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   Quark and gluon jet differences
Quark and gluon jet differences


We shall discuss in more detail the differences between hadron jets started by a quark and by a gluon. QCD predicts different coupling strengths for the radiation of an additional gluon from either a quark or a gluon. These coupling strengths are governed by the colour factors for gluon emission, which have the values  and  for radiation from a quark and a gluon, respectively. These are inclusive factors which need corrections to predict real jets. QCD predicts that a gluon is more likely to radiate a gluon than a quark. It is expected that a gluon jet will have a higher particle multiplicity, a softer particle spectrum and will be broader than a quark jet of equal energy [#Quark##1#].

Experimental tests of these predictions have been made using three-jet multihadron events in symmetric configurations. In the LEP experiments, a jet-finding algorithm is applied to the multihadronic events, and those events in which three jets are reconstructed are retained. Furthermore, events are only retained if two jets form angles of  to the third jet (Fig. 9a). This requirement gives a configuration in which two jets have approximately equal energies and the third jet has a significantly higher energy. The highest energy jet is a quark jet while the two lower energy jets are composed of a roughly equal mixture of quark and gluon jets. Using microvertex detectors, a subsample of the symmetric events is selected, in which one of the two lower energy jets contains a secondary vertex which signals a b-quark decay. Since b-quarks are almost entirely produced at the electroweak vertex, the presence of such a decay indicates that the tagged jet is very likely a b-quark jet. The remaining lower-energy jet is then a gluon jet. Only these anti-tagged gluon jets are considered further here (the tagged quark jets are biased and therefore are not used).

The heavy-flavour tagging allows gluon jets to be identified with a purity of about 93%; it is then possible to construct the distribution of interest, e.g. the fragmentation function for the two samples separately. Denoting the distribution for all lower-energy jets as  and that for the gluon jets in the tagged sample as  , two equations can be written:  ,  , where G and Q are the fragmentation function distributions for pure gluon and quark jets, respectively; the coefficients correspond to the jet purity. The linear equations can be solved for the pure jet properties. Since the jets have been selected from events with symmetric topologies, the quark and gluon jets have equal energies and identical jet environments, thus facilitating the comparison of jet properties.


Fig. 9c shows the fragmentation function, measured as a function of the quantity  , which is the scaled energy of a hadron with respect to the energy of the jet to which it is assigned. Gluon jets (open points) have fewer particles with large energies and more particles with low energies, and hence the particle energy spectrum is softer for gluon jets than for quark jets.

The angular spread of a jet may be studied by measuring the fraction of the jet energy contained in annular rings coaxial with the jet. This distribution is shown in Fig. 9b. Once again, clear differences between quark and gluon jet properties are seen. The gluon jets have more of their energy distributed at large angles to the jet axis, and therefore can be considered to be broader than the quark jets.

To leading order, the QCD prediction for the ratio of gluon to quark jet multiplicities is  , which becomes slightly smaller  in next-to-next-leading order. However, this prediction refers to the production of  or gg in a colour singlet state at asymptotically high energies, and therefore may not be directly comparable with the experimental measurements at finite centre-of-mass energies. The measurements of charged particle multiplicities in symmetric  configurations vary between  = 1.23 and 1.11 according to which jet algorithm is used.

We conclude that the measurements at LEP have confirmed that gluon jets have softer particle spectra, higher multiplicities, and are broader than quark jets of the same energy.



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