Observation of Λb⁰→ ψ(2S)pK⁻and Λb⁰→ J/ψπ⁺π⁻pK⁻decays and a measurement of the Λb⁰baryon mass

Abstract: The decays Λ _b ⁰→ ψ(2S)pK ⁻and Λ _b ⁰→ 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 Λ _b ⁰→ J/ψpK ⁻are measured to beℬ(Λ _b ⁰→ψ(2S)pK ⁻)/ℬ(Λ _b ⁰→J/ψpK ⁻)=(20.70±0.76±0.46±0.37)×10 ⁻², ℬ(Λ _b ⁰→ψ(2S)pK ⁻)/ℬ(Λ _b ⁰→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 Λ _b ⁰baryon is measured to be M(Λ _b ⁰)=5619.65±0.17±0.17MeV/c ², where the uncertainties are statistical and systematic.