Introduction to the Physics of Ultra-Relativistic Heavy-Ion Collisions

Abstract

The strong interaction between the elementary constituents of matter (quarks and gluons) is described by the theory of Quantum Chromodynamics (QCD). The basic ingredients of this quantum field theory will be explained in Sect. 1.1 and its peculiar properties driven by the running of the strong coupling constant will be addressed in Sect. 1.2. These properties lead to the prediction that strongly-interacting matter can exist in different phases depending on the temperature and the density of the system. Nuclear matter at extremely high temperatures and energy densities is obtained with ultra-relativistic heavy-ion collisions, which allow to create a state of matter where quarks and gluons are interacting without being confined into hadrons. According to the hot Big Bang model, this state of matter should have appeared after the electro-weak phase transition, a few microseconds after the Big Bang. The Lattice QCD approach, which is introduced in Sect. 1.3, allows to obtain quantitative predictions on the basic properties of the QCD phase diagram and on the phase transition, which are described in Sect. 1.4. The second part of the chapter (Sect. 1.5) is devoted to a review of the first results obtained by the experiments at the CERN Large Hadron Collider (LHC) in Pb–Pb collisions at the energy of \(\sqrt{s_\mathrm{NN}}=2.76\) TeV per nucleon–nucleon (NN) collision, also compared with the measurements performed at lower energies at the Relativistic Heavy-Ion Collider (RHIC) at the Brookhaven National Laboratory (BNL).