Phase transitions in quantum chromodynamics

Abstract

The current understanding of finite temperature phase transitions in QCD is reviewed. A critical discussion of refined phase transition criteria in numerical lattice simulations and of analytical tools going beyond the mean-field level in effective continuum models for QCD is presented. Theoretical predictions about the order of the transitions are compared with possible experimental manifestations in heavy-ion collisions. Various places in phenomenological descriptions are pointed out, where more reliable data for QCD's equation of state would help in selecting the most realistic scenario among those proposed. Unanswered questions are raised about the relevance of calculations which assume thermodynamic equilibrium. Promising new approaches to implement nonequilibrium aspects in the thermodynamics of heavy-ion collisions are described.

Phase transitions in the early and the present Universe: from the big bang to heavy ion collisions

“…The confinement-deconfinement transition for three flavors is likely to be of first order [20,21] and therefore occurs via the nucleation of hadronic bubbles in the background of a QGP. An estimate of whether the expansion of the Universe at the QCD phase transition results in non-equilibrium effects can be obtained in the same manner as in the case of the EW transition, i.e, by estimating the ratio Γ/H with Γ a typical strong interaction reaction rate.…”

“…This triplet of almost Goldstone bosons are the neutral and charged pions, which are the lightest pseudoscalar particles with m π ∼ 140 Mev. Lattice simulations [20] suggest that the confinement-deconfinement and chiral phase transitions occur at about the same temperature T ∼ 160 Mev (for a review see [21]). At this temperature only the lightest quark flavors influence the thermodynamics, the up, down and strange quarks with m s ∼ 150 Mev.…”

“…Under the assumption that the central region is baryon free, the conservation of entropy and energy and momentum lead to the following equations (for details see [56,21,53,55])…”

Nonequilibrium pion dynamics near the critical point in a constituent quark model

“…In contrast, the external sources η δ,π above are introduced to induce an expectation value for the fluctuations δ, π and allow to study the real-time evolution of these expectation values as an initial value problem as described in detail below. Perturbative calculations for the meson self-energies are carried out in terms of the following real-time free fermion propagators: 4) where · · · denotes expectation value with respect to the initial density matrix and a, b = ±. The Wightman functions S ≶ p (t, t ′ ) are expressed in terms of the spectral function as (with zero chemical potential) 5) where n(p 0 ) = 1/(e βp0 + 1) and the spectral function ρ(p, p 0 ) is given by eq.…”

“…This quark-gluon plasma then expands hydrodynamically and cools almost adiabatically, until the transition temperature is reached at a time scale 10 − 50 fm/c depending on the initial temperature [4]. At BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) initial temperatures of the quark-gluon plasma as high as T ∼ 300 and 500 MeV, respectively, are expected to be achieved in the most central collisions, hence providing the experimental possibility to study the restoration of chiral symmetry and its subsequent spontaneous breaking as the plasma expands and cools.…”