Searches for ${\mathrm{\Lambda }}_b^0$ and ${\mathrm{\Xi }}_b^0$ decays to $K_{\mathrm{S}}^{0}p{\pi }^{-}$ and $K_{\mathrm{S}}^{0}p{K}^{-}$ final states with first observation of the ${\mathrm{\Lambda }}_b^0\to K_{\mathrm{S}}^{0}p{\pi }^{-}$ decay

A search for previously unobserved decays of beauty baryons to the final states $K_{\mathrm{S}}^{0}p{\pi }^{-}$ and $K_{\mathrm{S}}^{0}p{K}^{-}$ is reported. The analysis is based on a data sample corresponding to an integrated luminosity of $1.0$fb${}^{-1}$ of $pp$ collisions. The $\Lambda^0_{b} \rightarrow \overline{\kern -0.2em K}^0_{\rm\scriptscriptstyle S}p \pi^{-}$ decay is observed with a significance of $8.6\sigma$, with branching fraction \begin{eqnarray*} {\cal{B}}(\Lambda^0_{b} \rightarrow \overline{\kern -0.2em K}^0 p \pi^{-}) & = & \left(1.26 \pm 0.19 \pm 0.09 \pm 0.34 \pm 0.05 \right) \times 10^{-5} ,, \end{eqnarray*} where the uncertainties are statistical, systematic, from the ratio of fragmentation fractions $f_{\mathrm{\Lambda }}\mathrm{/}{f}_d$, and from the branching fraction of the $B^0\to K_{\mathrm{S}}^0{\pi }^{+}{\pi }^{-}$ normalisation channel, respectively. A first measurement is made of the $CP$ asymmetry, giving \begin{eqnarray*} A_{C/!P} (\Lambda^0_{b} \rightarrow \overline{\kern -0.2em K}^0 p \pi^{-}) & = & 0.22 \pm 0.13$(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})$ \pm 0.03$(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$ , . \end{eqnarray*} No significant signals are seen for $\Lambda^0_{b} \rightarrow K^0_{\rm\scriptscriptstyle S}p K^{-}$decays,$\Xi^{0}{b}$decaystoboththe$K^0{\rm\scriptscriptstyle S}p \pi^{-}$and$K^0_{\rm\scriptscriptstyle S}p K^{-}$finalstates,andthe$\Lambda^0_{b} \rightarrow D^{-}{s} (\rightarrow K^0{\rm\scriptscriptstyle S} K^{-}) p$ decay, and upper limits on their branching fractions are reported.