Observation of two new Ξ^-_b baryon resonances

Two structures are observed close to the kinematic threshold in the Ξ⁰_bπ^- 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=(1/2)⁺ and J^P=(3/2)⁺ states, denoted Ξ^′_b^- 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^-)-m(Ξ⁰_b)-m(π^-)=3.653±0.018±0.006 MeV/c², m(Ξ^∗_b^-)-m(Ξ⁰_b)-m(π^-)=23.96±0.12±0.06 MeV/c², Γ(Ξ^∗_b^-)=1.65±0.31±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^-)<0.08 MeV at 95% confidence level. Relative production rates of these states are also reported.