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

The observation and measurement of the time-dependence of B0d↔B0d mixing are described. The B0d meson is reconstructed in final states that contain a D∗− and an ℓ+, where the b flavour of the B0d 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 B0d. From a sample of 556 D∗±ℓ∓ candidates reconstructed in the OPAL data collected during 1990–1993, the B0d↔B0d oscillation frequency is measured to be Δmd = 0.508 ± 0.075 (stat) ±0.025(syst) ps−1 giving an oscillation parameter of χd = 0.73 ± 0.11 (stat) ±0.08 (syst), where 0.076 of the systematic error on χd arises from the uncertainty on the B0d lifetime.