Search for D0-bar D0 Mixing Using Doubly Flavor Tagged Semileptonic Decay Modes

We have searched for D0 - (bar D)0 mixing in D*+ → π+D0 decays with D0 → K(*)e? in a sample of e+e- → 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 x 10-4. Using a frequentist method, we set corresponding 68% and 90% confidence intervals at (-5.6, 7.4) x 10-4 and (-13, 12) x 10-4, respectively