Different methods for quark-gluon jet classification on real data from the DELPHI detector

Different methods to separate quark jets from gluon jets have been investigated tested on data from the DELPHI experiment. A test sample of gluon jets was selected from bbbarg threejet events where the two b-jets had been identified using a lifetime tag and quark jet sample was obtained from qqbargamma events where the photon was required to have a high energy and to be well separated from the two jets. Three types of tests were made. Firstly, the jet energy, which is the variable most frequently used for quark/gluon jet separation, was compared with methods based of the differences in the fragmentation of quark and gluon jets. It was found that the fragmentation based classification provides significantly better identification than the jet energy only in events where the jets all have approximately the same energy. In Monte Carlo generated symmetric e$\sp+$e$\sp-\ \to$ qqbarg threejet events, where the jet energy does not provide any identification at all, the gluon jet was correctly assigned in 58% of the events...