Direct measurement of the pseudoscalar decay constant, fDs.

The Beijing Spectrometer (BES) experiment has observed purely leptonic decays of the D_s meson in the reaction e^+e^-→D_s^+D_s^- at a c.m. energy of 4.03 GeV. Three events are observed in which one D_s decays hadronically to φπ, K ^(*0)K, or K^0K, and the other decays leptonically to μν_μ or τν_τ. With the assumption of μ-τ universality, values of the branching fraction, B(D_s→μν_μ) = (1.5_(-0.6-0.2)^(+1.3+0.3))%, and the D_s pseudoscalar decay constant, f_D_s = (4.3_(-1.3-0.4)^(+1.5+0.4))×10^2 MeV, are obtained.