Narrow Results

First direct measurements of the weak decay constant $f_{D_s}(f_D)$ and the $D_s(D)$ branching fractions to $\mu {\nu }_{\mu }$, $\varphi \pi$, $eX$, $\varphi X(\mu {\nu }_{\mu })$ at the BES experiments in 1990s are reviewed. Referring to the BESIII results of $f_{D_s}$ and branching fraction $\mathrm{\text{Br}}(D_s\to \mu {\nu }_{\mu })$ with best precision¹ to date, prospect for challenge on precision measurements of $f_{D_s}$ and $\mathrm{\text{Br}}(D_s\to \mu {\nu }_{\mu })$ at BESIII is presented.

Purely leptonic decays of the D_s(D) meson in the reaction $e^{+}{e}^{-}\to D_s^{+}{D}_s^{-}\left(D^{*+}{D}^{-}\right)$ are observed by the BES group at the BEPC collider. Direct measurements of the weak decay constant f_Ds (f_D) and the D_s(D) branching fractions to μν_μ, ϕπ, eX, ϕX (μν_μ) are obtained for the first time.