Observation of the Ξb⁻→ Ξ⁻μ⁺μ⁻ and study of the Ξb⁻→Ξ⁻ψ(2S) decays

The rare decay Ξb⁻→Ξ⁻μ⁺μ⁻ 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~TeV, corresponding to integrated luminosities of $1$, $2$ and 6~fb⁻¹, respectively. Its branching fraction is reported in intervals of the dimuon invariant mass squared. The integrated value of B(Ξb⁻→Ξ⁻μ⁺μ⁻) is measured to be (118 ± 24 ± 7 ± 38 ) × 10⁻⁸, where the first uncertainty is statistical, the second is systematic and the third uncertainty arises from the normalisation branching fraction B(Ξb⁻→Ξ⁻J/ψ), whose uncertainty is dominated by SU(3)-breaking effects. In addition, the ratio of the branching fractions of the Ξb⁻→Ξ⁻ψ(2S) and the Ξb⁻→Ξ⁻J/ψ decays is measured to be 0.491 ± 0.041 ± 0.019.