Classical simulations and particle production in heavy-ion collisions
چکیده
One of the most important predictions of lattice QCD is the existence of a transition from ordinary hadronic matter, where quarks and gluons are confined, to a quark–gluon plasma at high temperatures. Current estimates are that this transition takes place at Tc ∼ 150 − 170 MeV, and is most likely a crossover at zero chemical potential. At higher chemical potential a firstorder phase transition is predicted, ending in a tricritical point at μB ∼ 100− 500 MeV. These predictions are currently being put to the test at RHIC and other heavy-ion colliders. However, interpreting the results of these experiments and comparing them to lattice QCD predictions is far from straightforward. One important reason for this is that the process to a large extent takes place out of thermal equilibrium, and equilibrium field theory methods such as lattice Monte Carlo are therefore not sufficient. One major, still unsolved puzzle of heavyion physics is whether the system ever actually reaches thermal equilibrium, and if so, what the equilibration time is. Many aspects of the collision can be successfully described (eg, using hydrodynamics [1]) by assuming a very short equilibration time of 1 fm/c, but it is far from understood how this would come about. Clearly, a proper understanding of the thermalisation process is essential if we are to have a coherent and reliable description of the heavy-ion collision. At late stages of the collision process, as the system expands, the particles eventually decouple. This typically occurs in two phases: first, inelastic collisions cease, causing the ratios of different particle species to be fixed (chemical freezeout); later, also the mean free path for elastic collisions becomes too large and the momentum distribution of the particles deviates from that of thermal equilibrium (thermal or kinetic freezeout). It is at this point that the final particle yields of the collision are fixed. Thus, nonequilibrium dynamics is needed to understand also this aspect of the process. Another field where non-equilibrium field theory is needed, is early-universe physics. Examples of non-equilibrium processes which require a fieldtheoretical approach include (p)reheating, electroweak and QCD phase transitions in the early universe, and baryogenesis. Field theories out of equilibrium is a notoriously difficult problem to study nonperturbatively. A large number of approaches have been employed, including Hartree and large-N approximations [2,3], Dyson–Schwinger-related approaches based on the 2PI effective action [4,5], and kinetic theory [6]. All of these have their strengths as well as drawbacks and limitations. Within its area of applicability, the classical approximation has the advantage of being fully nonperturbative, easy and relatively inexpensive to implement numerically, and straightforwardly applicable to gauge theories. The major drawback is obviously that quantum effects are not taken into account.