Measurement of the $B_s^0$ Lifetime Using Semileptonic Decays

We report a measurement of the $B_s^0$ lifetime in the semileptonic decay channel $B_s^0\to D_s^{-}{\mu }^{+}\nu X$ (and its charge conjugate), using approximately $0.4{\mathrm{fb}}^{-1}$ of data collected with the D0 detector during 2002–2004. Using 5176 reconstructed $D_s^{-}{\mu }^{+}$ signal events, we have measured the $B_s^0$ lifetime to be $\tau (B_s^0)=1.398\pm 0.044(\mathrm{stat}{)}_{-0.025}^{+0.028}(\mathrm{syst})\mathrm{ps}$. This is the most precise measurement of the $B_s^0$ lifetime to date.

Figure 1

The mass distribution of $\varphi {\pi }^{-}$ candidates. Points with errors bars show the “right-sign” $D_s^{-}{\mu }^{+}$ combinations, and the open squares show the corresponding “wrong-sign” distribution. The dashed curve represents the result of the fit to the “right-sign” combinations. The two peaks are associated with the $D^{-}$ and $D_s^{-}$ mesons, respectively.

Figure 2

Pseudoproper decay length distribution for $D_s^{-}{\mu }^{+}$ candidates with the result of the fit superimposed as the dashed curve. The dotted curve represents the combinatorial background and the filled area represents the $B_s^0$ signal.