A measurement of the branching fraction of the D(s) meson decay into a tau and a neutrino

The purely leptonic decay $Ds\to tn$ 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 BDs→ tne=0 .0586±0.0118stat ±0.0209 syst, BDs→tn m=0.0578±0.0085 stat± 0.0176syst . The combined result is BDs→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± 18stat±42 systMeV. ...