Universality in Hadron Production in Electron- Positron, Lepton-hadron and Hadron-hadron Reactions

Universality says that the mechanisms of hadron production are the same in all high-energy processes in which most of the final state hadrons arise from hard or semi-hard scattering of partons. Differences in the detailed composition of the final states are assumed to be due to differences in the flavours and configurations of the initial partons. Perturbative quantum chromodynamics (pQCD) has been rigorously tested in many high-energy experiments, and the theory is now well established as the one describing the strong interactions of quarks and gluons. However, while many aspects of the experimental data, such as scaling violations, event shapes, jet rates and so on, are amenable to description in terms of pQCD, the detailed mechanisms of hadron formation are much less well-understood. These processes occur at the scale of the hadron masses and are essentially non-perturbative in nature. For studies of the production of hadrons in high-energy reactions we therefore have to rely on models to help us understand what is going on. A number of models exist, some to a greater or lesser extent built around features of QCD, while others are purely phenomenological.