Measurement of the fraction of hadronic Z decays into charm quark pairs

The full statistics of hadronic Z decays collected with the ALEPH detector are analysed to measure, by three methods, the ratio, Rc, of the partial decay width of the Z into cc̄ quarks to the total hadronic width. One method uses the inclusive p and p⊥ spectra of electrons. The other two use a double-tagging technique to measure Rc independently of the charm-tagging efficiency. In one, the tagging procedure uses the reconstruction of high momentum charmed mesons (D*+, D+ and D0) to detect the charm quarks. In the other, a charm quark is tagged from the D*'s reconstructed in the D*+ → π+D0 decay channel and the other from the slow pion from the D*- → π-D̄0 decay. The combination of these measurements leads to Rc = Γ(Z → cc̄)/Γ(Z → hadrons)) = 0.1681 ± 0.0054(stat) ± 0.0062(syst).