Electroweak parameters of the z0 resonance and the standard model

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SEARCHES FOR NEW PARTICLES IN Z-DECAYS USING THE ALEPH DETECTOR

During 1989 and 1990 over 200 000 hadronic and leptonic events, corresponding to about 8.5 pb-1 of data, were collected in a scan of the Z peak using the ALEPH detector at the e+ e- collider, LEP. These data, at the highest centre-of-mass energy available to date in e+ e- collisions, have allowed a broad range of searches for new particles and new phenomena to be performed in a mass range significantly higher than previously attainable. The searches performed by ALEPH for Higgs bosons, supersymmetric particles and leptoquarks, and for evidence of compositeness are reviewed. No positive signals have been observed but a comprehensive set of mass and coupling limits is presented. Branching ratio limits are given for a number of "rare" Z decays

Improved measurements of electroweak parameters from Z decays into fermion pairs

The properties of the Z resonance are measured on the basis of 190000 Z decays into fermion pairs collected with the ALEPH detector at LEP. Assuming lepton universality, M(Z) = (91.182 +/- 0.009exp +/- 0.020L;P) GeV, GAMMA(Z) = (2484 +/- 17) MeV, sigma(had)0 = (41.44 +/- 0.36) nb, and GAMMA(had)/GAMMA(ll) = 21.00 +/- 0.20. The corresponding number of light neutrino species is 2.97 +/- 0.07. The forward-backward asymmetry in leptonic decays is used to determine the ratio of vector to axial-vector coupling constants of leptons: g(V)2(M(Z)2)/g(A)2(M(Z)2) = 0.0072 +/- 0.0027. Combining these results with ALEPH results on quark charge and bbBAR asymmetries, and tau-polarization, sin2 theta(W)(M(Z)2) = 0.2312 +/- 0.0018. In the context of the Minimal Standard Model, limits are placed on the top-quark mass

MEASUREMENT OF THE ABSOLUTE LUMINOSITY WITH THE ALEPH DETECTOR

We report on the absolute luminosity measurement performed with the ALEPH detector at LEP. The systematic errors of the measurements in 1990 are estimated to be 0.6% (experimental) and 0.3% (theoretical)

Measurement of B - anti-B mixing at the Z

From more than 175 000 hadronic Z decays observed with the ALEPH detector at LEP, we select 823 events with pairs of leptons in the final state. From these we measure χ, the probability thata b hadron which is observed to decay originated as a hadron. We find χ=0.132−0.026+0.027