First Measurement of [Γ(Ds(+) → μ(+)ν] / [Γ(Ds(+) → φπ(+)]

We present a measurement of $\frac{\ensuremath{\Gamma}({D}_{s}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}\ensuremath{\nu})}{\ensuremath{\Gamma}({D}_{s}^{+}\ensuremath{\rightarrow}\ensuremath{\varphi}{\ensuremath{\pi}}^{+}})=0.245\ifmmode\pm\else\textpm\fi{}0.052\ifmmode\pm\else\textpm\fi{}0.074$. Using this ratio and $\mathcal{B}({D}_{s}^{+}\ensuremath{\rightarrow}\ensuremath{\varphi}{\ensuremath{\pi}}^{+})=(3.7\ifmmode\pm\else\textpm\fi{}0.9)%$ we extract ${f}_{{D}_{s}}=344\ifmmode\pm\else\textpm\fi{}37\ifmmode\pm\else\textpm\fi{}52\ifmmode\pm\else\textpm\fi{}42$ MeV, where the last error is due to the uncertainty in the ${D}_{s}^{+}\ensuremath{\rightarrow}\ensuremath{\varphi}{\ensuremath{\pi}}^{+}$ absolute branching ratio. This result is larger than most theoretical predictions.