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 the first observation of the ${\mathrm{\Lambda }}_b^0\to K_S^{0}p{\pi }^{-}$ 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 $1.0{\mathrm{f}\mathrm{b}}^{-1}$ of $pp$ collisions. The $\Lambda^0_b \rightarrow K^0_{$\mathrm{S}$}p\pi^-$ decay is observed for the first time with a significance of $8.6\sigma$, and a measurement is made of the $CP$ asymmetry, which is consistent with zero. No significant signals are seen for $\Lambda^0_b \rightarrow K^0_{$\mathrm{S}$}pK^-$decays,$\Xi^0_b$decaystoboth$K^0_{$\mathrm{S}$}p\pi^-$and$K^0_{$\mathrm{S}$}pK^-$finalstates,andthe$\Lambda^0_b \rightarrow D^-s (K^0{$\mathrm{S}$}K^-)p$ decay, and upper limits on their branching fractions are reported.