Measurement of the semileptonic b branching fractions and average b mixing parameter in Z decays

The semileptonic branching fractions for primary and cascade b decays BR(b --> l(-)), BR(b --> c --> l(+)) and BR(b --> (c) over bar --> l(-) were measured in hadronic Z decays collected by the DELPHI experiment at LEP. The sample was enriched in b decays using the lifetime information and various techniques were used to separate leptons from direct or cascade b decays. By fitting the momentum spectra of di-leptons in opposite jets, the average b mixing parameter <(<chi>)over bar> was also extracted. The following results have been obtained: BR(b --> l(-)l(-)) = (10.70 +/- 0.08(stat) +/- 0.21(syst)(+0.44)(-0.30)(model))% BR(b --> c --> l(+)) = (7.98 +/- 0.22(stat) +/- 0.21(syst)(-0.20)(+0.14)(model))% BR(b --> (c) over bar --> l(-)) = (1.61 +/- 0.20(stat) +/- 0.17(syst)(-0.44)(+0.30)(model))% <(<chi>)over bar> = 0.127 +/- 0.013(stat) +/- 0.005(syst) +/- 0.004(model).