Observation of the Ξb−→J/ψΛK− decay

The observation of the decay Ξb−→J/ψΛK− is reported, using a data sample corresponding to an integrated luminosity of 3fb−1, collected by the LHCb detector in pp collisions at centre-of-mass energies of 7 and 8TeV. The production rate of Ξb− baryons detected in the decay Ξb−→J/ψΛK− is measured relative to that of Λb0 baryons using the decay Λb0→J/ψΛ. Integrated over the b-baryon transverse momentum pT<25GeV/c and rapidity 2.0<y<4.5, the measured ratio is [Formula presented][Formula presented]=(4.19±0.29 (stat)±0.15 (syst))×10−2, where fΞjavax.xml.bind.JAXBElement@174dd849javax.xml.bind.JAXBElement@60967187 and fΛjavax.xml.bind.JAXBElement@7f27e98cjavax.xml.bind.JAXBElement@22b2798a are the fragmentation fractions of b→Ξb− and b→Λb0 transitions, and B represents the branching fraction of the corresponding b-baryon decay. The mass difference between Ξb− and Λb0 baryons is measured to be M(Ξb−)−M(Λb0)=177.08±0.47 (stat)±0.16 (syst)MeV/c2.