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 distribution xE (b) for mesons containing a b quark is compared to the predictions of different fragmentation models. The mean value of xE (b) is found to be 〈xE (b)〉 = 0.715 ± 0.007(stat) ± 0.013(syst).