Observation of Two New Excited Ξ 0 b States Decaying to Λ 0 b K − π +

Two narrow resonant states are observed in the Λ0bK−π+ 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  fb−1. The minimal quark content of the Λ0bK−π+ system indicates that these are excited Ξ0b baryons. The masses of the Ξb(6327)0 and Ξb(6333)0 states are m[Ξb(6327)0]=6327.28+0.23−0.21±0.12±0.24 and m[Ξb(6333)0]=6332.69+0.17−0.18±0.03±0.22  MeV, respectively, with a mass splitting of Δm=5.41+0.26−0.27±0.12  MeV, where the uncertainties are statistical, systematic, and due to the Λ0b mass measurement. The measured natural widths of these states are consistent with zero, with upper limits of Γ[Ξb(6327)0]<2.20(2.56) and Γ[Ξb(6333)0]<1.60(1.92)  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 Ξ0b resonances.