Observation of Two New Excited ${\mathrm{\Xi }}_b^0$ States Decaying to ${\mathrm{\Lambda }}_b^0{K}^{-}{\pi }^{+}$

Two narrow resonant states are observed in the ${\mathrm{\Lambda }}_b^0{K}^{-}{\pi }^{+}$ mass spectrum using a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the LHCb experiment and corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$. The minimal quark content of the ${\mathrm{\Lambda }}_b^0{K}^{-}{\pi }^{+}$ system indicates that these are excited ${\mathrm{\Xi }}_b^0$ baryons. The masses of the ${\mathrm{\Xi }}_b(6327{)}^0$ and ${\mathrm{\Xi }}_b(6333{)}^0$ states are $m[{\mathrm{\Xi }}_b(6327{)}^0]=6327.28_{-0.21}^{+0.23}\pm 0.12\pm 0.24$ and $m[{\mathrm{\Xi }}_b(6333{)}^0]=6332.69_{-0.18}^{+0.17}\pm 0.03\pm 0.22\mathrm{MeV}$, respectively, with a mass splitting of $\mathrm{\Delta }m=5.41_{-0.27}^{+0.26}\pm 0.12\mathrm{MeV}$, where the uncertainties are statistical, systematic, and due to the ${\mathrm{\Lambda }}_b^0$ mass measurement. The measured natural widths of these states are consistent with zero, with upper limits of $\mathrm{\Gamma }[{\mathrm{\Xi }}_b(6327{)}^0]<2.20(2.56)$ and $\mathrm{\Gamma }[{\mathrm{\Xi }}_b(6333{)}^0]<1.60(1.92)\mathrm{MeV}$ 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 $1D$ ${\mathrm{\Xi }}_b^0$ resonances.

Figure 1

Invariant mass distributions of ${\mathrm{\Lambda }}_b^0$$K$$\pi$ candidates from (left) RS and (right) WS samples. The fit projections are overlaid. The black points with error bars correspond to the data, and the blue line shows the total fit projection. Individual fit components are listed in the legend.

Figure 2

Signal yields of the (left) ${\mathrm{\Xi }}_b(6327{)}^0$ and (right) ${\mathrm{\Xi }}_b(6333{)}^0$ states determined by mass fits to data samples in 5 MeV slices of the ${\mathrm{\Lambda }}_b^0$$\pi$ mass spectrum. The black points with error bars correspond to the yields of the ${\mathrm{\Xi }}_b(6327{)}^0$ or ${\mathrm{\Xi }}_b(6333{)}^0$ states, the blue solid lines are the total fit projections. Each individual component of the fit model is indicated in the legend.