The Quark-Gluon Plasma

Quarks, along with leptons, are the fundamental constituents of matter. In the standard model [1] there are six quark flavors, arranged in three families, as illustrated in fig. 1.1

**Figure 1.1:** The quarks are called up, down, charm, strange, top and bottom. For each flavor there is also a corresponding anti-quark. The top row has electric charge $q=\frac {2}{3}e$ and the bottom row has $q=-\frac {1}{3}e$ , where is the charge unit, i.e. the electric charge of the electron.
$\begin{figure}\centerline{ $\left( \begin{array}{c} u\\ d \end{array} \right)$ ... ...rray} \right)$ $\left( \begin{array}{c} t\\ b \end{array} \right)$} \end{figure}$

Quantum Chromo Dynamics [2], QCD, is the theory which describes the strong interaction. The force between quarks grows rapidly for large separations ( $\apprge 1$ fm), confining them into hadrons. The quarks are fermions and have to obey the Pauli principle. To account for all known hadrons, QCD introduces a quantum property called color charge. Each quark (anti-quark) is assigned a color of red, green or blue (anti-red, anti-green, anti-blue). Only color neutral objects are allowed. The mesons consist of two quarks ( $q_{color}\overline{q}_{\overline{color}}$ ), while the baryons consist of three ( $q_{red}q_{green}q_{blue}$ or $\overline{q}_{\overline{red}}\overline{q}_{\overline{green}}\overline{q}_{\overline{blue}}$ ).

The gluons are the exchange particles of the strong interaction. Since they - themselves - carry color charge, they couple not only to quarks but also to each other. This fact can be shown to imply color confinement (described above) and to a property called ``asymptotic freedom'', which means that the interaction gets weaker at short distances ( $\apprle 1$ fm). At large baryon densities (substantially larger than for ground state nuclei) and high temperatures, nuclear matter is therefore expected to undergo a phase transition to a state called the quark-gluon plasma, QGP [3,4]. In this state, the quarks are not confined to the hadrons. Quarks and gluons instead move over the whole, high density system. Besides this deconfinement, chiral symmetry is expected to be restored in a QGP, which means that the quark masses will approach zero. A predicted phase diagram of nuclear matter is shown in fig. 1.2.

**Figure 1.2:** QCD phase diagram. Lattice calculations at low baryon densities predict a phase transition at a temperature of about 150-200 MeV.
$\begin{figure}\centerline{\hbox{ \epsfxsize=12cm \epsffile{images/Phase9.eps} }} \end{figure}$

The quark-gluon plasma state is important in the Big Bang scenario. A few microseconds after Big Bang the hot and dense universe was in a state of freely moving quarks and gluons. As the universe expanded and cooled, the quarks and gluons became confined to hadrons. QGP may today exist in the core of neutron stars, which have extremely high baryon densities.

Accelerators designed to provide high-energy heavy-ion collisions offer a possible way to create and study the QGP. The collisions produce a hot and dense system that may reach energy densities and temperatures high enough for a phase transition to occur. The remaining sections of this chapter describe such experimental facilities and the physics that can be extracted from them.