Measurement of the time dependence of Bd0 ↔ Bd0 mixing using leptons and D∗± mesons

Data collected with the OPAL detector at LEP during 1990–1993 are used to measure the time dependence of Bd0 ↔ Bd0 mixing. From a sample of 153 ± 12 events with a charged D∗ and a lepton in the opposite hemisphere, we measure the Bd0↔Bd0 oscillation frequency to be Δmd = 0.57±0.11(stat.)±0.02(syst.) ps−1. This corresponds to an oscillation parameter of xd = ΔmdτBd0 = 0.82±0.16(stat.) ± 0.03(syst.) ± 0.09(syst. τB0d), where the second systematic uncertainty is due to the error in the measured Bd0 lifetime.