Measurement of the Z-boson branching fraction into hadrons containing bottom quarks

We use the Mark II detector to study Z decays into bottom quark-anti-quark pairs, leading to the production of bottom hadrons. The Z bosons are formed in e+e- annihilation at the SLC at center-of-mass energies between 89 and 93 GeV. We identify events containing semileptonic decays of bottom hadrons by detecting isolated leptons, i.e leptons with high transverse momenta relative to the nearest hadronic jet. Using isolated electrons and muons, we measure the B-hadron semileptonic branching ratio times the fraction of hadronic Z decays which contain bottom hadrons, B(B → X ell ν)·Γ(Z → b bar b)/Γ(Z → had) = 0.025 -0.009+0.100 ± 0.005, where we have listed the statistical errors followed by the systematic error. Assuming B(B → X(ell)ν) = 11% ± 1%, we measure Γ(Z → b bar b)/Γ(Z → had) = 0.23 -0.09+0.11, in good agreement with the standard-model prediction of 0.22. We find Γ(Z → b bar b) = 0.40 -0.16+0.19 GeV. 83 refs., 34 figs., 19 tabs