F. Badaud

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.

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.

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

Abstract Three different techniques are used to measure the mean decay length of the τ lepton with a high precision vertex detector in a sample of 11 800 τ pairs coming from Z decays, collected in1991 by ALEPH at LEP. Events in which both τs decay into one charged track are analyzed using two largely independent methods. Displaced vertices in three-prong decays yield another independent measurement. The derived lifetime is 295.5 ± 5.9 ± 3.1 fs, using m τ = 1777.1 ± 0.5 MeV/ c 2 . Including previous (1989–1990) ALEPH measurements, the combined τ lifetime is 294.7 ± 5.4 ± 3.0 fs.

Observation of the semileptonic decays of BS and Λb hadrons at LEP

The reactions have been used to search for the neutral Higgs bosons h and A of the MSSM in the case where the CP-odd A is lighter than 2mμ, taking into account the large h → AA decay branching ratio. No signal was found in the data sample collected until the end of 1991 by the ALEPH experiment at LEP. For tan β ⩾, mA < 2mμ is excluded at 95% CL for any mh.

A precise measurement of the

We report evidence for a narrow structure, X(5568), in the decay sequence X(5568)→B_{s}^{0}π^{±}, B_{s}^{0}→J/ψϕ, J/ψ→μ^{+}μ^{-}, ϕ→K^{+}K^{-}. This is evidence for the first instance of a hadronic state with valence quarks of four different flavors. The mass and natural width of this state are measured to be m=5567.8±2.9(stat)_{-1.9}^{+0.9}(syst)  MeV/c^{2} and Γ=21.9±6.4(stat)_{-2.5}^{+5.0}(syst)  MeV/c^{2}. If the decay is X(5568)→B_{s}^{*}π^{±}→B_{s}^{0}γπ^{±} with an unseen γ, m(X(5568)) will be shifted up by m(B_{s}^{*})-m(B_{s}^{0})∼49  MeV/c^{2}. This measurement is based on 10.4  fb^{-1} of pp[over ¯] collision data at sqrt[s]=1.96  TeV collected by the D0 experiment at the Fermilab Tevatron collider.