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

Abstract.

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. 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. 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*}