Determination of the Michel Parameters rho, xi, and delta in tau-Lepton Decays with tau --> rho nu Tags

Abstract

Using the ARGUS detector at the $e^+ e^-$ storage ring DORIS II, we have measured the Michel parameters $ρ$, $ξ$, and $ξδ$ for $τ^{\pm}\to l^{\pm} ν\barν$ decays in $τ$-pair events produced at center of mass energies in the region of the $Υ$ resonances. Using $τ^\mp \to ρ^\mp ν$ as spin analyzing tags, we find $ρ_{e}=0.68\pm 0.04 \pm 0.08$, $ξ_{e}= 1.12 \pm 0.20 \pm 0.09$, $ξδ_{e}= 0.57 \pm 0.14 \pm 0.07$, $ρ_μ= 0.69 \pm 0.06 \pm 0.08$, $ξ_μ= 1.25 \pm 0.27 \pm 0.14$ and $ξδ_μ= 0.72 \pm 0.18 \pm 0.10$. In addition, we report the combined ARGUS results on $ρ$, $ξ$, and $ξδ$ using this work und previous measurements.