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

Abstract

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