Observation of B[superscript 0][subscript s]-[bar over B][superscript 0][subscript s] mixing and measurement of mixing frequencies using semileptonic B decays

Abstract

The B[superscript 0][subscript s] and B[superscript 0] mixing frequencies, Δm[subscript s] and Δm[subscript d], are measured using a data sample corresponding to an integrated luminosity of 1.0 fb[superscript −1] collected by the LHCb experiment in pp collisions at a centre of mass energy of 7 TeV during 2011. Around 1.8×10[superscript 6] candidate events are selected of the type B[superscript 0][subscipt (s)]→D[superscript −][subscript (s)]μ[superscript +] (+ 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 Δm[subscript s]=(17.93 ± 0.22(stat) ±0.15(syst)) ps[superscript −1], Δm[subscript d]=(0.503 ± 0.011(stat) ±0.013(syst)) ps[superscript −1].The B[superscript 0][subscript s] and B[superscript 0] 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 B0(s)→D−(s)μ+B(s)0→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. 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 B[superscript 0][subscript s] and 13.0 standard deviations for B[superscript 0]. This is the first observation of B[superscript 0][subscript s] mixing to be made using only semileptonic decays.