Showing papers by "Claude Leroy published in 1992"

A Study of charged particle multiplicities in hadronic decays of the Z0

Abstract: We present an analysis of multiplicity distributions of charged particles produced inZ 0 hadronic decays. The results are based on the analysis of 82941 events collected within 100 MeV of theZ 0 peak energy with the OPAL detector at LEP. The charged particle multiplicity distribution, corrected for initial-state radiation and for detector acceptance and resolution, was found to have a mean 〈n ch〉=21.40±0.02(stat.)±0.43(syst.) and a dispersionD=6.49±0.02(stat.)±0.20(syst.). The shape is well described by the Lognormal and Gamma distributions. A negative binomial parameterisation was found to describe the shape of the multiplicity distribution less well. A comparison with results obtained at lower energies confirms the validity of KNO(-G) scaling up to LEP energies. A separate analysis of events with low sphericity, typically associated with two-jet final states, shows the presence of features expected for models based on a stochastic production mechanism for particles. In all cases, the features observed in the data are well described by the Lund parton shower model JETSET.

A global determination of {Mathematical expression} at LEP

Abstract: The value of the strong coupling constant, $$\alpha _s (M_{Z^0 } )$$ , is determined from a study of 15 different observables in hadronicZ 0 and τ decays. The study includes global event shape variables, jet production rates, energy correlations, theZ 0 line shape and decay asymmetries and the hadronic branching fraction of τ-leptons. Differences between the αs values from the different observables can be consistently attributed to unknown higher order contributions to the calculations. These uncertaities may be parametrized by variations of the renormalization scale and of the parton virtuality to which the data are corrected, separately for each observable, resulting in a consistent description of the event shapes, jet rates and energy correlations with the value $$\alpha _s (M_{Z^0 } ) = 0.122_{ - 0.005}^{ + 0.006} $$ in . The error is dominated by the theoretical uncertainties. Application of recent calculations which include the resummation of leading and next-to-leading logarithms to all orders for some observables confirm this result with a reduced sensitivity to renormalization scale variations. TheZ 0 line shapes and τ-lepton branching ratios yield $$\alpha _s (M_{Z^0 } ) = 0.148 \pm 0.021$$ and $$\alpha _s (M_{Z^0 } ) = 0.123_{ - 0.007}^{ + 0.006} $$ , respectively, in . These measurements and their uncertainties are entirely independent of each other and from event shape and jet observables; the good agreement of the resulting αs values thus constitutes an important consistency check of the reliability of perturbative QCD.

Inclusive neutral vector meson production in hadronic Z0 decays

Abstract: 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 o(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 o(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.

A measurement of electron production in hadronic Z0 decays and a determination of {Mathematical expression}

Abstract: The production rate of electrons with momentump>4 GeV/c and large momentum transverse to the jet containing the electron has been measured in 136000 hadronic decays of theZ 0 recorded with the OPAL detector at LEP in 1990. The dominant source of these electrons is the semileptonic decay of hadrons containingb quarks. If we assume that the semileptonic branching fraction ofb hadrons produced on theZ 0 resonance is the same as the branching fraction measured at the ϒ(4S) resonance, we determine $$\Gamma _{b\bar b} = 394 \pm 13 \pm 32$$ MeV, where the first error is statistical and the second error is systematic. The sensitivity of the result to this assumption is discussed. We have reduced the dependence of our result on the model ofb hadron semileptonic decay by taking into account the correlation between the model dependence of the branching fractions measured at the ϒ(4S) and of our kinematic acceptance for electrons.

Properties of multihadronic events with a final state photon at {Mathematical expression}

Abstract: The properties of final state photons in multihadronic decays of theZ0 and those of the recoiling hadronic system are discussed and compared with theoretical expectations. The yield of two and three jet events with final state photons is found to be in good agreement with the expectation from a matrix element calculation ofO(ααs. Uncertainties in the interpretation of the theoretical calculation do not yet permit a final assessment of events with just one reconstructed jet. Comparing the rates of two jet events with a photon to those of three jet events in the inclusive multihadronic sample, the strong coupling constant in second order is determined asαs\((M_{Z^0 } )\)=0.122±0.010, taking into account only the statistical and experimental systematic errors. It is found that an abelian model of the strong interaction does not describe the data. The comparison of the total yield and the jet rates with QCD shower programs shows better agreement with the ARIADNE model than with the JETSET model. Both programs are found to describe well the photon properties and the properties of the residual hadronic event.

Investigation of the local hardening effect produced by various low- Z materials in a Si/(Fe, Pb) electromagnetic calorimeter

Abstract: The condition for obtaining a calorimetric response linear with energy for hadronic showers and an energy resolution that improves as the incident energy increases is the equalization of the electromagnetic (e) and the hadronic (π) signal responses. This equalization is obtained by exploiting a local hardening effect realized through the insertion of low-Z thin plates between the low-Z absorbers and the active material in a hadronic calorimeter with silicon readout. This effect, which allows the reduction of the calorimeter response to the electromagnetic component of the incoming hadronic showers, has been investigated for different low-Z materials. The relevance of some aspects of this study to the radiation hardness of the calorimeters is also addressed.

Evidence for compensation in a Si hadron calorimeter

Abstract: The authors present results of measurements performed at the CERN Proton Synchrotron with a hadron Si/(Fe,Pb) calorimeter. In the hadron sampling calorimeter with silicon readout, a combination of plates of low-Z (Fe) and high-Z (Pb) materials used as absorbers enables the reduction of the calorimeter response to the electromagnetic component of the incoming hadronic showers. As a result, a ratio, varying from e/ pi >1 to e/ pi >

Study of High Energy Densities Over Extended Nuclear Volumes via Nucleus-Nucleus Collisions at the CERN SPS

Abstract: Nucleus–nucleus collisions at very high energy are believed to create the conditions for the existence of a new state of matter, the so-called quark–gluon plasma. This possibility depends on the magnitude of the energy density really achieved during the collisions. Estimates of the energy density are model dependent and have been attempted according to various approaches: fire-ball model and Bjorken model. The understanding of the transverse energy flow measurements and of the stopping power helps to define a proper way to estimate the energy density beyond naive approaches. A space- and time-averaged energy density is shown to be a more reliable probe for the QGP search than is a naive estimate of the maximum energy density. The computation is performed within the framework of hydrodynamics. The data considered in this paper were obtained by experiments performed to study nucleus–nucleus collisions at the CERN Super Proton Synchrotron (CERN-SPS). The incident nuclei were accelerated at energies of 60 and 200 GeV/nucleon.