Observation of the decays $Λ_b^0 \to χ_{c1} p K^-$ and $Λ_b^0 \to χ_{c2} p K^-$
The first observation of the decays $Λ_b^0 \to χ_{c1} p K^-$ and $Λ_b^0 \to χ_{c2} p K^-$ is reported using a data sample corresponding to an integrated luminosity of $3.0$ fb$^{-1}$, collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV. The following ratios of branching fractions are measured \begin{eqnarray*} \frac{{\cal B}(Λ_b^0 \to χ_{c1} p K^-)}{{\cal B}(Λ_b^0 \to J/ψp K^-)} = 0.242 \pm 0.014 \pm 0.013 \pm 0.009, \frac{{\cal B}(Λ_b^0 \to χ_{c2} p K^-)}{{\cal B}(Λ_b^0 \to J/ψp K^-)} = 0.248 \pm 0.020 \pm 0.014 \pm 0.009, \frac{{\cal B}(Λ_b^0 \to χ_{c2} p K^-)}{{\cal B}(Λ_b^0 \to χ_{c1} p K^-)} = 1.02 \pm 0.10 \pm 0.02 \pm 0.05, \end{eqnarray*} where the first uncertainty is statistical, the second systematic and the third due to the uncertainty on the branching fractions of the $χ_{c1}\to J/ψγ$ and $χ_{c2} \to J/ψγ$ decays. Using both decay modes, the mass of the $Λ_b^0$ baryon is also measured to be $m_{Λ_b^0} = 5619.44 \pm 0.28 \pm 0.26$ MeV/$c^2$, where the first and second uncertainties are statistical and systematic, respectively.