obtained. We observed h_ντ = -0.85_-0.17^+0.15 ± 0.05, ρ{variant} = 0.721 ± 0.040 ± 0.021, ξ = 1.26_-0.26^+0.30 ± 0.09, and ξδ = 0.77_-0.16^+0.18 ± 0.05. In addition, the combined ARGUS result on h_ντ, ρ{variant}, ξ, and ξδ are reported using this work and previous measurements.