Measurement of $\Delta m_s$ in the decay $B^0_s \rightarrow D^-_s(K^+K^-\pi^-)(3)\pi$

The $B^0_s-\overline{B^0_s}$ mixing frequency $\Delta m_s$ is measured on a dataset of 36 pb$^{-1}$ from the LHCb physics run in 2010. About 1350 $B^0_s$ signal candidates are constructed in $B^0_s \rightarrow D^-_s \pi$ and $B^0_s \rightarrow D_s 3\pi$ decays, with average proper time resolutions of 44 and 36~fs, respectively. Using opposite side flavour taggers with an effective tagging efficiency of $\epsilon_{eff}$ = 3.8 $\pm$ 2.1 \% a 4.6 $\sigma$ significant mixing signal (stat. only) is found. The preliminary mixing frequency measured is $\Delta m_s$ = 17.63 $\pm$ 0.11 (stat.) $\pm$ 0.04 (syst.)~ps$^{-1}$.