Observation of the time dependence of B0d−B0d mixing

Abstract The time dependence of B 0 d − B 0 d oscillations has been observed using events with a d ∗ , decaying into a D 0 π + , and a lepton in opposite hemispheres. The time dependence of the oscillations is derived from the displacement of the D 0 vertex and the D ∗ -lepton charge correlation. From a fit for the oscillation frequency the mass difference of the B 0 d states is measured: Δm = [3.44 +0.65 −0.70 (stat.) +0.26 −0.20 (syst.)] × 10 −4 eV/ c 2 .