Measurement of the partial decay width of the Z into $c\overline{c}$ quarks

Hadronic Z decays collected with the \A\ detector have been analysed to measure the partial decay width of the Z into $c \bar c$ quarks. Three methods have been investigated. The first two use a double tagging technique. In the first one the tagging procedure consists of the full reconstruction of high momentum D mesons ( $D^{*+}$, $D^+$ and $D^0$) to detect the two charmed quarks produced in the decay $Z \rightarrow c \bar c$. In the second one, a charm quark is tagged with the $D^*$'s reconstructed in the $D^{*+} \to \pi^+ D^0$ decay channel and the other one with the slow pion of the $D^{*-} \to \pi^- \bar D^0$ decay. The last analysis uses inclusive electrons and takes advantage of the high purity $b$ sample obtained using lifetime tagging. A $R_c$ measurement is obtained from a fit to the \qp\ and \qpt\ spectra. A combination of these measurements gives the preliminary result: \begin{eqnarray} \mathrm{R_c} & = & 0.1683 \, \pm 0.0091 \nonumber \end{eqnarray