Abstract
The study of quark jets in e(+)e(-) reactions at LEP has demonstrated that the hadronisation process is reproduced well by the Lund string model. However. our understanding of gluon fragmentation is less complete. In this study enriched quark and gluon jet samples of different purities are selected in three-jet events from hadronic decays of the Z collected by the DELPHI experiment in the LEP runs during 1994 and 1995. The leading systems of the two kinds of jets are defined by requiring a rapidity gap and their sum of charges is studied. An excess of leading systems with total charge zero is found for gluon jets in all cases, when compared to Monte Carlo simulations with JETSET (with and without Bose-Einstein correlations included) and ARIADNE. The corresponding leading systems of quark jets do not exhibit such an excess. The influence of the gap size and of the gluon purity on the effect is studied and a concentration of the excess of neutral leading systems at low invariant masses (less than or similar to 2 GeV/c(2)) is observed, indicating that gluon jets might have an additional hitherto undetected fragmentation mode via a two-gluon system. This could be an indication of a possible production of gluonic states as predicted by QCD. (c) 2006 Elsevier B.V. All rights reserved.
Highlights
The study of quark jets provides us with remarkable insights into the mechanism of hadronisation
No quantitative prediction exists up to now. This has triggered an experimental investigation by the DELPHI collaboration on gluon fragmentation in a leading system defined by a rapidity gap [13,14]
Three different Monte Carlo event samples have been created by using different generators: Model (1): JETSET with Bose-Einstein correlations (BEC) included (BE32 [26]) Model (2): JETSET without BEC Model (3): ARIADNE without BEC The number of events generated for each sample corresponds roughly to that of the data
Summary
The study of quark jets provides us with remarkable insights into the mechanism of hadronisation It gives strong evidence for chain-like charge ordered particle production in excellent agreement with string Monte Carlo models like JETSET [1]. The present study aims to consolidate the results of the preceding analyses [13,14,15,16] by studying the dependence of the excess of neutral leading systems in enriched gluon jets on the gap size and gluon content, by studying their mass spectra and by investigating if there are possible trivial origins for the observed effect This is especially important, since a significant failure of the string model to describe gluon jets might generally reveal the presence of hitherto undetected processes. As a cross-check, the same investigation is done for quark jets
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