Improved measurement of the $B^0_d$--$\overline{B}^0_d$ Oscillation frequency

The time dependence of $B^0_d--\bar{B}^0_d$ oscillations is measured by the ALEPH experimentat LEP, using three techniques, two of which are updates of previous measurements. In all cases the charge of the decaying $b$ quark and its decay time are measured in one hemisphere of the event the quark charge at production is tagged mainly using the opposite hemisphere. The first method uses the charge correlation between a $D^{*-}$ and a leptonin the opposite hemisphere; if no leptonis present, the produced quark charge is determined from the hemisphere charges. In the second method, the decay time is measured using a leptonin one hemisphere and the initial state is inferred from the opposite-hemisphere jet charge. The third method uses a leptonin each hemisphere. The data used were collected from 1991 to 1994. The combined result is $\Delta m_d = 0.436\pm 0.033\, {\rm ps}^{-1}$