A study of differences between quark and gluon jets using vertex tagging of quark jets

Quarkand gluonjets with equal energies are identified in three-jet hadronicZ0 events, using reconstructed secondary vertices from heavy quarkdecay in conjunction with energy ordering of the jets to anti-tag the gluonjets. Selection of jets from a symmetric event topology allows their properties to be compared in a simple and direct manner. The jets under study have an energy of about 24 GeV. It is observed that gluonjets have a larger angular width than quarkjets and yield a softer particle energy spectrum. Correspondingly, the mean particle multiplicity is found to be larger for gluonthan for quarkjets. Correcting the distributions for residual misidentification of the quarkand gluonjets, the ratio of mean particle multiplicty of gluonrelative to quarkjets is measured to be $$\frac{{\left\langle n \right\rangle _{ gluon} }}{{\left\langle n \right\rangle _{ quark} }} = 1.27 \pm 0.04(stat.) \pm 0.06(syst.),$$ where the jets are defined using thek⊥ jet finder. The numerical value of this ratio is found to be sensitive to the choice of the jet algorithm. The experimental results are compared to Monte Carlo calculations which incorporate perturbative QCDalong with different assumptions about the hadronization process.