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4.2 Phases of Strongly Interacting Matter
4.2.1 Introduction
Over the last two decades, the vigorous exploration of
the phase diagram of strongly interacting matter has
led to tremendous progress in the understanding of the
strong interaction.
Converging evidence from heavy ion collisions at various
energies is starting to form a coherent picture of
how nuclear matter evolves from the nuclear state at
zero temperature all the way to a deconfined plasma of
quarks and gluons, the state through which our universe
evolved shortly after the Big Bang. Due to the joint effort
of theory and experiment, a coherent interpretation of
the phenomenology of the different regions of the phase
diagram of strongly interacting matter has started and is
revealing the signs of a phase transition from hadronic
matter to a deconfined plasma of quarks and gluons,
and of a phase transition from a quantum liquid to a
hadron gas.
The study of many-body strongly interacting systems
exhibiting collective behaviour is one of the most powerful
tools for the advancement of nuclear physics. Collective
behaviour frequently reveals qualitatively novel features
of the complex system under study. Thermodynamics
provides a general framework for the understanding
of how properties of macroscopic matter and collective
phenomena emerge from the laws governing the
microscopic dynamics. The most dramatic example of
collective behaviour is the occurrence of phase transitions,
accompanied by qualitative changes in matter
properties. Experimentally accessible strongly interacting
systems exhibit transitions between characteristic
phases: a liquid-gas phase transition, a confinementdeconfinement
transition and a chiral transition between
massive hadrons and almost massless quarks. The
strong interaction itself allows for an even richer structure
(see also information Box 1).
In most many-body systems the macroscopic conditions
influence the microscopic properties. For example,
the effective mass of an electron is modified in a semiconductor
due to the presence of the crystal structure.
For strongly interacting systems such medium modifications
are significant, and they appear on a much more
fundamental level.
In practice, strongly interacting many-body physics
has to be studied in systems where the characteristic
length and coherence scales are very small. This requires
innovative approaches to studies of collective behaviour.
The strong coupling allows for effective multiple interactions
of particles even in small systems and on very
short time scales, which makes collective behaviour an
important characteristic of medium- and high-energy
nuclear reactions. It is by now well established that
one can create strongly interacting matter (as opposed
to assemblies of independent particles) in accelerator
based collision experiments and study its properties.
Quantitatively, the properties of an interaction are best
reflected on the ‘macroscopic’ level by the equation of
state of the matter produced.
Understanding baryonic matter at low energy density
constitutes a formidable challenge for strong interaction
theory. A microscopic approach to the effective interaction
acting among nucleons in the nuclear medium from
Lattice QCD is still in its infancy. Therefore, considerable
uncertainties still exist in the equation of state of nuclear
matter (EoS), particularly concerning its behaviour as a
function of the isospin asymmetry. At low density and
finite but small temperature of the order of the binding
energy of a nucleus, a first order liquid-gas phase
transition terminating in a second order critical point
is predicted for neutral nuclear matter. For a long time
this transition was believed to connect a homogeneous
dense liquid phase with a phase of homogeneous diluted
gas of neutrons and protons. We now understand that
in the diluted disordered phase many-body correlations
and clustering play an important role. This opens new
perspectives for the understanding of systems where this
phase transition occurs in nature, namely in the cores
of type II supernovae and in the crust of neutron stars.
Properties of such objects can now be closely linked to
experiments investigating the liquid-gas phase transition
in accelerator based heavy ion collisions.
At low temperatures, quarks and gluons are confined
inside colour neutral hadrons. The mass of a hadron
is much larger than the sum of the bare masses of its
constituents. This is due to the spontaneous breaking
of a fundamental symmetry, the chiral symmetry. Such
effects completely dominate the low-energy phenomenology
of the strong interaction, and make the theoretical
treatment extremely difficult. At high temperature, or at
high net baryon density, the strong interaction is radically
modified. The strong coupling decreases and the
confining part of the interaction potential is expected
to vanish. In this regime, chiral symmetry should be reestablished,
which in turn should manifest itself as an
observable modification of constituent masses. At very
high temperatures, a transition to a system of free and
massless quarks and gluons, the quark-gluon plasma
(QGP), is expected. This state of matter should have
existed in the very early universe, approximately 10 µs
after the Big Bang, when temperatures were extremely
high. The QGP phase transition is probably the only
phase transition of the early universe that can be studied
experimentally. The QGP should also exist in the core
of dense neutron stars, where the net baryon density is
very high. The determination of the corresponding equa-
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