Determination of the Michel parameters ρ, ξ, and δ in τ-lepton decays with τ→ρν tags

Abstract Using the ARGUS detector at the e + e − storage ring DORISII, we have measured the Michel parametersρ , ξ , and ξδ for τ ± →l ± ν ν decays in τ -pair events produced at center of mass energies in the region of the ϒ resonances. Using τ ∓ → ρ ∓ ν as spin analyzing tags, we find ρ e =0.68±0.04±0.08, ξ e =1.12±0.20±0.09, ξδ e =0.57±0.14±0.07, ρ μ =0.69±0.06±0.08, ξ μ =1.25±0.27±0.14 and ξδ μ =0.72±0.18±0.10. In addition, we report the combined ARGUS results on ρ , ξ , and ξδ using this work und previous measurements.