The ${\mathrm{\Xi }}_b^{-}{\pi }^{+}{\pi }^{-}$ invariant mass spectrum is investigated with an event sample of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$, collected by the CMS experiment at the LHC in 2016–2018 and corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$. The ground state ${\mathrm{\Xi }}_b^{-}$ is reconstructed via its decays to ${J/\psi \mathrm{\Xi }}^{-}$ and $J/\psi \mathrm{\Lambda }{K}^{-}$. A narrow resonance, labeled ${\mathrm{\Xi }}_b(6100{)}^{-}$, is observed at a ${\mathrm{\Xi }}_b^{-}{\pi }^{+}{\pi }^{-}$ invariant mass of $6100.3\pm 0.2(\mathrm{stat})\pm 0.1(\mathrm{syst})\pm 0.6({\mathrm{\Xi }}_b^{-})\mathrm{MeV}$, where the last uncertainty reflects the precision of the ${\mathrm{\Xi }}_b^{-}$ baryon mass. The upper limit on the ${\mathrm{\Xi }}_b(6100{)}^{-}$ natural width is determined to be $1.9\mathrm{MeV}$ at 95% confidence level. The low ${\mathrm{\Xi }}_b(6100{)}^{-}$ signal yield observed in data does not allow a measurement of the quantum numbers of the new state. However, following analogies with the established excited ${\mathrm{\Xi }}_c$ baryon states, the new ${\mathrm{\Xi }}_b(6100{)}^{-}$ resonance and its decay sequence are consistent with the orbitally excited ${\mathrm{\Xi }}_b^{-}$ baryon, with spin and parity quantum numbers $J^P=3/2^{-}$.

• Received 8 February 2021
• Revised 19 March 2021
• Accepted 23 April 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.252003

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