Measurement of the Absolute Branching Fractions for $D_s^{-}\to {\ell }^{-}{\bar{\nu }}_{\ell }$ and Extraction of the Decay Constant $f_{D_s}$

Abstract

The absolute branching fractions for the decays $D_s^{-}\to {\ell }^{-}{\bar{\nu }}_{\ell }$ ($\ell =e$, $\mu$, or $\tau$) are measured using a data sample corresponding to an integrated luminosity of 521 fb${}^{-1}$ collected at center of mass energies near 10.58 GeV with the \mbox{\slshape B\kern-0.1em{\smaller A}\kern-0.1em B\kern-0.1em{\smaller A\kern-0.2em R}} detector at the PEP-II $e^{+}{e}^{-}$ collider at SLAC. The number of $D_s^{-}$ mesons is determined by reconstructing the recoiling system $DKX\gamma$ in events of the type $e^{+}{e}^{-}\to DKX{D}_s^{\ast -}$, where $D_s^{\ast -}\to D_s^{-}\gamma$ and $X$ represents additional pions from fragmentation. The $D_s^{-}\to {\ell }^{-}{\bar{\nu }}_{\ell }$ events are detected by full or partial reconstruction of the recoiling system $DKX\gamma \ell$. The branching fraction measurements are combined to determine the $D_s^{-}$ decay constant $f_{D_s}=(258.6\pm 6.4\pm 7.5)$ MeV, where the first uncertainty is statistical and the second is systematic.

8 pages, 3 postscript figures

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