For a book of its genre, our previous book, An introduction to gauge theories and the “new physics” (1982) was a great success. It was not, alas, sold in airport lounges, but it did run to two additional printings (1983, 1985), and to extensively revised editions in Russian (1990), and in Polish (1991). More importantly, it seemed to achieve the principal goal which we had set ourselves, namely, to present a pedagogical account of modern particle physics with a balance of theory and experiment, which would be intelligible and stimulating for both theoretical and experimental graduate students. We did not try to write a profound book on field theory, nor a treatise on sophisticated experimental techniques. But we did wish to stress the deep, intimate and fruitful interaction between theoretical ideas and experimental results. Indeed, for us, it is just this aspect of physics which makes it seem so much more exciting than say pure mathematics. Our greatest pleasure came from the favourable reaction of students who were working through the book and from those reviewers who caught what we hoped was its essential flavour—‘the writing creates the feeling of an active progression of ideas arising from the repeated interaction of theoretical prejudice with experimental observation’, ‘unlike most textbooks, it is highly readable, and makes everything appear simple and obvious’. Well, the last comment is surely an exaggeration but that was our aim.

In the leptonic sector, once we are given the values of α, G and MZ, there are no free parameters left (at least at Born approximation level) so that the entire phenomenology of leptonic reactions is calculable.

In the hadronic sector, matters are not so clear cut. The coupling strengths Vij which involve four parameters are introduced at the quark level. Thus we have to find a convincing way to relate information at the quark level to the calculation of hadronic reactions. This step, in principle, involves the strong interactions and is therefore highly non-trivial. From this point of view the least difficult reactions are the semi-leptonic ones and we shall concentrate upon them in this chapter.

Clearly none of the results of the old Cabibbo theory pertaining to the classic low energy weak interactions discussed in Chapter 1 will be altered, so the most interesting information will come from reactions involving the ‘new’ particles and from high energy weak interactions. For this reason we shall not attempt an analysis of Vij in this chapter. Rather, we shall gather information on the Vij when discussing the discovery and properties of the new particles in the following chapters, in Chapter 18 and in the study of CP violation in Chapter 19.

As always with hadrons the strong interactions play a dominant role, which is largely not understood. Our weak interaction theory is very precise as far as quarks are concerned, but to translate this into precise statements about hadrons requires a model of the quark content of hadrons and the rôle of the quarks in scattering processes.

For a book of its genre, our previous book, An introduction to gauge theories and the “new physics” (1982) was a great success. It was not, alas, sold in airport lounges, but it did run to two additional printings (1983, 1985), and to extensively revised editions in Russian (1990), and in Polish (1991). More importantly, it seemed to achieve the principal goal which we had set ourselves, namely, to present a pedagogical account of modern particle physics with a balance of theory and experiment, which would be intelligible and stimulating for both theoretical and experimental graduate students. We did not try to write a profound book on field theory, nor a treatise on sophisticated experimental techniques. But we did wish to stress the deep, intimate and fruitful interaction between theoretical ideas and experimental results. Indeed, for us, it is just this aspect of physics which makes it seem so much more exciting than say pure mathematics. Our greatest pleasure came from the favourable reaction of students who were working through the book and from those reviewers who caught what we hoped was its essential flavour—‘the writing creates the feeling of an active progression of ideas arising from the repeated interaction of theoretical prejudice with experimental observation’, ‘unlike most textbooks, it is highly readable, and makes everything appear simple and obvious’. Well, the last comment is surely an exaggeration but that was our aim.

In thinking about a second edition we were faced with a serious conceptual problem. Ten years ago we were in a state of excited expectation.

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;

The recent story of narrow resonance discoveries (see Chapter 11) has shown how the properties of systems with quantum numbers of the photon (1−−) can best be studied with e+e- colliders. In this chapter we review some of the information that has come from e+e- machines. In particular, we shall see what support they provide for the SM. We also discuss the perspective for future e+e- colliders.

Electron-positron storage rings

The ideal tools for studying the spectroscopy of the new vector meson particles have undoubtedly been the various e+e- colliding beam machines: SPEAR at SLAC, DORIS at DESY (Deutsches Elektronen Synchrotron), PETRA at DESY and, more recently, SLC at Stanford and LEP 1 at CERN though the actual discovery (Aubert et al., 1974; Herb et al., 1977; UA1, 1983; UA2, 1983) of some of these particles occurred on the proton machines (Brookhaven, Fermilab and CERN).

The reason for the latter lies in the narrowness of these particles; one can simply miss them as one varies the energy. On the other hand, once discovered, the fact that J/Ψ(3097), ϒ(9.46) and the Z are vector particles 1––, and thus couple naturally to a virtual photon, makes an e+e- machine particularly efficacious since the main channel of e+e- annihilation is into a virtual photon.

Thus, it is rather difficult in an e+e- machine to sit right on top of one of these very narrow resonances whose width may be much smaller than the energy resolution of the machine itself.