Determination of $P(c \to D^{*+})$ and $BR(c \to l^{+})$ at LEP 1

The probability $P(c \rightarrow D^{*+})$ that a charm quark fragments into a $D^{*+}$ meson and the $c \rightarrow l^+$ semileptonic branching fraction were measured in $Z^0\ $ decay into $c\bar{c}$ events. \par % A double tag method based % on the detection of exclusively reconstructed $D$ mesons % accompanied by a slow pion or a lepton in the opposite hemisphere was used.\par From the analysis of 3.5 Million $Z^0\ $ events collected from 1992 to 1995, a sample of charm meson decays with $81\%$ $c\bar{c}$ purity was selected. % In this sample, a signal of $2371\pm140$ low momentum pions % originating from a $D^{*\pm}$ decay in the opposite hemisphere % was observed. The product of the $c \rightarrow D^{*+}$ fragmentation probability times the $D^{*+}\rightarrow D^0\pi^+$ branching fraction was measured to be: \begin{eqnarray*} P(c \rightarrow D^{*+})\cdot BR(D^{*+}\rightarrow D^0\pi^+) = 0.174\pm0.010(stat) \pm0.004(syst) \; . \end{eqnarray*} Using the world averaged value for $BR(D^{*+}\rightarrow D^0\pi^+)$, the fragmentation probability is inferred: \begin{eqnarray*} P(c \rightarrow D^{*+}) = 0.255\pm0.015(stat) \pm0.006(syst) \pm0.005 (syst.BR) \; . \end{eqnarray*} From the same sample, $1828\pm 51$ identified leptons in the opposite hemisphere were selected. From this sample, the charm semileptonic branching fraction was measured to be: \begin{eqnarray*} BR(c \rightarrow l^+) = 0.0958\pm0.0042(stat) \pm0.0028(syst) \; . \end{eqnarray*}