Measurement of the Bd0_{d}^{0} meson oscillation frequency

Time-dependent \Bd-\Bbd\ mixing is studied using 1.5 million hadronic Z decays collected by L3. Semileptonic B decays are selected by requiring at least one reconstructed lepton in both thrust hemispheres. Charge correlations between the tagged leptons are studied as a function of proper time. The proper time of the b-hadron decay is measured by reconstructing the production and decay vertices using a silicon microvertex detector. The measured \Bd\ meson oscillation frequency corresponds to a mass difference \Dmd\ between the two \Bd\ mass eigenstates of \begin{displaymath} \Dmd = \left( 0.496 ^{+ 0.055}_{- 0.051}\ \mathrm{(stat)} \pm{ 0.043}\ \mathrm{(syst)} \right)\ \mathrm{ps}^{-1}. \end{displaymath