Leptonic decays of the Ds meson

Abstract The purely leptonic decays D s → τν and D s → μν 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 D s → τν decays with τ→ e ν ν or μν ν , and the other optimized for D s → μν decays. The results are used to evaluate the D s decay constant, within the Standard Model: f D s =[285±19(stat)±40(syst)] MeV.