Measurement of the time dependence of B0d↔B0d mixing using a jet charge technique

Abstract The observation and measurement of the time-dependence of B 0 d ↔ B 0 d mixing are described. The B 0 d meson is reconstructed in final states that contain a D ∗− and an l + , where the b flavour of the B 0 d at decay time is tagged by the electric charge of the lepton. A new and efficient method, using a jet charge technique, is developed for identifying the b flavour of the produced B 0 d . From a sample of 556 D ∗± l ∓ candidates reconstructed in the OPAL data collected during 1990–1993, the B 0 d ↔ B 0 d oscillation frequency is measured to be Δm d = 0.508 ± 0.075 (stat) ±0.025(syst) ps −1 giving an oscillation parameter of χ d = 0.73 ± 0.11 (stat) ±0.08 (syst), where 0.076 of the systematic error on χ d arises from the uncertainty on the B 0 d lifetime.