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4.2 Phases of Strongly Interacting Matter
of gluons into two lower momentum gluons; and 2) the
recombination of low momentum gluons into a single
high momentum gluon. The gluon density therefore
is expected to saturate at a characteristic value. This
saturated gluon matter can be regarded as a state
representing the classical field limit of the strong interactions.
In summary, the investigation of the phases of strongly
interacting matter addresses some of the most important
open questions of fundamental physics today. These
include:
• What are the fundamental properties of matter interacting
via the strong interaction as a function of
temperature and density
• What are the microscopic mechanisms responsible
for the properties of high density strongly interacting
matter
• How do hadrons acquire mass
• How is mass modified by the medium it moves in
• What is the structure of nuclei when observed at the
smallest scales, i.e., with the highest resolution
We are now on the verge of a significant new revolution
in the fi eld, owing to the recent and future availability
of very high energy nuclear beams at the Large Hadron
Collider (LHC) at CERN and very high intensity beams
at the Facility for Antiproton and Ion Research (FAIR) at
GSI. These two central facilities, which are at the forefront
of the European research arena, will pave the way
for the exploration of completely unexplored regimes of
the strong interaction. The new generation of powerful
state of the art experiments will provide unprecedented
resolving power. The LHC will provide an energy increase
as compared to the Relativistic Heavy Ion Collider (RHIC)
by a factor of almost 30. We note that, in the past, each
major boost in energy scale has invariably been accompanied
by significant discoveries.
In the subsequent sections of this chapter the fundamental
questions which will be addressed in the coming
years by theory and experiment will be discussed, based
on a short review of some of the most salient accomplishments
of the past years. The strategies necessary
to ensure that the European Nuclear Physics community
will be able to play a leading role in this fi eld will be
delineated. In the fi nal section we will summarise the
prospects within reach and indicate the efforts that the
community considers as its priority.
4.2.2 The QCD Phase Diagram
Many of the features of the phase diagram of nuclear
matter, relevant for all energy scales, can be understood
from the collective properties of the system. In this section
we give an overview of some of the general features;
in subsequent subsections of this chapter we address
more specific probes of the conditions prevailing in the
different phases.
The properties of nuclear matter at fi nite temperature
T and net baryon density (or chemical potential µ B )
are describable from the theory of strong interactions,
QCD. The thermal properties and the equation of state
(EoS) of nuclear matter are best quantified within Lattice
QCD (LQCD), a numerical formulation of the theory on a
space-time grid, though analytical methods can provide
complementary insights. Experimentally, hot and dense
nuclear matter is explored through the study of collisions
of heavy ions at ultra-relativistic energies.
The phase diagram and the EoS from LQCD – LQCD
with dynamical quarks has provided new results on the
EoS and on collective phenomena in nuclear matter
at fi nite temperature and chemical potential. Recent
results on the equation of state at finite T and vanishing
chemical potential are shown in Figure 1. The energy
density increases rapidly in a narrow temperature interval.
Such behaviour is generally interpreted as being
due to deconfinement, i.e., the liberation of quark and
gluon degrees of freedom. The pressure also exhibits
Figure 1. Recent LQCD calculation showing the energy density
and pressure of nuclear matter as a function of temperature. The
calculations were performed in (2+1)-flavour QCD, i.e., they include
effects of two light (u, d) quarks and one strange. The predicted
QCD transition temperature is in the range 170-190 MeV.
(Courtesy of F. Karsch et al.)
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