Direct Measurement of the Pseudoscalar Decay Constant, $f_{D_s}$

The Beijing Spectrometer (BES) experiment has observed purely leptonic decays of the $D_s$ meson in the reaction $e^{+}{e}^{-}\to D_s^{+}{D}_s^{-}$ at a c.m. energy of 4.03 GeV. Three events are observed in which one $D_s$ decays hadronically to $\phi \pi$, ${\overline{K}}^{*0\mathrm{}}K$, or ${\overline{K}}^{0}K$, and the other decays leptonically to $\mu {\nu }_{\mu }$ or $\tau {\nu }_{\tau }$. With the assumption of $\mu -\tau$ universality, values of the branching fraction, $B(D_s\to \mu {\nu }_{\mu })=\left({1.5}_{-0.6-0.2\mathrm{}}^{+1.3+0.3\mathrm{}}\right)\%$, and the $D_s$ pseudoscalar decay constant, $f_{D_s}=\left({4.3}_{-1.3-0.4\mathrm{}}^{+1.5+0.4\mathrm{}}\right)\times {10}^2$ MeV, are obtained.