Observation of the ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}^{-}{\mu }^{+}{\mu }^{-}$ and study of the ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}^{-}\psi (2S)$ decays

The rare decay ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}^{-}{\mu }^{+}{\mu }^{-}$ is observed for the first time using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of $7$, $8$ and~$13\text{ TeV}$, corresponding to integrated luminosities of $1$, $2$ and $6{\text{fb}}^{-1}$, respectively. Its branching fraction is reported in intervals of the dimuon invariant mass squared. The integrated value of $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}^{-}{\mu }^{+}{\mu }^{-})$ is measured to be $(118\pm 24\pm 7\pm 38)\times {10}^{-8}$, where the first uncertainty is statistical, the second is systematic and the third uncertainty arises from the normalisation branching fraction $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}^{-}J\mathrm{/}\psi )$, whose uncertainty is dominated by SU(3)-breaking effects. In addition, the ratio of the branching fractions of the ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}^{-}\psi (2S)$ and the ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}^{-}J\mathrm{/}\psi$ decays is measured to be $0.491\pm 0.041\pm 0.019$.