Measurement of theDSemileptonic Branching Ratio ine+e−Annihilation at theψ′′(3770)

We have observed the ${\psi }^{\prime \prime }$ resonance in the cross section for $e^{+}{e}^{-}\to \mathrm{hadrons}$ at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=3770\mathrm{}\pm 6.0$ MeV, of total width $\Gamma =24\mathrm{}\pm 5$ MeV and partial width to electron pairs ${\Gamma }_{\mathrm{ee}}=180\mathrm{}\pm 60$ eV. The cross section for hadronic events which contain anomalous electron provides both unambiguous evidence of $D$ semileptonic decays and a branching ratio measurement of (11±2)%.