First observation of the beauty baryon Λb in the decay channel Λb→J/ψΛ at the CERN proton-antiproton collider

Abstract We report on the first observation of the beauty baryon Λ b in an exclusive decay channel at the CERN p p collider. Using 4.7 pb −1 of muon data collected in the 1988/89 collider runs we reconstruct 16±5 Λ b 's in the decay mode Λ b → J ψΛ above a background of 9±1 events, corresponding to a significance of about five standard deviations. We measure the Λ b mass to be m Λ b =5640±50±30 MeV/ c 2 . Using the beauty cross-section measured by UA1 we deduce for the product of the production fraction and branching ratio f Λ b Br (Λ b → J ψΛ )=(1.8±1.0)×10 -10 . Our sample contains three-muon event in which the beauty particle opposite to the Λ b is tagged by the third muon. We also observe an indication of a signal in the decay channel B 0 → J ψ K 0∗ with a significance of three standard deviations.