First observation of excited $\hat{I}{\text{c◯}}_b^{-}$ states

Abstract

We report four narrow peaks in the $\hat{I}{ž}_b^0{K}^{-}$ 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 \begin{align*} m(Ω_b(6316)^-) &= 6315.64\pm0.31\pm0.07\pm0.50 {\rm MeV}, \\ m(Ω_b(6330)^-) &= 6330.30\pm0.28\pm0.07\pm0.50 {\rm MeV}, \\ m(Ω_b(6340)^-) &= 6339.71\pm0.26\pm0.05\pm0.50 {\rm MeV}, \\ m(Ω_b(6350)^-) &= 6349.88\pm0.35\pm0.05\pm0.50 {\rm MeV}, \end{align*}where the uncertainties are statistical, systematic and the last is due to the knowledge of the $\hat{I}{ž}_b^0$ 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 $\hat{I}“(\hat{I}{\text{c◯}}_b(6316{)}^{-})<2.8$ MeV, $\hat{I}“(\hat{I}{\text{c◯}}_b(6330{)}^{-})<3.1$ MeV and $\hat{I}“(\hat{I}{\text{c◯}}_b(6340{)}^{-})<1.5$ MeV. The natural width of the $\hat{I}{\text{c◯}}_b(6350{)}^{-}$ peak is $1.4_{-0.8}^{+1.0}\pm 0.1$ MeV, which is 2.5$\ddot{I}ƒ$ from zero and corresponds to an upper limit of 2.8 MeV. The peaks have local significances ranging from 3.6$\ddot{I}ƒ$ to 7.2$\ddot{I}ƒ$. After accounting for the look-elsewhere effect, the significances of the $\hat{I}{\text{c◯}}_b(6316{)}^{-}$ and $\hat{I}{\text{c◯}}_b(6330{)}^{-}$ peaks are reduced to 2.1$\ddot{I}ƒ$ and 2.6$\ddot{I}ƒ$ respectively, while the two higher mass peaks exceed 5$\ddot{I}ƒ$. The observed peaks are consistent with expectations for excited $\hat{I}{\text{c◯}}_b^{-}$ resonances.

7 pages, 2 figures, All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2019-042.html