Observation of a New Excited Beauty Strange Baryon Decaying to Ξ$^{-}$$_{b}$π$^{+}$π$^{-}$

The Ξ$\substack{-\\b}$π$^{+}$π$^{-}$ invariant mass spectrum is investigated with an event sample of proton-proton collisions at √s=13  TeV, collected by the CMS experiment at the LHC in 2016–2018 and corresponding to an integrated luminosity of 140  fb$^{-1}$. The ground state Ξ$\substack{-\\b}$ is reconstructed via its decays to J/ψΞ$^{-}$ and J/ψΛK$^{-}$. A narrow resonance, labeled Ξ$_{b}$ (6100) $^{-}$, is observed at a Ξ$\substack{-\\b}$ π$^{+}$π$^{-}$ invariant mass of 6100.3±0.2(stat)±0.1(syst)±0.6(Ξ$\substack{-\\b}$)  MeV, where the last uncertainty reflects the precision of the Ξ$\substack{-\\b}$ baryon mass. The upper limit on the Ξ$_{b}$(6100)$^{-}$ natural width is determined to be 1.9  MeV at 95% confidence level. The low Ξ$_{b}$(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 Ξ$_{c}$ baryon states, the new Ξ$_{b}$ (6100)$^{-}$ resonance and its decay sequence are consistent with the orbitally excited Ξ$\substack{-\\b}$ baryon, with spin and parity quantum numbers JP=3/2$^{-}$.