First observation of the $$\overline{B}_{s}^{0}\toΛ_{c}^{+}\overlineΛ_{c}{}^{-}$$ decay and evidence for the $\overline{B}^{0}\toΛ_{c}^{+}\overlineΛ_{c}{}^{-}$ decay

Abstract: A search is presented for the two-body charmed baryonic decays $$\overline{B}<em>{(s)}^{0}\toΛ</em>{c}<sup>{+}\overlineΛ_{c}{}<sup>{-}$$, using a data sample collected by the LHCb experiment during 2011--2012 and 2015--2018 corresponding to an integrated luminosity of $9\,\mathrm{fb}<sup>{-1}$. The first observation of the $$\overline{B}<em>{s}^{0}\toΛ</em>{c}<sup>{+}\overlineΛ_{c}{}<sup>{-}$$ decay is reported with $6.2σ$ significance, along with $4.3σ$ evidence for the $$\overline{B}<sup>{0}\toΛ<em>{c}^{{+}\overlineΛ}</em>{c}{}<sup>{-}$$ decay. The branching fractions are measured to be $$\cal{B}(\overline{B}^{{0}\toΛ<em>{c}^{{+}\overlineΛ}</em>{c}^{-})} = (1.01^{{+0.27}_{-0.28}} \pm 0.08 \pm 0.15) \times 10<sup>{-5}$$ and $$\cal{B}(\overline{B}<em>{s}^{0}\toΛ</em>{c}^{{+}\overlineΛ_{c}{}^{-})} = (5.0 \pm 1.3 \pm 0.5 \pm 0.8) \times 10<sup>{-5}$$, 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.