A search for $C\!P$ violation in the decay $\Lambda^0_b \to p K^- \mu^+ \mu^-$ is presented. This decay is mediated by flavour-changing neutral-current transitions in the Standard Model and is potentially sensitive to new sources of $C\!P$ violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of $3$ $\mathrm{ fb}^{ -1}$. The $\Lambda^0_b \to p K^- \mu^+ \mu^-$ decay is observed for the first time, and two observables that are sensitive to different manifestations of $C\!P$ violation are measured, $\Delta\mathcal{ A}_{ C\!P} \equiv \mathcal{ A}_{ C\!P}(\Lambda^0_b \to p K^- \mu^+ \mu^-)-\mathcal{ A}_{ C\!P}(\Lambda^0_b\to pK^- J/\psi)$ and $a_{ C\!P}^{ \widehat{ T}-odd}$, where the latter is based on asymmetries in the angle between the $\mu^+\mu^-$ and $p K^-$ decay planes. These are measured to be $\Delta\mathcal{ A}_{ C\!P} = (-3.5 \pm 5.0(stat) \pm 0.2(syst))\times 10^{ -2}$, $a_{ C\!P}^{ \widehat{ T}-odd} = (1.2 \pm 5.0(stat) \pm 0.7(syst))\times 10^{ -2}$, and no evidence for $C\!P$ violation is found.
Categories: hep-ex