18 research outputs found

    Measurement of lambda(b) production and lifetime in Z0 hadronic decays

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    Evidence for the production of b-flavoured baryons in Z0 decays is reported from the analysis of 365 000 hadronic events collected by the DELPHI detector. Assuming that most of the weakly decaying b flavoured baryons produced by the Z0 are Λb, the observed signal of (Λ-+Λl+) pairs, where l is a high transverse momentum lepton in the same jet as the Λ, leads to a production rate: f(b→Λb)×Br(Λb→ Λlν X) = (0.41±0.13±0.09)%. Using a sample of 18 reconstructed Λb decay vertices, the lifetime is determined to be τ(Λb) = (1.04+0.48-0.38±0.10) ps.0info:eu-repo/semantics/publishe

    A study of the decays of tau leptons produced on the Z resonance at LEP

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    From the analysis of a data sample corresponding to an integrated luminosity of 4.63 pb-1 taken during the 1990 run of LEP at centre of mass energies between 88.2 GeV an 94.2 GeV, the tau decays {Mathematical expression} and their charge conjugates have been studied. The following branching ratios have been measured; {Mathematical expression}, {Mathematical expression}, Br(τ- → π- (K-)vτ)=11.9±0.7±0.7%, BR (τ- → ρ- vτ)= 22.4±0.8±1.3%, in good agreement with world averages. The measured electronic and muonic branching ratios lead to a measurement of the strong coupling constant, αs (mτ) = 0.26-0.12+0.09. Extrapolating the αs value from mτ to mZ yields αs (mZ) = 0.109-0.028+0.012. The average polarization Pτ of taus produced in Z → τs τs decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes is Pτ=-0.24±0.07. This value of Pτ gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of υτ/aτ = 0.12 ± 0.04, and hence a value of the effective electroweak mixing parameter sin2 θW(mZ2). © 1992 Springer-Verlag

    MEASUREMENT OF THE TRIPLE-GLUON VERTEX FROM 4-JET EVENTS AT LEP

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    From the combined data of 1990 and 1991 of the DELPHI experiment at LEP, 13057 4-jet events are obtained and used for determining the contribution of the triple-gluon vertex. The relevant variables are the generalized Nachtmann Reiter angle theta(NR)* and the opening angle of the two least energetic jets. A fit to their two-dimensional distribution yields C(A)/C(F)=2.12+/-0.35 and N(C)/N(A)=0.46+/-0.19, where C(A)/C(F) is the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung from quarks, and N(C)/N(A), the ratio of the number of quark colours to the number of gluons. This constitutes a convincing model-independent proof of the existence of the triple-gluon vertex, since its contribution is directly proportional to C(A)/C(F). The results are in agreement with the values expected from QCD: C(A)/C(F)=2.25, and N(C)/N(A)=3/8

    Determination of alpha-s using the next-to-leading log approximation of QCD

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    A new measurement of αs is obtained from the distributions in thrust, heavy jet mass, energy-energy correlation and two recently introduced jet broadening variables following a method proposed by Catani, Trentadue, Turnock and Webber. This method includes the full calculation of O(s 2) terms and leading and next-to-leading logarithms resummed to all orders of αs. The analysis is based on data taken with the DELPHI detector at LEP during 1991. I its found that the inclusion of the resummed leading and next-to-leading logarithms reduces the scale dependence of αs and allows an extension of the fit range towards the infrared limit of the kinematical range. The combined value for αs obtained at the scale μ2=MZ 2 is: {Mathematical expression} © 1993 Springer-Verlag

    Invariant mass dependence of particle correlations in hadronic final states from the decay of the Z0

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    Two-particle correlations in invariant mass are studied separately for like-sign and unlike-sign pions. Strong correlations are observed only at small masses. The correlations decrease very rapidly for like-sign pions and slowly for unlike-sign pions, manifesting different particle dynamics. The like-sign correlations exhibit a power-law behaviour as a function of mass, compatible with the previously observed intermittency effect. The predictions of the parton shower model (JETSET 7.3) deviate significantly from the data. However, good agreement is obtained with the mass correlations, both for the unlike-sign and for the like-sign pairs if the production of the η′ and ρ0 mesons is reduced and Bose-Einstein correlations are included in the model. The value of the Bose-Einstein correlation strength is consistent with unity for pions which are produced directly or in the decays of short-lived resonances. © 1994 Springer-Verlag.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    Charged particle multiplicity distributions for fixed number of jets in Z0 hadronic decays

