Observation of the Decays Λ[superscript 0][subscript b] → χ[subscript c1]pK[superscript −] and Λ[subscript 0][subscript b] → χ[subscript c2]pK−

The first observation of the decays Λ[superscript 0][subscript b] →χ[subscript c1]pK[superscript -] and Λ[superscript 0][subscript b] →χ[subscript c2]pK[superscript -] is reported using a data sample corresponding to an integrated luminosity of 3.0 fb[superscript -1], collected by the LHCb experiment in pp collisions at center-of-mass energies of 7 and 8 TeV. The following ratios of branching fractions are measured: B(Λ[superscript 0][subscript b] →χ[subscript c1]pK[superscript -])/B(Λ[superscript 0][subscript b]→J/ψpK[superscript -]) = 0.242±0.014±0.013±0.009, B(Λ[superscript 0][subscript b] → χ[subscript c2]pK[superscript -])/B(Λ[subscript 0][subscript b] →J/ψpK[superscript -]) = 0.248±0.020±0.014±0.009, B(Λ[subscript 0][subscript b] →χ[subscript c2]pK[superscript -])/B(Λ[superscript 0][subscript b] → χ[subscript c1]pK[superscript -]) = 1.02±0.10±0.02±0.05, where the first uncertainty is statistical, the second systematic, and the third due to the uncertainty on the branching fractions of the χ[subscript c1] → J/ψγ and χ[subscript c2] → J/ψγ decays. Using both decay modes, the mass of the Λ[superscript 0][subscript b] baryon is also measured to be m[subscriprt Λ[superscript 0][subscript b0]] = 5619.44±0.28±0.26 MeV/c[superscript 2], where the first and second uncertainties are statistical and systematic, respectively.