Showing papers by "Georges Azuelos 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.

Radiative muon capture on Al, Si, Ca, Mo, Sn, and Pb

Abstract: The branching ratio for radiative muon capture (RMC), relative to the nonradiative process, is sensitive to {ital g}{sub {ital p}}, the induced pseudoscalar coupling constant of the weak hadronic current. The photon energy spectra from RMC on {sup 27}Al, {sup 28}Si, {sup 40}Ca, {sup nat}Mo, {sup nat}Sn, and {sup nat}Pb have been measured using a high-acceptance pair spectrometer. The measured partial branching ratios, {ital R}{sub {gamma}}, for photons of {ital E}{sub {gamma}}{gt}57 MeV are 1.43{plus minus}0.13, 1.93{plus minus}0.18, 2.09{plus minus}0.19, 1.11{plus minus}0.11, 0.98{plus minus}0.09, and 0.60{plus minus}0.07 respectively, in units of 10{sup {minus}5}. The results confirm the previously observed suppression of {ital R}{sub {gamma}} with increasing {ital Z} for {ital Z}{gt}20. For {sup 40}Ca the present result is in good agreement with previous measurements. For the heavier nuclei, the results are compared with two recent calculations performed in the Fermi-gas model. In one case the data indicate a complete quenching of {ital g}{sub {ital p}}, but the more recent calculation does not reproduce the data for any value of {ital g}{sub {ital p}}.

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.

The TRIUMF radiative muon capture facility

Abstract: Radiative muon capture (RMC) on hydrogen produces photons with a yield of ∼10−8 per stopped muon. To measure RMC at TRIUMF we have constructed a large-solid-angle photon pair-spectrometer which surrounds the liquid hydrogen target. The spectrometer consists of a cylindrical photon converter and a large-volume cylindrical drift chamber to track the e+e− pairs. It is enclosed in a spectrometer magnet which produces a highly uniform axial magnetic field. The detector subsystems, the hardware trigger and the data acquisition system are described, chamber calibration and tracking techniques are presented, and the spectrometer performance and its Monte Carlo simulation are discussed.

Radiative muon capture on hydrogen

Abstract: The radiative capture of negative muons by protons can be used to measure the weak induced pseudoscalar form factor. Brief arguments why this method is preferable to ordinary muon capture are given followed by a discussion of the experimental difficulties. The solution to these problems as attempted by experiment #452 at TRIUMF is presented together with preliminary results from the first run in August 1990. An outlook on the expected final precision and the experimental schedule is also given.

Radiative muon capture on hydrogen

Abstract: The radiative capture of negative muons by protons can be used to measure the weak induced pseu-doscalar form factor Brief arguments why this method is preferable to ordinary muon capture are given followed by a discussion of the experimental difficulties The solution to these problems as attempted by experiment #452 at TRIUMF is presented together with preliminary results from the first run in August 1990 An outlook on the expected final precision and the experimental schedule is also given