Observation of two new $Ξ_b^-$ baryon resonances

Two structures are observed close to the kinematic threshold in the $Ξ_b^0 π^-$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content $bds$ are expected in this mass region: the spin-parity $J^P = \frac{1}{2}^+$ and $J^P=\frac{3}{2}^+$ states, denoted $Ξ_b^{\prime -}$ and $Ξ_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m(Ξ_b^{\prime -}) - m(Ξ_b^0) - m(π^{-}) = 3.653 \pm 0.018 \pm 0.006$ MeV$/c^2$, $m(Ξ_b^{*-}) - m(Ξ_b^0) - m(π^{-}) = 23.96 \pm 0.12 \pm 0.06$ MeV$/c^2$, $Γ(Ξ_b^{*-}) = 1.65 \pm 0.31 \pm 0.10$ MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of $Γ(Ξ_b^{\prime -}) < 0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.