Observation of the decay Λb⁰ → χc1pπ⁻

The Cabibbo-suppressed decay Lambda b0-> chi (c1)p(-) 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 Lambda b0-> chi (c2)p(-) decay is also found. Using the Lambda b0-> chi (c1)pK(-) decay as normalisation channel, the ratios of branching fractions are measured to beBLambda b0-> chi c1p pi-BLambda b0-> chi c1pK-=6.59 +/- 1.01 +/- 0.22x10-2,BLambda b0 -> chi c2p pi-BLambda b0 -> chi c1p pi-=0.95 +/- 0.30 +/- 0.04 +/- 0.04,BLambda b0 -> chi c2pK-BLambda b0 -> chi c1pK-=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 chi (c1,2)-> J/psi gamma decays.