Measurement of Cross Sections for $D^0 {\bar D}^0$ and $D^+D^-$   Production in $e^+e^-$ Annihilation at $\sqrt{s}=3.773$ GeV
  The cross sections for $D^0 {\bar D}^0$ and $D^+D^-$ production at 3.773 GeV have been measured with BES-II detector at BEPC. These measurements are made by analyzing a data sample of about 17.3 $\rm pb^{-1}$ collected at the center-of-mass energy of 3.773 GeV. Observed cross sections for the charm pair production are radiatively corrected to obtain the tree level cross section for $D\bar D$ production. A measurement of the total tree level hadronic cross section is obtained from the tree level $D \bar D$ cross section and an extrapolation of the $R_{uds}$ below the open charm threshold. 
BES Collaboration