Observation of ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\bar{D}}^{(\ast )0}{K}^{-}$ and ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{\ast -}$ decays

The decays ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\bar{D}}^{(\ast )0}{K}^{-}$ and ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{\ast -}$ are observed for the first time, in proton-proton collision data at $\sqrt{s}=13$TeV corresponding to an integrated luminosity of 5.4 fb${}^{-1}$ collected with the LHCb detector. Their ratios of branching fractions with respect to the ${\mathrm{\Lambda }}_b^0\text{\hspace{0.17em}⁣}\to {\mathrm{\Lambda }}_c^{+}{\mathrm{D}}_s^{-}$ mode are measured to be \begin{align*} \begin{split} \frac{$\mathcal{B}$(Λ_b^0 \to Λ_c^+\bar{D}^0 K^-)}{$\mathcal{B}$(Λ_b^0 \to Λ_c^+ D_s^-)} & = 0.1908 {}{-0.0034}^{+0.0036} {}{-0.0018}^{+0.0016} \pm 0.0038 \frac{$\mathcal{B}$(Λ_b^0 \to Λ_c^+\bar{D}^{0} K^-)}{$\mathcal{B}$(Λ_b^0 \to Λ_c^+ D_s^-)} & = 0.589 {}{-0.017}^{+0.018} {}{-0.018}^{+0.017} \pm 0.012 \frac{$\mathcal{B}$(Λ_b^0 \to Λ_c^+ D_s^{-})}{$\mathcal{B}$(Λ_b^0 \to Λ_c^+ D_s^-)} & = 1.668 \pm 0.022 {}_{-0.055}^{+0.061}\ , \end{split} \end{align*} where the first uncertainties are statistical, the second systematic, and the third, for the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\bar{D}}^{(\ast )0}{K}^{-}$ decays, are due to the uncertainties on the branching fractions of the $D_s^{-}\to K^{-}{K}^{+}{\pi }^{-}$ and ${\bar{D}}^0\to K^{+}{\pi }^{-}$ decay modes. The measured branching fractions probe factorization assumptions in effective theories and provide the normalization for future pentaquark searches in ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\bar{D}}^{(\ast )0}{K}^{-}$ decay channels.