Observation of the decay ${\mathrm{\Lambda }}_b^0\to p{K}^{-}{\mu }^{+}{\mu }^{-}$ and a search for $C\text{\hspace{0.17em}⁣}P$ violation

A search for $C\text{\hspace{0.17em}⁣}P$ 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 $C\text{\hspace{0.17em}⁣}P$ violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of $3$ ${\mathrm{f}\mathrm{b}}^{-1}$. The ${\mathrm{\Lambda }}_b^0\to p{K}^{-}{\mu }^{+}{\mu }^{-}$ decay is observed for the first time, and two observables that are sensitive to different manifestations of $C\text{\hspace{0.17em}⁣}P$ 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}${C!P} = (-3.5 \pm 5.0(stat) \pm 0.2(syst))\times 10^{-2}$,$a{C!P}^{\widehat{T}-odd} = (1.2 \pm 5.0(stat) \pm 0.7(syst))\times 10^{-2}$,andnoevidencefor$C!P$ violation is found.