Search forD0D¯0Mixing in Semileptonic Decay Modes

We report the result of a search for $D^0\overline{D}{}^0$ mixing in the data from hadroproduction experiment E791 at Fermilab. We use the charge of the pion from the strong decay $D^{*+}\to D^0{\pi }^{+}$ (and charge conjugate) to identify the charm quantum number of the neutral $D$ at production, and the charge of the lepton and the kaon in the semileptonic decays $D^0\to \mathrm{Ke}\nu$ and $K\mu \nu$ to identify the charm at the time of decay. No evidence of mixing is seen. We set a 90% confidence level upper limit on mixing of $r<\mathrm{}0.50\mathrm{}\%$, where $r=\gamma (D^0\to \overline{D}{}^0\to K^{+}{l}^{-}{\overline{\nu }}_l)/\gamma (D^0\to K^{-}{l}^{+}{\nu }_l)$.