First Observation of Excited Omega_{b}^{-} States.

We report four narrow peaks in the Xi_{b}^{0}K^{-} mass spectrum obtained using pp collisions at center-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of 9 fb^{-1} recorded by the LHCb experiment. Referring to these states by their mass, the mass values are m[Omega_{b}(6316)^{-}]=6315.64+/-0.31+/-0.07+/-0.50 MeV, m[Omega_{b}(6330)^{-}]=6330.30+/-0.28+/-0.07+/-0.50 MeV, m[Omega_{b}(6340)^{-}]=6339.71+/-0.26+/-0.05+/-0.50 MeV, m[Omega_{b}(6350)^{-}]=6349.88+/-0.35+/-0.05+/-0.50 MeV, where the uncertainties are statistical, systematic, and the last is due to the knowledge of the Xi_{b}^{0} mass. The natural widths of the three lower mass states are consistent with zero, and the 90% confidence-level upper limits are determined to be Gamma[Omega_{b}(6316)^{-}]<2.8 MeV, Gamma[Omega_{b}(6330)^{-}]<3.1 MeV and Gamma[Omega_{b}(6340)^{-}]<1.5 MeV. The natural width of the Omega_{b}(6350)^{-} peak is 1.4_{-0.8}^{+1.0}+/-0.1 MeV, which is 2.5sigma from zero and corresponds to an upper limit of 2.8 MeV. The peaks have local significances ranging from 3.6sigma to 7.2sigma. After accounting for the look-elsewhere effect, the significances of the Omega_{b}(6316)^{-} and Omega_{b}(6330)^{-} peaks are reduced to 2.1sigma and 2.6sigma, respectively, while the two higher mass peaks exceed 5sigma. The observed peaks are consistent with expectations for excited Omega_{b}^{-} resonances.