Observation of the Decay Λ0b → p K− μ+ μ− and a Search for CP Violation

A search for CP violation in the decay Λ0b → p K− 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 → p K− μ+ μ− decay is observed for the first time, and two observables that are sensitive to different manifestations of CP violation are measured, ΔA_CP ≡ A_CP(Λb0 → p K− μ+ μ−) − A_CP(Λ0b → p K− J/ψ) and a_CP^T-odd, where the latter is based on asymmetries in the angle between the μ+ μ− and p K− decay planes. These are measured to be ΔA_CP = (−3.5 ± 5.0(stat) ± 0.2(syst)) × 10^−2, a_CP^T-odd = (1.2 ± 5.0(stat) ± 0.7(syst)) × 10^−2, and no evidence for CP violation is found.