Observation of a new Xi(b) baryon
The observation of a new b baryon via its strong decay into Ξ − b π + (plus charge conju-gates) is reported. The measurement uses a data sample of pp collisions at √ s = 7 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 5.3 fb − 1 . The known Ξ − b baryon is reconstructed via the decay chain Ξ − b → J/ ψ Ξ − → µ + µ − Λ 0 π − , with Λ 0 → p π − . A peak is observed in the distribution of the difference between the mass of the Ξ − b π + system and the sum of the masses of the Ξ − b and π + , with a signiﬁcance exceeding 5 standard deviations. The mass difference of the peak is 14.84 ± 0.74 (stat.) ± 0.28 (syst.) MeV. The new state most likely corresponds to the J P = 3/2 + companion of the Ξ b .