Abstract
Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from hadronic Z0 decay events produced in e+e- annihilations. A subsample of these jets is identified which exhibits a large gap in the rapidity distribution of particles within the jet. After imposing the requirement of a rapidity gap, the gluon jet purity is 86%. These jets are observed to demonstrate a high degree of sensitivity to the presence of color reconnection, i.e. higher order QCD processes affecting the underlying color structure. We use our data to test three QCD models which include a simulation of color reconnection: one in the Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection models can describe our gluon jet measurements only if very large values are used for the cutoff parameters which serve to terminate the parton showers, and that the description of inclusive Z0 data is significantly degraded in this case. We conclude that color reconnection as implemented by these two models is disfavored. The signal from the Herwig color reconnection model is less clear and we do not obtain a definite conclusion concerning this model. In a separate study, we follow recent theoretical suggestions and search for glueball-like objects in the leading part of the gluon jets. No clear evidence is observed for these objects.
Highlights
Rapidity y, defined by y = 2 lnE+p E−p with E the energy of a particle and p the component of its 3-momentum along an axis1, is one of the most common variables used to characterize the phase space distribution of particles in high energy collisions
To identify gluon jets with a rapidity gap, we examine the charged and neutral particles assigned to the selected gluon jets by the jet finder
A sample of 12 611 gluon jets with a mean energy of 22 GeV and estimated purity of 95% is identified in e+e− hadronic Z0 decay events using b quark jet tagging
Summary
E+p E−p with E the energy of a particle and p the component of its 3-momentum along an axis, is one of the most common variables used to characterize the phase space distribution of particles in high energy collisions. The reason for this is partly that the detector level distributions are better described by Ariadne, as stated above, and partly that Ariadne is used to determine the correction factors. The Herwig-CR model exhibits a similar excess with respect to Herwig, with less significance We correct the measurements for the effects of initial-state radiation, detector acceptance and resolution, and gluon jet impurity, and compare the predictions of the models to the data at the hadron level. -0.5 0 0.5 1 1.5 2 2.5 3 3.5 ymin (Rathsman-CR − Jetset)/Jetset (Ariadne-CR − Ariadne)/Ariadne (Herwig-CR − Herwig)/Herwig
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