Measurement of the effective b quark fragmentation function at the Z resonance

Abstract Using a sample of about 1.46 million hadronic Z decays collected between 1991 and 1993 with the ALEPHdetector at LEP, the energy distribution of the B 0 and B± mesonsproduced at the Z resonance is measured by reconstructing semileptonic decays B→ lν l D(X) or B → lν l D ∗+ (X) . The charmed mesons are reconstructed through the decay modes D 0 → K − π + , D 0 → K − π + π − π + , D + → K − π + π + and D ∗+ → D 0 π + . The neutrino energy is estimated from the missing energyin the lepton hemisphere. Accounting for B ∗ and B ∗∗ production, the shape of the scaled energy distribution x E (b) for mesons containing a b quark is compared to the predictions of different fragmentation models. The mean value of x E (b) is found to be 〈 x E (b) 〉 = 0.715 ± 0.007(stat) ± 0.013(syst).