53 research outputs found

    Update of electroweak parameters from Z decays

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    Based on 520000 fermion pairs accumulated during the first three years of data collection by the ALEPH detector at LEP, updated values of the resonance parameters of the Z are determined to be M(Z) = (91.187 +/- 0.009) GeV, GAMMA(Z) = (2.501 +/- 0.012) GeV, sigma(had)0 = (41.60 +/- 0.27) nb, and R(l) = 20.78 +/- 0.13. The corresponding number of light neutrino species is N(v) = 2.97 +/- 0.05. The forward-backward asymmetry in lepton-pair decays is used to determine the ratio of vector to axial-vector couplings of leptons: g(V)2(M(Z)2)/g(A)2 (M(Z)2) = 0.0052 +/- 0.00 1 6. Combining this with ALEPH measurements of the b and c quark asymmetries and tau polarization gives sin^2(theta(W))eff = 0.2326 +/- 0.0013. Assuming the minimal Standard Model, and including measurements of M(W)/M(Z) from p(p)over bar colliders and neutrino-nucleon scattering, the mass of the top quark is M(top)=156 +/-22/25 +/-17/22 Higgs GeV

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

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    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

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

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    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 the strong coupling constant using tau decays

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    The strong coupling constant is determined from the leptonic branching ratios, the lifetime, and the invariant mass distribution of the hadronic final state of the τ lepton, using data accumulated at LEP with the ALEPH detector. The strong coupling constant measurement, αs(mτ2) = 0.330±0.046, evolved to the Z mass yields αs(MZ2) = 0.188±0.005. The error includes experimental and theoretical uncertainties, the latter evaluated in the framework of the Shifman, Vainshtein and Zakharov (SVZ) approach. The method allows the non-perturbative contribution to the hadronic decay rate to be determined to be 0.3±0.5%

    Searches for the standard Higgs boson

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    A data sample corresponding to about 100000 hadronic Z decays collected by ALEPH at LEP has been used to search for the standard Higgs boson produced in the reaction e+e- -> H0 Z*. No indication for any signal was found, and a 95% CL lower limit on the Higgs boson mass has been set at 41.6 GeV

    Measurement of prompt photon production in hadronic Z decays

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    The production of isolated photons in hadronicZ 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

    Search for a nonminimal Higgs boson produced in the reaction e+ e- ---> h Z*

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    A data sample corresponding to 1.23 million hadronic Z decays collected by the ALEPH detector at LEP has been searched for signals of the production of a non-minimal CP-even Higgs boson h in the reaction e+e- --> hZ*. The h decay modes considered were: those of the minimal standard model Higgs boson, with modified branching ratios; decays into a pair of CP-odd Higgs bosons A; and decays into invisible final states. Only one event was found, a very acoplanar e+e- pair which could originate from the standard model background process e+e- --> e+e- nu (nu) over bar. Upper limits for the cross-section of the reaction e+e- --> hZ* have been derived as a function of m(h), the mass of the Higgs boson h. In the case of invisible decays, the 95% CL lower limit on m(h) is 65 GeV/c(2) for a production cross-section equal to that of a minimal standard model Higgs boson. When combined with previous ALEPH results on the reaction e+e- --> hA, these cross-section upper limits exclude a domain in the (m(h), M(A)) plane of the MSSM such that, if invisible h and A decays can be neglected, 95% CL lower limits of 44 and 21 GeV/c(2) result for m(h) and m(A), respectively, independent of the other parameters of the model

    Measurement of the anti-B0 and B- meson lifetimes

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    The lifetimes of the and B− mesons have been measured with the ALEPH detector at LEP. Semileptonic decays of and B− mesons were partially reconstructed by identifying events containing a lepton with an associated meson. The proper time of the B meson was estimated from the measured decay length and the momentum and mass of the D-lepton system. A fit to the proper time of 77 D∗+ℓ− and 77 D0ℓ− candidates, combined with a constraint on the lifetime ratio () arising from the relative rates of observed D∗+ℓ− and D0ℓ− events, yielded the following lifetimes:

    Properties of hadronic Z decays and test of QCD generators

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    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
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