First Observation of the B ¯ s 0 → Λ c + Λ ¯ c − Decay and Evidence for the B ¯ 0 → Λ c + Λ ¯ c − Decay

A search is presented for the two-body charmed baryonic decays, B[over ¯]_{(s)}^{0}→Λ_{c}^{+}Λ[over ¯]_{c}^{-}, using a data sample collected by the LHCb experiment during 2011-2012 and 2015-2018, corresponding to an integrated luminosity of 9 fb^{-1}. The first observation of the B[over ¯]_{s}^{0}→Λ_{c}^{+}Λ[over ¯]_{c}^{-} decay is reported with 6.2σ significance along with 4.3σ evidence for the B[over ¯]^{0}→Λ_{c}^{+}Λ[over ¯]_{c}^{-} decay. The branching fractions are measured to be B(B[over ¯]^{0}→Λ_{c}^{+}Λ[over ¯]_{c}^{-})=(1.01_{-0.28}^{+0.27}±0.08±0.15)×10^{-5} and B(B[over ¯]_{s}^{0}→Λ_{c}^{+}Λ[over ¯]_{c}^{-})=(5.0±1.3±0.5±0.8)×10^{-5}, where the first uncertainty is statistical, the second systematic, and the third due to external inputs. These results provide novel experimental inputs for the theoretical framework describing two-body baryonic decays of B mesons via W-emission and W-exchange mechanisms.