Observation of $B^{0}_{s}$ – $\overline{B}{}^{0}_{s}$ mixing and measurement of mixing frequencies using semileptonic B decays

The B0s and B 0 mixing frequencies, Δm s and Δm d , 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 B0(s)→D−(s)μ+ (+ 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 Δms=(17.93±0.22(stat)±0.15(syst)) ps−1,Δmd=(0.503±0.011(stat)±0.013(syst)) ps−1. The hypothesis of no oscillations is rejected by the equivalent of 5.8 standard deviations for B0s and 13.0 standard deviations for B 0. This is the first observation of B0s mixing to be made using only semileptonic decays.