Using a data sample of 17.7 $\rm pb^{ -1}$ collected at 3.773 GeV with the BES-II detector at the BEPC, the cross sections for $D^0 \bar D^{ 0}$ and $D^+D^-$ productions at 3.773 GeV have been measured. From the data sample about 33 $\rm pb^{ -1}$ taken around 3.773 GeV, 3 purely leptonic decay events of $D^+ \to \mu^+\nu$ are observed. The branching fraction and the decay constant are measured to be $BF(D^+ \to \mu^+\nu)=(0.120^{ +0.092+0.010}_{ -0.063-0.009}) %$ and $f_D=(365^{ +121+32}_{ -113-28})$ MeV. The semileptonic decay branching fractions for $D^0 \to K^-e^+\nu$, $D^0 \to \pi^-e^+\nu$ and $D^+ \to \bar K^0 e^+\nu$ are measured to be $BF(D^0 \to K^-e^+\nu_e)=(3.52 \pm 0.36\pm 0.25)%$, $BF(D^0 \to \pi^-e^+\nu_e)=(0.36 \pm 0.14\pm 0.03)%$ and $BF(D^+ \to \bar K^0 e^+\nu_e)=(8.64 \pm 1.51\pm 0.72)%$. The vector form factors are determined to be $|f^K_+(0)| = 0.75 \pm 0.04 \pm 0.03$ and $|f^{ \pi}_+(0)| = 0.76 \pm 0.15 \pm 0.06$. The ratio of the two form factors is extracted to be $|f^{ \pi}_+(0)/f^K_+(0)|= 1.01 \pm 0.20 \pm 0.08$. The ratio of the decay widths is measured to be $\Gamma(D^0 \to K^-e^+\nu)/\Gamma(D^+ \to \bar K^0 e^+\nu) =1.04\pm 0.21 \pm 0.08$. From the data sample of about 27 $\rm pb^{ -1}$, the evidence of $\psi(3770) \to J/\psi \pi^+\pi^-$ non-${ D \bar D}$ decay is observed. The branching fraction is determined to be $BF(\psi(3770) \to J/\psi \pi^+\pi^-)=(0.34\pm 0.14 \pm 0.08)%$, corresponding to the partial width of $\Gamma(\psi(3770) \to J/\psi \pi^+\pi^-) = (80 \pm 32 \pm 21)$ keV.
Categories: hep-ex