Observation of a New Excited Beauty Strange Baryon Decaying to Ξb−π+π−

Abstract

The Ξ−bπ+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 140fb−1. The ground state Ξ−b is reconstructed via its decays to J/ψΞ−and J/ψΛK−. A narrow resonance, labeled Ξb(6100)−, is observed at a Ξ −bπ+π−invariant mass of 6100.3±0.2(stat)±0.1(syst)±0.6(Ξ−b)MeV, where the last uncertainty reflects the precision of the Ξ−b baryon mass. The upper limit on the Ξb(6100)−natural width is determined to be 1.9 MeV at 95% confidence level. The low Ξb(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 Ξc baryon states, the new Ξb(6100)−resonance and its decay sequence are consistent with the orbitally excited Ξ−b baryon, with spin and parity quantum numbers JP=3/2−.