Study of Leading Hadrons in Gluon and Quark Fragmentation

TL;DR

This paper investigates the fragmentation of gluon and quark jets in e+e- collisions, revealing an excess of neutral leading systems in gluon jets that may suggest new gluonic states predicted by QCD.

Contribution

It provides experimental evidence of an excess of neutral leading systems in gluon jets, indicating a potential new gluonic fragmentation mode not accounted for in existing models.

Findings

Gluon jets show an excess of neutral leading systems compared to simulations.

The excess is concentrated at low invariant masses (< 2 GeV/c^2).

Quark jets do not exhibit this excess.

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 (<~ 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.