A Measurement Of The Branching Fraction Of The D(s) Meson Decay Into A Tau And A Neutrino

The purely leptonic decay Ds-->tn is studied in a sample of four million hadronic Z0 decays collected in 1991-1995 with the ALEPH detector at LEP. A linear discriminant analysis is performed to form an optimal set of discriminant variables. The total number of signal events is extracted from an unbinned maximum likelihood fit to the two dimensional discriminant variable distribution. The branching fractions are measured from the two leptonic decay channels of the t lepton to be B(Ds--> tn)e=0 .0586+/-0.0118(stat )+/-0.0209( syst), B(Ds-->tn )m=0.0578+/-0.0085 (stat)+/- 0.0176(syst) . The combined result is B(Ds-->tn )=0.0579+/-0.0076( stat)+/-0.0178( syst). Within the Standard Model, the result corresponds to a D s decay constant of fDs=273+/- 18(stat)+/-42( syst)MeV.