0000000000089087
AUTHOR
Qian Z
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(…
First Measurement of the Absolute Branching Fraction of Λ→pμ−ν¯μ
The absolute branching fraction of Λ→pμ−ν¯μ is reported for the first time based on an e+e− annihilation sample of 10×109 J/ψ events collected with the BESIII detector at s=3.097 GeV. The branching fraction is determined to be B(Λ→pμ−ν¯μ)=[1.48±0.21(stat)±0.08(syst)]×10−4, which is improved by about 30% in precision over the previous indirect measurements. Combining this result with the world average of B(Λ→pe−ν¯e), we obtain the ratio {[Γ(Λ→pμ−ν¯μ)]/[Γ(Λ→pe−ν¯e)]} to be 0.178±0.028, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of Λ→pμ−ν¯μ and Λ¯→p¯μ+νμ is also determined, and no evidence for CP violation is fo…
Measurements of mean lifetime and branching fractions of b hadrons decaying to J/ψ
From a data sample of 450 000 hadronic events recorded with the ALEPH detector at LEP, 92 +/- 10 events are observed containing a J/psi meson decaying to mu+mu- or e+e-. From these data the measured inclusive branching fraction for a b flavoured hadron to decay to a J/psi is BR(b --> J/psiX) = (1.21 +/- 0.13 (stat.) +/- 0.08 (syst.))%, and the average b hadron lifetime in the events tagged with a J/psi is tau(b) = 1.35(-0.17)+0.19 +/- 0.05 ps. Five events are observed consistent with the exclusive decay B+/- --> J/psiK+/- and from these events the exclusive branching fraction is measured to be BR(B+/- --> J/psiK+/-) = (0.22 +/- 0.10 +/- 0.02)%. Upper limits for other exclusive branching rat…
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 hadronic Z 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 alpha(s) 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 JETSET O(alpha(s)2) matrix elements (ME) even when the renormalization scale is optimized.
A measurement of the b baryon lifetime
In 451 000 hadronic Z0 decays, recorded with the ALEPH detector at LEP, the yields of lambdal- and lambdal+ combinations are measured. Semileptonic decays of b baryons result in a signal of 122 +/- 18(stat.)+22/-23(syst.) lambdal- combinations. From a fit to the impact parameter distribution of the leptons in the lambdal- sample, the lifetime of b baryons is measured to be 1.12+0.32/-0.29(stat.) +/- 0.16(syst.) ps.
Evidence for b baryons in Z decays
Abstract In 160 000 hadronic Z decays recorded with the ALEPH detector at LEP, the yields of Λl− and Λl+ combinations have been measured. The observed excess of Λl− over Λl+ of 53±13 is interpreted as evidence for b baryons and their semileptonic decay. Assuming that three body decay processes such as Λ b →Λ c + l − v dominate the semileptonic decay of b baryons, this ex cess corresponds to a product branching ratio BR ( b →Λ b )· BR (Λ b →Λ c + l − v )· BR (Λ c + →ΛX) = (0.95±0.22 ( stat. )±0.21( syst. ))% , where Λb and Λc+ denote the bottom and charm baryons respectively.
Search for CP violation in ZmF ττ
Abstract Using the 18.8 pb−1 of data accumulated at LEP in 1990 and 1991 with the ALEPH detector, a direct test of neutral current CP-invariance is performed by a search for CP-odd correlations in Z decays to τ pairs where both τ decay modes are identified. No evidence for CP-violation is observed. The weak dipole moment of the τ has been measured to be dτ(mZ) = (1.3 ± 1.4 ± 0.1) × 10−17 e·cm which results in an upper limit on the weak dipole moment of |dτ(mZ)| ⩽ 3.7 × 10−17 e·cm with 95% confidence level.
MEASUREMENT OF THE PRODUCTION-RATES OF ETA AND ETA' IN HADRONIC-Z DECAYS
The decays eta --> gammagamma and eta' --> etapi+pi- have been observed in hadronic decays of the Z produced at LEP. The fragmentation functions of both the eta and eta' have been measured. The measured multiplicities for x > 0.1 are 0.298 +/- 0.023 +/- 0.021 and 0.068 +/- 0.018 +/- 0.016 for eta and eta' respectively. While the fragmentation function for the eta is fairly well described by the JETSET Monte Carlo, it is found that the production rate of the eta' is a factor of four less than the corresponding prediction.
AN INVESTIGATION INTO INTERMITTENCY
The results of an investigation based on ALEPH data, e+ e- --> hadrons at square-root s = 91 GeV, into fluctuations in rapidity space are presented. It is found that the behaviour of the factorial moments is well represented by the Lund parton shower model. An estimate is made of the scale of fluctuations needed to describe the data. Differential moments are introduced and are used to demonstrate that within the average represented by the traditional factorial moments the pattern of fluctu10.1007/BF01483869ations is itself a strong function of rapidity. This pattern is shown to be primarily associated with the emission of hard gluons. The implied structure between the hadron clusters and pa…