Measurements of inclusive semileptonic branching fractions of b hadrons inZ(0) decays

A measurement of inclusive semileptonic branching fractions of b hadrons pr oduced in Z(0) decays is presented. An enriched Z(0) --> b (b) over bar sam ple is obtained with a lifetime flavour-tagging technique. The leptonic eve nts are then selected from, this sample, and classified according to their origin, which is determined by comparing the distribution of several kinema tic variables using artificial neural network techniques. Using 3.6 million multihadronic events collected with the OPAL detector at energies near the Z(0) resonance, the values BR(b --> lX) = (10.84 +/- 0.09 (stat.) +/- 0.21 (syst.) (+0.21)(-0.13) (mod el)) BR(b --> c --> lX) = (8.39 +/- 0.15 (stat.) +/- 0.22 (syst.) -(+0.33)(0.29) (model)). are measured, where b denotes all weakly decaying b hadrons and l represents either e or mu The second error includes all experimental syst ematic uncertainties whereas the last error is due to uncertainties in mode lling of the lepton momentum spectrum in semileptonic decays and b quark fr agmentation. The average fraction of the beam energy carrried by the weakly decaying b hadron, < x(E)> is measured to be < x(E)> = 0.709 +/- 0.003 (stat.) +/- 0.003 (syst.) +/- 0.013 (model) where the modelling error is dominated by the choice of b fragmentation mod el. The agreement between data and various semileptonic decay models and fr agmentation functions is also investigated.