First observation of the ${\bar{B}}_s^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c{}^{-}$ decay and evidence for the ${\bar{B}}^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c{}^{-}$ decay

A search is presented for the two-body charmed baryonic decays ${\bar{B}}_{(s)}^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c{}^{-}$, using a data sample collected by the LHCb experiment during 2011–2012 and 2015–2018 corresponding to an integrated luminosity of $9{\mathrm{f}\mathrm{b}}^{-1}$. The first observation of the ${\bar{B}}_s^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c{}^{-}$ decay is reported with $6.2\sigma$ significance, along with $4.3\sigma$ evidence for the ${\bar{B}}^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c{}^{-}$ decay. The branching fractions are measured to be $\mathcal{B}({\bar{B}}^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c{}^{-})=(1.01_{-0.28}^{+0.27}\pm 0.08\pm 0.15)\times {10}^{-5}$ and $\mathcal{B}({\bar{B}}_s^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c{}^{-})=(5.0\pm 1.3\pm 0.5\pm 0.8)\times {10}^{-5}$, 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.