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

Abstract.

Time-dependent ${{\rm B^{0}}\mbox{-}\bar{{\rm B}}^{0}}$ 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 ${{\rm B_d^0}}$ meson oscillations is \[ \Delta m_d = 0.444 \pm 0.040 {\rm ps}^{-1}. \]