Observation of the decay Λb0 → χ_c1pπ⁻

The Cabibbo-suppressed decay Λb0→ χ_c1pπ⁻ is observed for the first time using data from proton-proton collisions corresponding to an integrated luminosity of 6 fb⁻¹, collected with the LHCb detector at a centre-of-mass energy of 13 TeV. Evidence for the Λb0→ χ_c2pπ⁻ decay is also found. Using the Λb0→ χ_c1pK⁻ decay as normalisation channel, the ratios of branching fractions are measured to beB(Λb0→χc1pπ−)B(Λb0→χc1pK−)=(6.59±1.01±0.22)×10−2,B(Λb0→χc2pπ−)B(Λb0→χc1pπ−)=0.95±0.30±0.04±0.04,B(Λb0→χc2pK−)B(Λb0→χc1pK−)=1.06±0.05±0.04±0.04, where the first uncertainty is statistical, the second is systematic and the third is due to the uncertainties in the branching fractions of χ_c1,2→ J/ψγ decays.