From hadrons to quarks

Abstract

In this chapter, we look at some simple models to illustrate how a phase transition from hadronic matter to a quark-gluon plasma can occur. First we determine when hadrons start forming clusters which can be considered as quark matter. After considering the thermodynamics of an ideal hadron gas and of an ideal quark-gluon medium, we introduce bag pressure and baryon repulsion as interaction features to specify under which conditions strongly interacting matter prefers to consist of hadrons and when it wants to turn into a plasma of unbound quarks and gluons. Finally we show that also a simple string model yields localized hadrons at low density, while at high density color charges can move around freely by changing partners. In all cases, very basic physical notions are found to lead to a two-phase structure of matter.