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

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 CONTACT INTERACTIONS IN THE REACTIONS E+E--]L+L- AND E+E--]GAMMA-GAMMA

Contact interactions are searched for using the differential cross sections for the reactions e+e- --> e+e-, e+e- --> mu+mu-, e+e- --> tau+tau- and e+e --> gammagamma measured at 12 energies around the Z peak and corresponding to about 20 pb-1 of cumulated luminosity. Four-fermion contact term models assuming various chiralities of lepton currents are fitted to the lepton data and lower limits on the energy scale LAMBDA of such terms are set at 95% c.l. The limits vary in the range 0.9-4.7 TeV, depending on the model and on the lepton flavour. The eegammagamma contact terms are searched for assuming various chiralities. Limits on the energy scale LAMBDA between 79 and 130 GeV are extracted from the data. The results are compared and combined with those reported at lower energies

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 PROMPT PHOTON PRODUCTION IN HADRONIC-Z DECAYS

The production of isolated photons in hadronic Z decays is measured with the ALEPH detector at LEP using a sample of 450 000 hadronic events. The corrected rate is given for several values of the minimum invariant mass squared cut between the photon and the jets. This measurement of final state radiation from the quarks is compared with the predictions of parton shower models JETSET, ARIADNE and HERWIG as well as with the predictions of QCD matrix element calculations

Observation of the semileptonic decays of B(s) and LAMBDA(b) hadrons at LEP

In 450 000 hadronic Z decays recorded with the ALEPH detector at LEP, the yields of Ds−ℓ+ and Λc+ℓ− combinations have been measured. 16.0 ± 4.3 Ds−ℓ+ combinations were observed in the Ds− → ππ− channel and 17.0 ± 4.5 combinations were observed in the channel. 21.0 ± 5.0 Λc+ℓ− combinations were observed, with the Λc+ reconstructed in the decay mode Λc+ → pK−π+. These events provide evidence for the decays . Assuming that the Bs and Λb semileptonic decays are dominantly three-body, these observed yields, after background subtraction, translate into the following product branching ratios: ,

MEASUREMENT OF TAU BRANCHING RATIOS

Using the data accumulated at LEP in 1989 and 1990 with the ALEPH detector, the inclusive and exclusive branching ratios of the tau-lepton have been measured assuming lepton universality in Z0 decays. The inclusive branching fractions for the tau-decay into one, three, and five charged particles have been determined to be (85.45 +/- 0.97)%, (14.35 +/- 0.48)%, and (0.10 +/- 0.05)%, respectively, in agreement with the world averages. New undetected decay modes are determined to have a branching fraction of less than 2.1% at 95% CL. The measured branching ratios for quasi-exclusive channels are slightly larger than, but consistent with the world averages, except for the modes tau --> 3 hadrons + nu(tau) and tau --> hadron + 2-pi(0)nu(tau), which are significantly larger. These latter branching ratios have been found to be (9.5 +/- 0.7)% and (10.2 +/- 1.1)%, respectively. The sum of all the measured quasi-exclusive branching ratios is (100.4 +/- 1.8)%. A fully exclusive analysis of modes with neutral pions shows no evidence for new photonic decay modes with a branching fraction limit of 3.4% at 95% CL

A STUDY OF BOSE-EINSTEIN CORRELATIONS IN E+E- ANNIHILATION AT 91 GEV

This paper describes a study of Bose-Einstein correlations made using the ALEPH detector at LEP. The correlations are found to enhance the two particle differential cross section for pairs of identical pions by a factor which can be roughly parametrized by R(Q) = 1 + lambda exp(-Q2-sigma(2)), where Q is the difference in the 3-momenta of the two pions in their centre of mass frame, lambda = 0.51 +/- 0.04 +/- 0.11 and sigma = 3.3 +/- 0.2 +/- 0.8 GeV-1, which corresponds to a source size of 0.65 +/- 0.04 +/- 0.16 fm. The large systematic errors on these results reflect their strong dependence on the choice of the reference sample used in the analysis. This problem is believed to occur primarily because of uncertainties in the rates of resonance production and a lack of knowledge about the pion-pion strong interaction. No significant correlations are seen amongst like-charged pion-kaon pairs

Properties of hadronic Z decays and test of QCD generators

Distributions are presented of event shape variables, jet roduction rates and charged particle momenta obtained from 53 000 hadronicZ decays. They are compared to the predictions of the QCD+hadronization models JETSET, ARIADNE and HERWIG, and are used to optimize several model parameters. The JETSET and ARIADNE coherent parton shower (PS) models with running αs and string fragmentation yield the best description of the data. The HERWIG parton shower model with cluster fragmentation fits the data less well. The data are in better agreement with JETSET PS than with JETSETO(αS2) matrix elements (ME) even when the renormalization scale is optimized