First observation of the ${\Lambda }_b^0\text{\hspace{0.17em}⁣}\to {\Lambda }_c^{+}{D}_s^{-}{K}^{+}{K}^{-}$ decay and search for pentaquarks in the ${\Lambda }_c^{+}{D}_s^{-}$ system

The ${\Lambda }_b^0\text{\hspace{0.17em}⁣}\to {\Lambda }_c^{+}{D}_s^{-}{K}^{+}{K}^{-}$ decay is observed for the first time using the data sample from proton-proton collisions recorded at a center-of-mass energy of $13\text{TeV}$ with the LHCb detector, corresponding to an integrated luminosity of $6{\text{fb}}^{-1}$. The ratio of branching fraction to that of ${\Lambda }_b^0\text{\hspace{0.17em}⁣}\to {\Lambda }_c^{+}{D}_s^{-}$ decays is measured as $0.0141\pm 0.0019\pm 0.0012$, where the first uncertainty is statistical and the second systematic. A search for hidden-charm pentaquarks with strangeness is performed in the ${\Lambda }_c^{+}{D}_s^{-}$ system. No evidence is found, and upper limits on the production ratio of $P_{c\bar{c}s}(4338{)}^0$ and $P_{c\bar{c}s}(4459{)}^0$ pentaquarks relative to the ${\Lambda }_c^{+}{D}_s^{-}$ final state are set at the $95\mathrm{\%}$ confidence level as $0.12$ and $0.20$, respectively.