Direct Measurement of Charmed<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>D</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>D</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>Semileptonic Branching Ratios

Measurements of the semileptonic branching ratios of charmed ${D}^{+}$ and ${D}^{0}$ mesons are presented, from data collected at the $\ensuremath{\psi}(3770)$ in the Mark III detector at SPEAR. From a sample of events kinematically selected as charmed ${D}^{+}{D}^{\ensuremath{-}}$ or ${D}^{0}{\overline{D}}^{0}$ pairs, the branching ratios $R({D}^{+}\ensuremath{\rightarrow}{e}^{+}+X)=(17.0\ifmmode\pm\else\textpm\fi{}1.9\ifmmode\pm\else\textpm\fi{}0.7)%$ and $R({D}^{0}\ensuremath{\rightarrow}{e}^{+}+X)=(7.5\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\pm\else\textpm\fi{}0.4)%$ are obtained. The ratio of these measurements can be interpreted as the ratio of ${D}^{+}$ and ${D}^{0}$ lifetimes $\frac{{\ensuremath{\tau}}^{+}}{{\ensuremath{\tau}}^{0}}={2.3}_{\ensuremath{-}0.4\ensuremath{-}0.1}^{+0.5+0.1}$ by neglect of the contribution of Cabibbo-suppressed decays to the total decay width.