Measurements of Z 0→bb̄ decays and the semileptonic branching ratio BR(b→l+X)

We have measured the decay properties of Z 0→bb̄ from a study of inclusive muon and electron events. The average (e and μ) b-quark semileptonic branching ratio has been determined to be BR(b→l+X)=0.119±0.003(stat.)±0.006(syst.), assuming the standard model prediction of Γ bb̄=378±3 MeV. From the ratio of the number of dilepton events to single lepton events, we find BR(b→l+X)=0.113±0.010(stat.)±0.006(syst.), without assumptions on Γ bb̄. The partial decay width of the Z 0 into bb̄ has been measured to be Γ bb̄=385±7(stat.)±11(syst.) MeV with an additional 19 MeV error from the uncertainty in BR(b→l+X). The average fractional energy of bottom hadrons in Z 0→bb̄ events has been determined to be 〈X E〉=0.686± 0.006(stat.)±0.016(syst.).