Production of strange B-baryons decaying into Ξ∓−ℓ∓ pairs at LEP

An excess of events containing, in a jet, a samesign Ξ^∓−ℓ^∓ pair as compared to those with an oppositesign Ξ^∓−ℓ^± pair has been observed in an analysis of 1.7 million hadronic Z⁰ decays collected by the DELPHI detector at LEP between 1991 and 1993 inclusive. The probability for this signal to come from non B-baryon decays is less than 5×10⁻⁴. The measured production fraction corresponds to: per lepton species, averaged for electrons and muons and assuming that the two channels have an equal contribution. Semileptonic decays of Λ_b baryons can account for less than 10% of these events and the major part of the signal has to originate from Ξ_b semileptonic decays. Using the subsample of these events where the Ξ^∓ trajectory has been measured in the Vertex Detector, the lifetime of B-baryons producing a Ξ^∓ in their semileptonic decay final state is found to be: $$\tau (B - baryon \to \Xi ^ - \ell ^ - X) = 1.5_{ - 0.4}^{ + 0.7} \pm 0.3ps.$$