Observation of two new $\Xi_b^-$ baryon resonances

Two structures are observed close to the kinematic threshold in the $\Xi_b^0\pi^-$ 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 $\Xi_b^{\prime -}$ and $\Xi_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be \begin{eqnarray*} m(\Xi_b^{\prime -}) - m(\Xi_b^0) - m(\pi^{-}) &=& 3.653 \pm 0.018 \pm 0.006~{\rm MeV}/c^2, \\ m(\Xi_b^{*-}) - m(\Xi_b^0) - m(\pi^{-}) &=& 23.96 \pm 0.12\pm 0.06~{\rm MeV}/c^2, \\ \Gamma(\Xi_b^{*-}) &=& 1.65 \pm 0.31 \pm 0.10~{\rm MeV}, \end{eqnarray*} 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 $\Gamma(\Xi_b^{\prime -}) < 0.08$ MeV at 95 % confidence level. Relative production rates of these states are also reported.