Search for $D^0-{\bar D}^0$ Mixing Using Doubly Flavor Tagged Semileptonic Decay Modes

We have searched for $D^0 - \bar{D}^0$ mixing in $D^{*+} \to π^+ D^0$ decays with $D^0 \to K^{(*)} e ν$ in a sample of $e^+ e^- \to {c\bar c}$ events produced near 10.58 GeV. The charge of the slow pion from charged $D^*$ decay tags the charm flavor at production, and it is required to be consistent with the flavor of a fully reconstructed second charm decay in the same event. We observe 3 mixed candidates compared to 2.85 background events expected from simulation. We ascribe a 50% systematic uncertainty to this expected background rate. We find a central value for the mixing rate of $0.4 \times 10^{-4}$. Using a frequentist method, we set corresponding 68% and 90% confidence intervals at $(-5.6, 7.4) \times 10^{-4}$ and $(-13, 12) \times 10^{-4}$, respectively.

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