Measurement of B0B0 mixing in hadronic Z0 decays: OPAL collaboration

Abstract From a sample of approximately 135 000 hadronic Z 0 decays recorded with the OPAL detector, 1 536 events were selected with two lepton candidates, either electrons or muons. A signal for B 0  B 0 mixing was observed using the sign of the lepton charge to tag the charge of the b quark in decaying b-flavoured hadrons. A flavour discriminating variable was constructed from the lepton momentum and its component perpendicular to the jet axis. By fitting the fraction of events in which the two lepton charges are of the same sign, as a function of this variable, the average mixing parameter was measured to be χ = 0.145 +0.041 -0.035 ± 0.018, where the first error is statistical and the second is systematic.