Measurement of Z⁰ → bb̅ decays and the semileptonic branching ratio BR(b → ℓ + X)

Abstract

We have measured the decay properties of Z⁰ → bb̅ 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_bb = 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 λ_bb. The partial decay width of the Z⁰ into bb̅ has been measured to λ be λ_bb = 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 Z⁰ → bb̅ events has been determined to be ℓx_Eℓ = 0.686 ± 0.006 (stat.) ± 0.016 (syst.).