Introduction

Although the earliest evidence for jets came from hadronic physics, it is in e+e− collisions that jets can be studied most cleanly. Thus, the first hint of planar (i.e. three-jet) configurations implying the existence of gluon jets came from PETRA in 1979. The field is, by now, dominated by LEP data at which energies several jets can be produced.

In this chapter we briefly discuss the historical developments and then turn to the most recent results. Some mention is made of models designed to bridge the gap between the QCD perturbative hard reactions and the non-perturbative process of hadronization. All these models use Monte Carlo simulations based on probabilistic descriptions; thus they cannot describe the subtle quantum mechanical effects that might occur. One should be able to attack this problem analytically but it may be a long time before a viable approach is found.

General outline of e+e− jets

Evidence for two-jet formation in the multihadronic final states in e+e− reactions above 5 GeV/c first emerged from the SLAC-LBL magnetic detector (Hanson et al., 1975; many excellent reviews of this group's data have appeared, see, for example, Hanson, 1976) at SPEAR and was based on the following pieces of data: (i) analysis of the mean sphericity variable (see below) to reconstruct the jet axis and comparison between a pure (isotropic) phase space model and a jet model;