Observation of B⁰_s – B⁰_s mixing and measurement of mixing frequencies using semileptonic B decays

The B⁰_s and B⁰ 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 initialstate flavour and so to resolve oscillations due to mixing. We obtain Δm_s = (17.93 ± 0.22 (stat) ± 0.15 (syst)) ps⁻¹, Δm_d = (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 B⁰_s and 13.0 standard deviations for B⁰. This is the first observation of B⁰_s mixing to be made using only semileptonic decays.