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

H. Albrecht, T. Hamacher, R. P. Hofmann, T. Kirchhoff, R. Mankel, A. Nau, S. Nowak, D. Reßing, H. Schröder, H. D. Schulz, M. Walter, R. Wurth, C. Hast, H. Kapitza, H. Kolanoski, A. Kosche, A. Lange, A. Lindner, M. Schieber, T. SiegmundB. Spaan, H. Thurn, D. Töpfer, D. Wegener, P. Eckstein, C. Frankl, J. Graf, M. Schmidtler, M. Schramm, K. R. Schubert, R. Schwierz, R. Waldi, K. Reim, H. Wegener, R. Eckmann, H. Kuipers, O. Mai, R. Mundt, T. Oest, R. Reiner, W. Schmidt-Parzefall, J. Stiewe, S. Werner, K. Ehret, W. Hofmann, A. Hüpper, K. T. Knöpfle, J. Spengler, P. Krieger, D. B. MacFarlane, J. D. Prentice, P. R.B. Saull, K. Tzamariudaki, R. G. Van de Water, T. S. Yoon, M. schneider, S. Weseler, G. Kernel, P. Križan, E. Križnič, T. Podobnik, T. Živko, V. Balagura, S. Barsuk, I. Belyaev, R. Chistov, M. Danilov, L. Gershtein, Yu Gershtein, A. Golutvin, I. Korolko, G. Kostina, D. Litvintsev, P. Pakhlov, S. Semenov, A. Snizhko, I. Tichomirov, Yu Zaitsev

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29 Scopus citations


Using the ARGUS detector at the e+e- storage ring DORIS II, we have determined the Michel parameters ξ and δ of τ → lν 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 -) (ππ+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 → lν ν -. Simultaneously to the determination of the Michel parameters, a measurement of the τ-neutrino helicity hντ in the decay τ → ππ+π-ν was obtained. We observed hντ = -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 hντ, ρ{variant}, ξ, and ξδ are reported using this work and previous measurements.

Original languageEnglish (US)
Pages (from-to)576-584
Number of pages9
JournalPhysics Letters B
Issue number4
StatePublished - Apr 27 1995

All Science Journal Classification (ASJC) codes

  • Nuclear and High Energy Physics


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