Measurement of the Bd0 meson oscillation frequency

Time-dependent Bd0-Bd0 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 Bd0 meson oscillation frequency corresponds to a mass difference Δmd between the two Bd0 mass eigenstates of Δmd = (0.496−0.051+0.055 (stat) ± 0.043 (syst)) ps−1.