Comparison of three-jet and radiative two-jet events in electron-positron annihilation at 29 GeV

Abstract

By comparing 3-jet (e+ e--+ qqg) and radiative 2-jet (e+ e--+ qq"Y) events from electron-positron annihilation, we have studied the local and global effects of the presence of a hard bremsstrahlung gluon in hadronic events. Detector and event selection efficiencies and biases affect these two kinds of events almost equally because they have very similar kinematics and topologies. Accurate comparisons of qqg and qq"Y events can therefore be made.Globally, we observe a depletion of hadrons in qqg events relative to qq"Y events on the opposite side of the event plane from the gluon, in the angular region between the q and q jets. This depletion is shown to be in agreement with the predictions of Quantum Chromo dynamics (QCD). The ,existence of this effect demonstrates that the presence of a gluon significantly alters the color forces and hence the fragmentation process in hadronic events.We also use these qq"Y and qqg events to compare low energy (4.5 Ge V) gluon and quark jets. Our data indicate that gluon jets have softer xp distributions than quark jets, while the transverse momentum distributions of these two types of jets are identical within our errors. Although we are unable to determine if the multiplicities of gluon (ng) and quark (nq) jets are different, the ratio ng / nq = 9/4 predicted asymptotically in QCD would not be consistent with our data.