Properties of hadronic events in e+e− annihilation at S(1/2)S^(1/2) = 91-GeV

We report on properties of hadronic events from e+e− annihilation observed by the ALEPH detector at the large Electron Positron Collider at CERN. The center-of-mass energy was . Measured distributions of the global event-shape variables sphericity, aplanarity, thrust and minor value, and of the inclusive variables xp, p⊥in, p⊥out and y are presented. We measure a mean charged multiplicity in hadronic events of 〈Nch〉=21.3±0.1 (statistical)±0.6 (systematic). The data are in good agreement with QCD-based models which use the leading-logarithm approximation, and are less well described by a model using O(αs2) QCD

Search for supersymmetric particles using acoplanar charged-particle pairs from Z0 decays

We have performed a search for supersymmetric particles using acoplanar pairs of oppositely-charged particles in decays of the Z0. In 0.53 pb−1 of integrated luminosity near the Z0 peak, we observe two events where approximately four are expected from background, allowing limits to be extended on combined photino and slepton masses, and also on combined photino and chargino masses

Heavy flavour production in Z decays

From an analysis of inclusive leptons in data collected by the ALEPH detector at LEP, we measure the fractions of b and c events in hadronic Z decays. The b fraction times semileptonic branching ratio is measured to be . Assuming a b semileptonic branching ratio of 0.102 ± 0.010 gives , in good agreement with the standard model prediction of 0.217. The c fraction times semileptonic branching ratio is measured to be . Assuming a c semileptonic branching ratio of 0.090 ± 0.013 gives , in agreement with the standard model prediction of 0.171

Search for neutral Higgs bosons from supersymmetry in Z decays

The light scalar Higgs boson h and the pseudoscalar Higgs boson A of the minimal supersymmetric standard model have been searched for in the processes e+ e- -> h f fbar and e+e−->hA using data collected by ALEPH at the LEP e+e− collider, with center of mass energies at and near the Z peak. Using a variety of signatures adapted to various mass ranges for h and A, we have excluded a large domain in the parameter space. For large values of nu_2/nu_1, the ratio of the vacuum expectation values of the two Higgs fields, the whole range from 0 to 38.8 GeV is excluded for Mh and MA at 95% CL

Search for decays of the Z0 into a photon and a pseudoscalar meson

A search is reported for decays of the Z0 into π0γ, ηγ and η′ (958)γ in e+e− collisions using data collected during a scan around the Z0 mass. In order to search for π0γ final states, in which the two photons from the π0 decay are unresolved, the production of pairs of high-energy electromagnetic clusters is studied. The data are compared with the expectations from the pure QED process e+e− → γγ, and a 95% confidence level upper limit on the branching ratio of the Z0 into π0γ of 4.9 × 10−4 is derived. For η′γ, the decay modes of the mesons that contain two charged particles are largely free from QED background. These modes are used to place upper limits of 4.6 × 10−4 and 2.2 × 10−4 on the corresponding branching ratios of the Z0

Measurement of electroweak parameters from Z decays into Fermion pairs

We report on the properties of theZ resonance from 62 500Z decays into fermion pairs collected with the ALEPH detector at LEP, the Large Electron-Positron storage ring at CERN. We findM Z=(91.193±0.016exp±0.030LEP) GeV, Γ Z =(2497±31) MeV, σ had 0=(41.86±0.66)nb, and for the partial widths Γinv=(489±24) MeV, Γhad(1754±27) MeV, Γ ee =(85.0±1.6)MeV, Γ μμ =(80.0±2.5) MeV, and Γττ=(81.3±2.5) MeV, all in good agreement with the Standard Model. Assuming lepton universality and using a lepton sample without distinction of the final state we measure Γu=(84.3±1.3) MeV. The forward-backward asymmetry in leptonic decays is used to determine the vector and axial-vector weak coupling constants of leptors,g v 2(M Z 2)=(0.12±0.12)×10−2 andg A 2(M Z 2)=0.2528±0.0040. The number of light neutrino species isN ν=2.91±0.13; the electroweak mixing angle is sin2θW(M Z 2)=0.2291±0.0040

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)

Search for the neutral Higgs bosons of the MSSM and other two doublet models

Large radiative corrections modify the predictions of the minimal supersymmetric extension of the standard model (MSSM) sufficiently for the constraints on this model, formerly derived from the searches for the CP-even h and for the CP-odd A neutral Higgs bosons, to be invalidated. In particular, the new h → AA decay mode has to be considered. The results presented here have been obtained from a data sample corresponding to about 185000 hadronic Z decays collected by the ALEPH experiment at LEP. No indication for any signal of the reactions or e+e− → hA was found. A domain in the (mh,mA) plane is thus excluded at 95% CL in a large class of two-Higgs-doublet models. More restrictive results are derived in the MSSM, with one loop radiative corrections to the Higgs potential taken into account. It is found that mh > 41 GeV/c2 and mA > 20 GeV/c2 at 95% CL when the other parameters of the model are varied in their allowed ranges

Measurement of the b ---> tau- anti-tau-neutrino X branching ratio

Using a missing energy tag, evidence is presented for the decay b ---> tau- anti-tau-neutrino X, and its branching ratio is measured to be (4.08 ± 0.76 ± 0.62)%