Results are reported of a study of neutral vector meson production in multihadronicZ0 decays in the OPAL experiment at LEP. Pions and kaons have been identified by specific ionisation energy loss andK±π∓ andK+K− mass spectra have been fitted, in bins of the scaled momentum variablexp, to combinations of resonance signals and non-resonant backgrounds. Rates are given forK*(892)° and ø(1020), and production cross sections are compared to the predictions of Monte Carlo models. Overall multiplicities have been determined as 0.76±0.07±0.06K*(892)° and 0.086±0.015±0.010 ø(1020) per hadronicZ0 decay (the quoted errors are respectively statistical and systematic). Momentum dependent distortions of the ππ mass spectra, possibly associated indirectly with Bose-Einstein effects, have prevented reliable measurement of the ρ(770)° cross section in this study.
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We present a measurement of the forward-backward charge asymmetry in hadronic decays of the Z 0 using data collected with the OPAL detector at LEP. The forward-backward charge asymmetry was measured using a weight function method which gave the number of forward events on a statistical basis. In a data sample of 448 942 hadronic Z 0 decays, we have observed a charge asymmetry of A h = 0.040±0.004 (stat.)±0.006 (syst.)±0.002 (B 0 B 0 mix.), taking into account the effect of B 0 B 0 mixing. In the framework of the standard model, this asymmetry corresponds to an effective weak mixing angle averaged over five quark flavours of sin 2 θ W = 0.2321 ± 0.0017 ( stat. ) ± 0.0027 ( syst. ) ± 0.0009 (B 0 B 0 mix.). The result agrees with the value obtained from the Z 0 line shape and lepton pair forward-backward asymmetry.
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The second systematic error is due to the uncertainty in the correction for B.BBAR mixing which had been applied to the data.
The second systematic error is due to the uncertainty in the correction for B.BBAR mixing which had been applied to the data.
The production of the octet and decuplet baryons Λ, Ξ − , Σ (1385) ± , Ξ(1530) 0 and Ω − and the corresponding antibaryons has been measured in a sample of 485 000 hadronic Z 0 decays. Results on differential and integrated cross sections are presented. The differential cross section of Λ baryons is found to be softer than the one predicted by the Jetset and Herwig Monte Carlo generators. The measured decuplet yields are found to disagree with the simple diquark picture where only one tuning parameter for spin 1 diquarks is used. Comparisons of the momentum spectra for Λ and Ξ − with the predictions of an analytical QCD formula are also presented.
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The decays η → γγ and η ′ → ηπ + π − have been observed in hadronic decays of the Z produced at LEP. The fragmentation functions of both the η and η ′ have been measured. The measured multiplicities for x > 0.1 are 0.298±0.023±0.021 and 0.068±0.016 for η and η ′ respectively. While the fragmentation function for the η is fairly well described by the JETSET Monte Carlo, it is found that the production rate of the η ′ is a factor of four less than the corresponding prediction.
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Additional 7 pct systematic error.
Additional 23 pct systematic error.
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.
Sphericity distribution.
Sphericity distribution.
Aplanarity distribution.
The value of the strong coupling constant,$$\alpha _s (M_{Z^0 } )$$, is determined from a study of 15 d
Differential jet mass distribution for the heavier jet using method T. The data are corrected for the finite acceptance and resolution of the detector and for initial state photon radiation.
Differential jet mass distribution for the jet mass difference using methodT. The data are corrected for the finite acceptance and resolution of the detec tor and for initial state photon radiation.
Differential jet mass distribution for the heavier jet using method M. The data are corrected for the finite acceptance and resolution of the detector and for initial state photon radiation.
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Three different methods are used for extraction Alphas value (see text for details). Systematical errors with C=HADR and C=THEOR are due to hadronization correction and theoretical uncertainties.
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NC, CF, and TF are the color factors for SU(N) group. For SU(3) they are equal to: NC = 3, CF = 4/3, and TF = 1/2.
We observe evidence for the production of b-flavoured baryons in decays of the Z 0 boson with the OPAL detector at LEP. We find 68 Λl − , Λ l + candidates in 458 583 hadronic Z 0 decays. We interpret this as a signal of 55 ± 9 +0.3 −3.1 events from the semi-leptonic decays of b baryons. Assuming weakly decaying b baryons produced in Z 0 decays are mostly Λ b particles, we measure the product branching ratio (Γ b b /Γ had ) f ( b →Λ b ) B (Λ b →Λl − v X ) , averaged over the electron and muon channels, to be (6.2±1.0±1.5)×10 −4 .
FD is considered as a quark fragmentation fraction. Charge conjugated state is understood.
Cross sections are presented for the inclusive production of Λ hyperons in electron-positron annihilations at s=29 GeV based on the full 291-pb−1 sample of data taken in the High Resolution Spectrometer experiment at the SLAC e+e− storage ring PEP. These results, and the associated correlation analyses, are consistent with the Lund model predictions with the strange diquark suppression ratio δ fixed at 0.59±0.10±0.18, as compared to the standard Lund value of 0.32. The Λ multiplicity has been found to be 0.182±0.020 per event. The opposite-strangeness multiplicity 〈nΛΛ¯〉 has been measured to be 0.046±0.020, whereas the like-strangeness multiplicity 〈nΛΛ+Λ¯Λ¯〉 is 0.009±0.028. A strong correlation is found between Λ's and Λ¯'s; when one is found in an event, the other is found in the same event with a probability that exceeds 50%.
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Extrapolate to full z interval using Lund fit.
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