A Monte Carlo Study on the Identification of Quark and Gluon Jets

Three jets events in the ee collisions at 91.2 GeV are investigated using both HERWIG and JETSET Monte Carlo generators. The angles between the three jets are used to identify the quark and gluon jets. The analysis at parton level is done to ensure the reasonableness of this method and an angular cut is ultilized to improve the purity of this identification. The multiplicity inside the identified quark or gluon jets agree with the QCD predictions. In the theory of QCD, unobserved quarks and gluons are confined by color force. They eventually hadronize into observed final state jets, which in a sense inherit the information about original partons. In order to study the difference of nonpertubative hadronization processes between quarks and gluons, it is important to identify whether a final state jet is originated from a quark or gluon. Many works have been done on this topic, see for example 1 2 . Recently mostly used way are the so called b-tagged events in which b (and anti-b) quarks are well tagged and the rest jet is certainly a gluon jet 4 5 . This method, apart from its high accuracy, is restricted in quark flavour and is inapplicable to the gluon jets produced together with the light-quark jets. In this paper we analyse the events of ee annihilations at 91.2 GeV generated by HERWIG 5.9 7 and JETSET 7.4 8 to find a method for the identification of quark and gluon jets without flavour restriction. Durham jet algorithm 9 is used to form jets both at parton level and at hadron level. R2 and R3 are two and three jets fraction, respectively. Distributions of R2, R3 at parton level and hadron level, are obtained by varying ycut value from 10 −4 to 10, cf. Fig.1, in which there exists a ycut value (log (ycut) = −2.5), where R3 both at parton level and at hadron level get to the maximum. We choose this ycut for all the later analysis. We assume that the three jets in one event are coming from quark, antiquark and radiated gluon, respectively. Because of energy-momentum conservation, the three jets in one event must lie in a plan. These three jets are ordered according to the angles between two neighbouring jets. Jet-1 is THIS WORK IS SUPPORTED IN PART BY THE NSFC UNDER PROJECT