UC Santa Cruz

The $$B^{0}_{s}$$ and B0 mixing frequencies, Δms and Δmd, are measured using a data sample corresponding to an integrated luminosity of 1.0 fb−1 collected by the LHCb experiment in pp collisions at a centre of mass energy of 7 TeV during 2011. Around 1.8×106 candidate events are selected of the type $$B^{0}_{(s)} \to D^{-}_{(s)} \mu^{+}$$ (+ anything), where about half are from peaking and combinatorial backgrounds. To determine the B decay times, a correction is required for the momentum carried by missing particles, which is performed using a simulation-based statistical method. Associated production of muons or mesons allows us to tag the initial-state flavour and so to resolve oscillations due to mixing. We obtain $$\begin{aligned} &\Delta m_s = \bigl( 17.93 \pm0.22\textrm{(stat)} \pm0.15 \textrm{(syst)}\bigr)~\textrm{ps}^{-1}, \ &\Delta m_d = \bigl( 0.503 \pm0.011\textrm{(stat)} \pm0.013 \textrm{(syst)}\bigr)~\textrm{ps}^{-1}. \end{aligned}$$The hypothesis of no oscillations is rejected by the equivalent of 5.8 standard deviations for $$B^{0}_{s}$$ and 13.0 standard deviations for B0. This is the first observation of $$B^{0}_{s}$$ mixing to be made using only semileptonic decays.