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

Abstract

Using the ARGUS detector at the e + e − storage ring DORIS II the Michel parameter ξ and ξδ are determined. The integrated luminosity of the ARGUS experiment around √ s = 10 GeV is 445 pb −1 . This corresponds to approximatly 415 000 produced τ -pairs. From this data sample, 3262 events are selected with e + → τ + τ − → ( l ± υυ¯ )( π ± π + π − υ . The semihadronic decay is used as an analyser of the τ -spin and allows — owing to the spin-correlation — to determine the Michel parameters ξ and ξδ in the decay τ ± → l ± υυ¯ . Simultanously to the determination of the Michel parameters, a measurement of the τ -neutrino helicity h ν τ in the decay τ ∓ → π ∓ π + π − ν is provided. Thus, preliminary results of the Michel parameters ϱ, ξ, ξ δ , and the τ -neutrino helicity h ν τ will be presented. In addition a preliminary combined ARGUS result on ϱ, ξ, ξ δ , and h ν τ will be given using this work and previous measurements.