Observation of two new ${\Xi }_b^{-}$ baryon resonances

Abstract

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 ${\Xi }_b^{\ast -}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m({\Xi }_b^{\prime -})-m({\Xi }_b^0)-m({\pi }^{-})=3.653\pm 0.018\pm 0.006$ MeV$/c^2$, $m({\Xi }_b^{\ast -})-m({\Xi }_b^0)-m({\pi }^{-})=23.96\pm 0.12\pm 0.06$ MeV$/c^2$, $\Gamma ({\Xi }_b^{\ast -})=1.65\pm 0.31\pm 0.10$ 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 $\Gamma ({\Xi }_b^{\prime -})<0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.

17 pages, 2 figures

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