Measurement of the $B_{d}^{0} - \overline{B}_{d}^{0}$ oscillation frequency

Time-dependent ${\mathrm{B^{0}}\mbox{-}\bar{\mathrm{B}} ^{0}}$ mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of ${\mathrm{B_d^0}}$ meson oscillations is \begin{displaymath} {\Delta m_d} = 0.444 \pm 0.040 \ \mathrm{ps}^{-1}. \end{displaymath} \end{abstract} \end{document}