Observation of the decay Λb0 → pK- μ+ μ- and a search for CP violation

A search for CP violation in the decay Λ0b → pK − μ + μ − is presented. This decay is mediated by flavour-changing neutral-current transitions in the Standard Model and is potentially sensitive to new sources of CP violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of 3 fb−1. The Λ0b → pK − μ + μ − decay is observed for the first time, and two observables that are sensitive to different manifestations of CP violation are measured, ΔACP≡ACP(Λ0b→pK−μ+μ−)−ACP(Λ0b→pK−J/ψ)ΔACP≡ACP(Λb0→pK−μ+μ−)−ACP(Λb0→pK−J/ψ) and a aTˆ-oddCPaCPT^-odd, where the latter is based on asymmetries in the angle between the μ + μ − and pK − decay planes. These are measured to be ΔACP=(−3.5±5.0(stat)±0.2(syst))×10−2,aTˆ-oddCP=(1.2±5.0(stat)±0.7(syst))×10−2,ΔACP=(−3.5±5.0(stat)±0.2(syst))×10−2,aCPT^-odd=(1.2±5.0(stat)±0.7(syst))×10−2, and no evidence for CP violation is found.