Lawrence Berkeley National Laboratory

Abstract: The decays Λb0→ ψ(2S)pK−and Λb0→ J/ψπ+π−pK−are observed in a data sample corresponding to an integrated luminosity of 3 fb−1, collected in proton-proton collisions at 7 and 8 TeV centre-of-mass energies by the LHCb detector. The ψ(2S) mesons are reconstructed through the decay modes ψ(2S) → μ+μ−and ψ(2S) → J/ψπ+π−. The branching fractions relative to that of Λb0→ J/ψpK−are measured to beℬ(Λb0→ψ(2S)pK−)/ℬ(Λb0→J/ψpK−)=(20.70±0.76±0.46±0.37)×10−2, ℬ(Λb0→ψ(2S)pK−)/ℬ(Λb0→J/ψpK−)=(20.86±0.96±1.34)×10−2 where the first uncertainties are statistical, the second are systematic and the third is related to the knowledge of J/ψ and ψ(2S) branching fractions. The mass of the Λb0baryon is measured to be M(Λb0)=5619.65±0.17±0.17MeV/c2, where the uncertainties are statistical and systematic.