Observation of Λ_b ⁰→ ψ(2S)pK⁻and Λ_b ⁰→ J/ψπ⁺π⁻pK⁻decays and a measurement of the Λ_b ⁰baryon 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/c², where the uncertainties are statistical and systematic.