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, \({\rm R_c}\), of the partial decay width of the Z into \({\rm c\bar c}\) quarks to the total hadronic width. One method uses the inclusive \(p\) and \(p_{\perp}\) spectra of electrons. The other two use a double-tagging technique to measure \({\rm R_c}\) independently of the charm-tagging efficiency. In one, the tagging procedure uses the reconstruction of high momentum charmed mesons (\({\rm D^{*+}}\), \({\rm D^+}\) and \({\rm D^0}\) to detect the charm quarks. In the other, a charm quark is tagged from the \({\rm D^*}\)'s reconstructed in the \({\rm D^{*+} \to \pi^+ D^0}\) decay channel and the other from the slow pion from the \({\rm D^{*-} \to \pi^- \bar{D}^0}\) decay. The combination of these measurements leads to \({\rm R_c = \Gamma(Z \to c\bar c) / \Gamma(Z \to hadrons)}) = 0.1681\pm0.0054({\rm stat})\pm0.0062({\rm syst})\).