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

Using a sample of about 1.46 million hadronic Z decays collected between 1991 and 1993 with the ALEPH detector at LEP, the energy distribution of the B0 and B+/- mesons produced at the Z resonance is measured by reconstructing semileptonic decays B -> lνlD(X) or B -> lνlD*+ (X). The charmed mesons are reconstructed through the decay modes D0 -> K- π+, D0 -> K- π+ π- π+, D+ -> K- π+ π+ and D*+ -> D0π+. The neutrino energy is estimated from the missing energy in the lepton hemisphere. Accounting for B* and B** production, the shape of the scaled energy distributionx(b)E for mesons containing a b quark is compared to the predictions of different fragmentation models. The mean value ofx(b)E is found to be <x(b)E> = 0.715 +/- 0.007 (stat) +/- 0.013 (syst).