Observation of Bs0-B̄s0 mixing and measurement of mixing frequencies using semileptonic B decays

The Bs0 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(s)0 → D̄(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 (Formula present) The hypothesis of no oscillations is rejected by the equivalent of 5.8 standard deviations for Bs0 and 13.0 standard deviations for B0. This is the first observation of Bs0 mixing to be made using only semileptonic decays. © 2013 CERN for the benefit of the LHCb collaboration.