Abstract
The purely leptonic decays Ds -> tau nu and Ds -> mu nu are studied in a sample of four million hadronic Z decays collected with the ALEPH detector at the LEP e+e- collider from 1991 to 1995. The branching fractions are extracted from a combination of two analyses, one optimized to select Ds -> tau nu decays with tau -> e nu nubar or mu nu nubar, and the other optimized for Ds-> mu nu decays. The results are used to evaluate the Ds decay constant, within the Standard Model: fDs = [285 +- 19(stat) +- 40 (syst)] MeV
Highlights
The leptonic decays of the Ds meson are interesting because they open a window onto the strong interactions of the constituent quarks of the Ds
The goodness of fit is evaluated by comparing the likelihood value obtained from each fit to the data with the distribution of likelihoods obtained from fits to many toy Monte Carlo samples
As in the Ds → τ ν analysis, the signal component consists of eight contributions from Ds and D+ decays to τ ν and μν in cc and bbevents; the relative normalizations of these contributions are again fixed according to the Standard Model expectations (Table 3)
Summary
It is worthwhile to measure fDs because it characterizes the structure of the Ds meson and can be calculated in various theoretical models. Lattice QCD is generally considered to be the most reliable method for calculating the pseudoscalar meson decay constants; recent results yield a prediction of fDs = 255 ± 30 MeV [1]. An important application of the lattice QCD calculations of decay constants is in the evaluation of the third-generation Cabibbo-Kobayashi-Maskawa matrix elements. Constraints on these elements are obtained from measurements of B0-B 0 mixing and rely on theoretical estimates of fB, which is experimentally inaccessible at present. The helicity suppression in these decays leads to an extremely small branching fraction for the electron mode.
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