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, R(c) of the partial decay width of the Z into c (c) over bar quarks to the total hadronic width. One method uses the inclusive p and pT spectra of electrons. The other two use a double-tagging technique to measure R(c) independently of the charm-tagging efficiency. In one, the tagging procedure uses the reconstruction of high momentum charmed mesons (D*(+), D(+) and D(0)) to detect the charm quarks. In the other, a charm quark is tagged from the D*'s reconstructed in the D*(+) --> pi(+)D(0) decay channel and the other from the slow pion from the D*(-) --> pi(-)(D) over bar(0) decay. The combination of these measurements leads to R(c) = Gamma(Z --> c (c) over bar)/Gamma(Z --> hadrons) = 0.1681 +/- 0.0054(stat) +/- 0.0062(syst)