derived.leading to the limit Am, > 12 >< 10"* eV/cz (95% CL), from which (Am/I`), > 2.0 is Allowing a second frequency component for the Bf a high value for Am, is favoured, a mass difference for the B2 mass eigenstates Amd =·(3.3 fgji ;k0.7) >< 10'4 eV/c2 nature of Bgg; mixing. The frequency measured for the oscillation corresponds to function of the measured decay time, and clear evidence is seen for the time-dependent method. The fraction of events in which the leptons have the same charge is studied as a decay length of the b hadrons, and their momentum is determined using an energy-iiow dominantly from b decays; a topological vertexing technique is applied to measure the sides of the event and with high transverse momentum. The leptons are expected to be ALEPH experiment at LEP. Events are selected with two leptons present, on opposite B°FU oscillation is studied using almost a million hadronic Z decays collected by the and thus the measurement of both BQ and BQ mixing would constrain the matrix elements: the B3 and Bf are expected to be close to unity, however, with rather smaller uncertainty, quark mass, and other poorly determined hadronic factors. The ratios of such factors for Maskawa matrix elements Wd and W, respectively, but also on the as yet unmeasured top mass differences for the B3 and Bf have been calculated. They depend on the KobayashiIn the Standard Model mixing occurs via second-order W-exchange, and the expected

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