Jets have by now become a major component of high energy particle physics. It is widely (if not universally) accepted that jets are implied by perturbative QCD and that they are the simplest and perhaps best evidence supporting it. This means that jets are to be found in all large pT phenomena irrespective of whether they are initiated by hadronic or by leptonic processes. In spite of this, for pedagogical and historical reasons we shall present the discussion of jets in hadronic and in e+e− reactions separately.

A drastic selection has had to be made to reduce the material on jets to an acceptable size. For additional material see Cavasinni (1990), Altarelli (1989) and Jacob (1990) and references therein. [See also the various contributions in the CERN 89-08 (vol 1–3) edited by Altarelli, Kleiss and Verzegnassi, 1989].

Note that a complete set of formulae for the cross-sections of all the 2 → 2 partonic reactions discussed herein can be found in Appendix 7.

Introduction

We here discuss the evidence that hadronic reactions involving large transfers of transverse momentum (pT) are controlled by the direct collision of constituents within the colliding hadrons, with subsequent fragmentation of these constituents into showers of hadrons. Naturally, it is supposed that these constituents are the quarks and gluons discussed in previous chapters.

It is well known that high energy hadronic interactions are dominated by the production of a large number of particles, mostly confined to the nearly forward direction, i.e. dominated by events in which the produced secondaries have small PT leading to the conclusion that strong interactions at high energy are generally rather ‘soft’.