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Abstract (English)

Three jet events arising from decays of the Z boson, collected by the Delphi detector at Lep, were used to compare properties of gluon and quark jets. Gluon jets were anti-tagged by identifying heavy (b) quark jets. Quark and gluon jet properties of different jet scale ranges were compared for the first time within the same detector. Quark and gluon jets of nearly the same topology in symmetric Y- and Mercedes events were also compared. Gluon jets are broader and produce fragments with a softer energy spectrum than quark jets of equivalent scale. The string effect was observed for the first time in fully symmetric three jet events. The measured ratio of the charged particle flow in the quark-anti-quark inter-jet region of the quark-anti-quark-gluon and quark-ant-quark-gamma samples agrees with the perturbative QCD expectation. The charged hadron multiplicity in a cone perpendicular to the event plane of symmetric three jet events was determined. coherence property of QCD and of LPHD. The production spectra of the identified particles K+-, Pi+-, proton, and anti-proton were found to be softer in gluon jets compared to quark jets, with a higher multiplicity in gluon jets as observed for inclusive charged particles. A significant proton enhancement in gluon jets is observed indicating that baryon production proceeds directly from colour objects. The maxima, xi*, of the xi-distributions for kaons in gluon and quark jets are observed to be different. The study of isoscalar resonance production shows no indication of an excess of phi(1020) production in gluon jets. The splitting processes of quarks and gluons are investigated using longitudinal and transverse observables. The primary splitting into subjets agrees with fragmentation models and, for specific regions of the jet resolution y, with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is 2.66+-0.09+-0.10. Due to non-perturbative effects, the data are below the expectation at small y. The transition from the perturbative to the non-perturbative domain appears at smaller y for quark jets than for gluon jets. Combined with the observed behaviour of the higher rank splittings, this explains the relatively small multiplicity ratio between gluon and quark jets. The fragmentation functions of quarks and gluons are measured in various three jet topologies. The results at different values of transverse momentum-like scales are compared. Parameterizations of the quark and gluon fragmentation functions at a fixed reference scale and also for a wide range of scales are given. The fragmentation functions show the predicted pattern of scaling violations. Scaling violations in identified gluon jets are observed for the first time. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy e+e- annihilation experiments. For gluon jets it appears to be significantly stronger. The scale dependences of the gluon and quark fragmentation functions agree with the prediction of the DGLAP evolution equations from which the colour factor ratio C_A/C_F is measured to be: C_A/C_F = 2.26 +- 0.09_{stat.} +- 0.06_{sys.} +- 0.12_{clus.,scale} . The charged hadron multiplicity in gluon and quark jets was determined as a function of a transverse momentum-like scale. The dependence of the charged hadron multiplicity on the opening angle in symmetric three jet events is compared with theoretical predictions.

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