Observation of two new excited ${\Xi }_b^0$ states decaying to ${\Lambda }_b^0{K}^{-}{\pi }^{+}$

Abstract

Two narrow resonant states are observed in the ${\Lambda }_b^0{K}^{-}{\pi }^{+}$ mass spectrum using a data sample of proton-proton collisions at a center-of-mass energy of 13$\mathrm{TeV}$, collected by the LHCb experiment and corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$. The minimal quark content of the ${\Lambda }_b^0{K}^{-}{\pi }^{+}$ system indicates that these are excited ${\Xi }_b^0$ baryons. The masses of the ${\Xi }_b(6327{)}^0$ and ${\Xi }_b(6333{)}^0$ states are $m({\Xi }_b(6327{)}^0)=6327.28_{-0.21}^{+0.23}\pm 0.12\pm 0.24\mathrm{MeV}$ and $m({\Xi }_b(6333{)}^0)=6332.69_{-0.18}^{+0.17}\pm 0.03\pm 0.22\mathrm{MeV}$, respectively, with a mass splitting of $\Delta m=5.41_{-0.27}^{+0.26}\pm 0.12\mathrm{MeV}$, where the uncertainties are statistical, systematic and due to the ${\Lambda }_b^0$ mass measurement. The measured natural widths of these states are consistent with zero, with upper limits of $\Gamma ({\Xi }_b(6327{)}^0)<2.20(2.56)\mathrm{MeV}$ and $\Gamma ({\Xi }_b(6333{)}^0)<1.60(1.92)\mathrm{MeV}$ at a $90\%(95\%)$ credibility level. The significance of the two-peak hypothesis is larger than nine (five) Gaussian standard deviations compared to the no-peak (one-peak) hypothesis. The masses, widths and resonant structure of the new states are in good agreement with the expectations for a doublet of 1$D$ ${\Xi }_b^0$ resonances.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2021-025.html

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