Abstract

QCD coherence effects are studied based on measurements of correlations of particles with either restricted transverse momenta, pT<pTcut, where pT is defined with respect to the thrust axis, or restricted absolute momenta, p equiv |p| < pcut, using about four million hadronic Z decays recorded at LEP with the OPAL detector. The correlations are analyzed in terms of normalized factorial and cumulant moments. The analysis is inspired by analytical QCD calculations which, in conjunction with Local Parton-Hadron Duality (LPHD), predict that, due to colour coherence, the multiplicity distribution of particles with restricted transverse momenta should become Poissonian as pTcut decreases. The expected correlation pattern is indeed observed down to pTcut approx 1GeV but not at lower transverse momenta. Furthermore, for pcut to 0 GeV a strong rise is observed in the data, in disagreement with theoretical expectation. The Monte Carlo models reproduce well the measurements at large pTcut and pcut but underestimate their magnitudes at the lowest momenta. The e+e- data are also compared to the measurements in deep-inelastic e+p collisions. Our study indicates difficulties with the LPHD hypothesis when applied to many-particle inclusive observables of soft hadrons.

Highlights

  • At high energies, the annihilation process e+e−→ hadrons proceeds through the creation of a highly virtual primary quark and anti-quark which initiate a cascade of partons through successive parton emissions

  • Analytical perturbative Quantum Chromodynamics (QCD) calculations, in agreement with parton-level Monte Carlo calculations, show that gluons produced in a jet become uncorrelated when the gluon transverse momentum relative to the jet axis, pT, is restricted to small values

  • The approach to a Poisson regime is a direct consequence of colour coherence, or angular ordering of gluon emissions in the QCD cascade and is expected to hold for soft hadrons if Local Parton-Hadron Duality (LPHD) is valid

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Summary

Introduction

The annihilation process e+e−→ hadrons proceeds through the creation of a highly virtual primary quark and anti-quark which initiate a cascade of partons through successive parton emissions. It is remarkable that inclusive characteristics of hadrons measured in a variety of hard processes show a behaviour similar to that expected from perturbative parton-level calculations [3,4,5] This indicates that perturbative QCD effects and colour coherence in particular leave their imprint on the hadronic final state even for quantities which are not infra-red safe, such as particle multiplicity. The multiplicity distribution of partons in a jet is generally broader than a Poisson distribution (which corresponds to uncorrelated production) and obeys asymptotic KNO-scaling [11] It was pointed out in [10] that, due to colour coherence, gluons produced with bounded transverse momenta, pT < pTcut, where pT is defined with respect to the primary parton in a jet, become, for small pTcut, independently emitted from the primary parton. The measurements are based on a data sample of about four million Z hadronic decays recorded with the OPAL detector at the LEP e+e− collider at CERN

Analysis
The OPAL detector
Data and Monte Carlo samples
Factorial moments
Factorial cumulants
Comparison with deep-inelastic scattering
Summary and conclusions
Full Text
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