A search for neutral Higgs bosons has been performed using the full sample of Z 0 decays collected by the OPAL detector at LEP up to 1995. The data were taken at centre-of-mass energies between 88 GeV and 95 GeV and correspond to an integrated luminosity of approximately 160 pb 1. The present search addresses the processes Z 0 !H 0 Z and h 0 Z , where H 0 is the Higgs boson predicted by the Standard Model and h 0 the lightest neutral scalar Higgs boson predicted in the framework of the Minimal Supersymmetric Standard Model. For the virtual Z 0 boson, Z , the following decay c hannels are considered: Z ! , e + e and +. One candidate event

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The spectral functions of the vector current and the axial-vector current have been measured in hadronic τ decays using the OPAL detector at LEP. Within the framework of the Operator Product Expansion a simultaneous determination of the strong coupling constant α s , the non-perturbative operators of dimension 6 and 8 and of the gluon condensate has been performed. Different perturbative descriptions have been compared to the data. The Contour Improved Fixed Order Perturbation Theory gives α s (m 2 τ ) = 0.348±0.009 exp ±0.019 theo at the τ -mass scale and α s (m 2 Z ) = 0.1219±0.0010 exp ±0.0017 theo at the Z 0 -mass scale. The values obtained for α s (m 2 Z ) using Fixed Order Perturbation Theory or Renormalon Chain Resummation are 2.3 % and 4.1 % smaller, respectively. The 'running' of the strong coupling between s 0 ≃ 1.3 GeV 2 and s 0 = m 2 τ has been tested from direct fits to the integrated differential hadronic decay rate R τ (s 0 ). A test of the saturation of QCD sum rules at the τ -mass scale has been performed.

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An event sample enriched in semileptonic decays of b hadrons is selected using an inclusive lepton selection from approximately 3.0 million hadronic Z 0 decays collected with the OPAL detector. This sample is used to investigate B meson oscillations by reconstructing a proper decay time for the parent of each lepton, using a jet charge method to estimate the production flavour of this parent, and using the lepton charge to tag the decay flavour. We measure the mass difference between the two B 0 d mass eigenstatesFor the B 0 s system, we find ∆m s > 3.1 ps −1 at the 95% confidence level. This limit varies only a little if alternative limit setting approaches are adopted. Regions at higher ∆m s values are also excluded with some methods for setting the limit.By studying the charge symmetry of the B 0 d mixing structure, we are able to constrain possible CP and CPT violating effects. We measure the CP violation parameter Re ǫ B = −0.006 ± 0.010 ± 0.006 and the indirect CPT violating parameter Im δ B = −0.020 ± 0.016 ± 0.006 . If we invoke CPT symmetry, then we obtainRe ǫ B = 0.002 ± 0.007 ± 0.003 .

The production rates of D * ± mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary cc pairs in hadronic Z 0 decays have been measured at LEP using almost 4.4 million hadronic Z 0 decays collected with the OPAL detector between 1990 and 1995. Using a combination of several charm quark tagging methods based on fully and partially reconstructed D * ± mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into D * ± mesons have been found to be f (b → D * + X) = 0.173 ± 0.016 ± 0.012 and f (c → D * + X) = 0.222 ± 0.014 ± 0.014 .The fraction of cc events in hadronic Z 0 decays, Γ cc /Γ had = Γ(Z 0 → cc)/Γ(Z 0 → hadrons), is determined to be Γ cc /Γ had = 0.180 ± 0.011 ± 0.012 ± 0.006 .In all cases the first error is statistical, and the second one systematic. The last error quoted for Γ cc /Γ had is due to external branching ratios.

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This paper describes a search for the Standard Model Higgs boson using data from e + e − collisions collected at center-of-mass energies of 161, 170 and 172 GeV by the OPAL detector at LEP. The data collected at these energies correspond to integrated luminosities of 10.0, 1.0 and 9.4 pb −1 , respectively. The search is sensitive to the main final states from the process in which the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electron, muon or tau leptons. One candidate event is observed, in agreement with the Standard Model background expectation. In combination with previous OPAL searches at center-of-mass energies close to the Z 0 resonance and the revised previous OPAL searches at 161 GeV , we derive a lower limit of 69.4 GeV for the mass of the Standard Model Higgs boson at the 95% confidence level.