LHCb: Measurement of $\Delta m_s$ in the decay $B_s \to D_s(3)\pi$ at LHCb

An analysis of the $B_s$-$\overline{B}_s$ mixing frequency $\Delta m_s$ was performed on a dataset of ~35 pb$^{-1}$ obtained from the LHCb physics run in 2010. After reconstruction about 1350 $B_s$ signal candidates are found in the decay $B_s \to D_s (3)\pi$ with an average proper time resolution of 36-44 fs. Opposite side flavour taggers with a tagging performance of $e_{eff} = 3.8 \pm 2.1$ % were used. The observed mixing frequency is $\Delta m_s = 17.63 \pm 0.11(stat.) \pm 0.04 (syst.) ps^{-1}$