Observation of the decay $\Lambda^0_b \to p K^- \mu^+ \mu^-$ and a search for $C\!P$ violation

A search for CP violation in the decay Λ$_{b}^{0}$ → 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 Λ$_{b}^{0}$ → pK$^{−}$ μ$^{+}$ μ$^{−}$ decay is observed for the first time, and two observables that are sensitive to different manifestations of CP violation are measured, $ \Delta {\mathcal{A}}_{CP}\equiv {\mathcal{A}}_{CP}\left({\Lambda}_b^0\to p{K}^{-}{\mu}^{+}{\mu}^{-}\right)-{\mathcal{A}}_{CP}\left({\Lambda}_b^0\to p{K}^{-}J/\psi \right) $ and a $ {a}_{CP}^{\widehat{T}\hbox{-} odd} $ , where the latter is based on asymmetries in the angle between the μ$^{+}$ μ$^{−}$ and pK$^{−}$ decay planes.