B. Bloch-Devaux

Measurement of the charged particle multiplicity distribution in hadronic Z decays

The charged particle multiplicity distribution of hadronic Z decays was measured on the peak of the Z resonance using the ALEPH detector at LEP. Using a model independent unfolding procedure the distribution was found to have a mean 〈n〉=20.85±0.24 and a dispersion D=6.34±0.12. Comparison with lower energy data supports the KNO scaling hypothesis in the energy range GeV. At GeV the shape of the multiplicity distribution is well described by a log-normal distribution, as predicted from a cascading model for multi-particle production. The same model also successfully describes the energy dependence of the mean and width of the multiplicity distribution. A next-to-leading order QCD prediction in the framework of the modified leading-log approximation and local parton-hadron duality is found to fit the energy dependence of the mean but not the width of the charged multiplicity distribution, indicating that the width of the multiplicity distribution is a sensitive probe for higher order QCD or non-perturbative effects.

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

The aim of this paper is to present the French approach to Information and Communication, and to sketch out some arguments pro and con for their amalgamation into a unique scientific body. Since its creation in 1975, the French academic field of Information-Communication has proved several advantages in the development of a new scientific corpus, but also some drawbacks. These are going to be reviewed and the question will be posed on the opportunity to generalize that model or to abandon it. The research concludes that a dichotomy between information and communication is certainly not representative of the French field of information and communication; it would rather be a continuum or a multi-polar space. Furthermore it is suspected that Anglo-Saxon separation of information science from communication science is not clear either. International comparison and research program in information – communication are advocated.

Production and decay of charmed mesons at the Z resonance

In a sample of 190 000 hadronic Z decays, three signals of charm production are observed: two from the exclusive decays and one in the transverse-momentum distribution of soft hadrons relative to the nearest jet. The features of these signals are in good agreement with expectations based on the standard model and previous measurements of the branching fractions. The number of per hadronic decay of the Z is measured to be (5.11±0.34) × 10−3, and the branching ratio B(D0 → K−π+) is (3.62 ± 0.34 ± 0.44)%. Charm hadronization has been studied. The average fraction of the beam energy carried by the D∗ meson is found to be 〈XE〉c = 0.504−0.017+0.013±0.008, and implications of the measurements on the pseudoscalar-to-vector production ratio of charmed mesons are discussed.

Measurements of mean lifetime and branching fractions of b hadrons decaying to J/PSI

From a data sample of 450 000 hadronic events recorded with the ALEPH detector at LEP, 92±10 events are observed containing a J/ψ meson decaying to μ+μ− or e+e−. From these data the measured inclusive branching fraction for a b flavoured hadron to decay to a J/ψ is BR(b→ J/ψX) = (1.21±0.13 (stat.)±0.08 (syst.))%, and the average b hadron lifetime in the events tagged with a J/ψ is τb = 1.35+0.19−0.17±0.05 ps. Five events are observed consistent with the exclusive decay B± → J/ψK± and from these events the exclusive branching fraction is measured to be BR(B± → J/ψK±) = (0.22±0.10±0.02)%. Upper limits for other exclusive branching ratios are given.

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.

Measurement of isolated photon production in hadronic Z decays

The production of high energy isolated photons in hadronic Z decays is measured with the ALEPH detector at LEP using a sample of 180 000 hadronic events. Such photons are mainly radiated by quarks, thus giving direct insights into the early parton showering mechanism. The observed rate is compared with a QCD calculation of final state radiation from quarks.

Evidence for b baryons in Z decays

Evidence for {Lambda}{sub b} baryon production at LEP using ALEPH data is reported. In 450,000 hadronic Z decays recorded with the ALEPH detector at LEP, the yields of {Lambda}l{sup {minus}} and {Lambda}{sub c}{sup +}l{sup {minus}} combinations have been measured, providing the first evidence for {Lambda}{sub b} baryons in Z decays and measurement of {Lambda}{sub b} semileptonic decay rate. Updated results using data recorded with the ALEPH detector in 1992 shall also be presented.

Measurement of

Abstract Results on B 0 − B 0 mixing in e + e − annihilation at LEP are reported. A new method is used, where the charge of one b quark is tagged by a high- p , high- p ⊥ electron or muon, and the charge of the other is extracted from the momentum-weighted average of the charges of jet fragments. Based on the analysis of 180 000 hadronic Z decays produced in the ALEPH detector, ⨍ d χ d +0.72⨍ s χ s =0.113±0.018( stat. )±0.027( syst. ) is obtained. Combining this result with the ALEPH dipleton measurement yields χ =0.129±0.022.

B\bar{B}

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: , .

mixing at the Z using a jet-charge method

Using the data accumulated at LEP in 1989 and 1990 with the ALEPH detector, the inclusive and exclusive branching ratios of the τ lepton have been measured assuming lepton universality inZ0 decays. The inclusive branching fractions for the τ 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 τ→3 hadrons+vτ andτ→hadron+2π0vτ, 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.