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    The multiplicity distributions of charged particles in full phase space and in restricted rapidity intervals for events with a fixed number of jets measured by the DELPHI detector are presented. The data are well reproduced by the Lund Parton Shower model and can also be well described by fitted negative binomial distributions. The properties of these distributions in terms of the clan model are discussed. In symmetric 3-jet events the candidate gluon jet is found not to be significantly different in average multiplicity than the mean of the other two jets, thus supporting previous results of the HRS and OPAL experiments. Similar results hold for events generated according to the LUND PS and to the HERWIG models, when the jets are defined by the JADE jet finding algorithm. The method seems to be insensitive for measuring the color charge ratio between gluons and quarks. © 1992 Springer-Verlag.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    A Measurement of D meson production in Z0 hadronic decays

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    A study of the fragmentation properties of charm and bottom quarks into D mesons is presented. From 263 700 Z0 hadronic decays collected in 1991 with the DELPHI detector at the LEP collider, D0, D+ and D*+ are reconstructed in the modes K-π+, K-π+K+ and D0π+ followed by D0→K-π+, respectively. The fractional decay widths {Mathematical expression} are determined, and first results are presented for the production of D mesons from {Mathematical expression} and {Mathematical expression} events separately. The average energy fraction of D*± in charm quark fragmentation is found to be 〈XE(D*)〉c=0.487± 0.015 (stat)±0.005 (sys.). Assuming that the fraction of Ds and charm-baryons produced at LEP is similar to that around 10 GeV, the Z0 partial width into charm quark pairs is determined to be Γc/Γh=0.187±0.031 (stat)±0.023 (sys). The probability for a b quark to fragment into {Mathematical expression} or b-baryons is inferred to be 0.268±0.094 (stat)±0.100 (sys) from the measured probability that it fragments into a {Mathematical expression} or B-. © 1993 Springer-Verlag.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe

    A Precision measurement of the average lifetime of B hadrons

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    The average lifetime of B hadrons was measured using data collected with the DELPHI detector at the LEP collider during 1991 and 1992. The measurement was performed using two different anayses. The first method was an improvement on a previous technique, which used charged particle impact parameter distributions. This analysis measured an average lifetime for B hadrons of {Mathematical expression} The second method was based on a new technique, which used inclusively reconstructed secondary vertices to measure {Mathematical expression} Taking into account both the statistical and systematic corelations, the results of these two methods were combined to give an average lifetime for B hadrons of {Mathematical expression} © 1994 Springer-Verlag

    Production rate and decay lifetime measurements of B(s)0 mesons at LEP using D(s) and phi mesons

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    The study of the properties of inclusive production of Ds mesons and of events in which a φ{symbol} and a muon are present in the same jet provides two independent measurements of the probability, fs w, for a heavy quark to hadronize into a strange B or D meson. The data sample analysed corresponds to 243,000 hadronic Z0 decays. The combined value of these measurements is fs w=0.19±0.06±0.08. From the flight distance distributions of Ds and of (φ{symbol}-lepton) secondary vertices, with the lepton emitted at high transverse momentum relative to the jet axis, two values are obtained for the Bs 0 meson lifetime. Combining these measurements with a previous result based on the study of Ds-μ events, the Bs 0 meson lifetime is measured to be: 0.96±0.37 ps. © 1994 Springer-Verlag

    Evidence for B(s)0 meson production in Z0 decays

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    Seven unambiguous events out of a sample of 270 000 Z0 decays, contain in the same jet a Ds meson and a muon at large transverse momentum relative to the jet axis. These events are direct evidence for Bs 0 meson production in hadronic Z0 decays. The production rate of these events, relative to all hadronic Z0 decays is (18 ± 8) × 10-5, this number including the relevant branching fractions of the Bs 0 and Ds. The value of the Bs 0 meson lifetime relative to the average B meson lifetime is measured to be 0.8 ± 0.4.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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