First observation of the $\Lambda_b^0 \to \Lambda_c^+ D_s^- K^+ K^-$ decay and search for pentaquarks in the $\Lambda_c^+ D_s^-$ system

The Λb0→Λc+Ds−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 TeV with the LHCb detector, corresponding to an integrated luminosity of 6 fb−1. The ratio of branching fraction to that of Λb0→Λc+Ds− decays is measured as 0.0141±0.0019±0.0012, where the first uncertainty is statistical and the second systematic. A search for hidden-charm pentaquarks with strangeness is performed in the Λc+Ds− system. No evidence is found, and upper limits on the production ratio of Pcc¯s(4338)0 and Pcc¯s(4459)0 pentaquarks relative to the Λc+Ds− final state are set at the 95% confidence level as 0.12 and 0.20, respectively.