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Figure 7. Isotopic dependence of the break-up temperature
extracted experimentally from the relative population of isotopes,
the so-called isotopic thermometer. (Courtesy of C. Sfienti et al.)
finite temperatures. This information can be extracted
from isotopic ratios of light particles and fragments from
carefully selected space-time emission regions. These
regions will be separated using collective observables
and imaging techniques, which are already currently
developed in the field.
Isospin fractionation – The comparison of data with
different asymmetries and similar centrality will also allow
the different isotopic composition of coexisting phases to
be quantified for isospin asymmetric systems, a phenomenon
known as isospin fractionation. Up to now this has
only been studied for stable systems. Fractionation is a
generic feature of phase separation in multi-component
systems. In particular, since an increased fractionation
is expected, if fragmentation occurs out of equilibrium, a
quantitative study of fractionation will elucidate the role
of spinodal instabilities in the as yet unclear mechanism
of fragment production.
Modelling the nucleonic regime – At the theoretical
level, extraordinary progress has been achieved in
the past years connected to microscopic calculations
for nuclear matter at sub-saturation densities, where
correlations and clustering of nucleons into fragments
dominate. The extension of these calculations to neutron
rich systems will be available in the next few years
and will need to be confronted with experimental data.
New progress in the studies of dynamical non-relativistic
transport theories (TDHF and its extension) will be
essential for the interpretation of transport observables.
Such calculations are required to quantitatively extract
the EoS from experimental data, and should also be
strongly encouraged.
Requirements – The availability of intermediate energy
beams up to several hundreds of MeV per nucleon is
essential to test regions of different baryon (isoscalar) and
isospin (isovector) density during the collision process.
For this reason it is important to complete the superFRS
programme at FAIR, and to begin the construction of
adequate post-accelerators at SPIRAL2 and possibly
SPES, steps towards a future EURISOL facility.
To extract the isovector part, one needs to measure
differential quantities (i.e., ratios of proton-neutron, or
3 He- 3 H). To minimise theoretical as well as experimental
uncertainties, it is important to compare systems
of similar size but markedly different N/Z, since in this
case the difference between the predictions of different
EoS is amplified.
Measuring collective observables necessitates full
event reconstruction with low thresholds as well as A
and Z identification for heavy elements, as in the FAZIA
project.
4.2.4 Exploring the QCD
Phase Diagram at Large
Baryon-Chemical Potentials
One of the goals of future heavy ion collision experiments
at relativistic beam energies is the precise scanning of
the QCD phase diagram in the region of high net-baryon
densities.
Such experiments address fundamental physics questions:
What are the properties of very dense nuclear
matter Is there a first order phase transition between
hadronic and partonic matter Is there a critical or a triple
point and, if yes, where are these points located Is there
a chiral phase transition and, if yes, does it coincide with
the deconfinement phase transition Are there new QCD
phases such as ‘quarkyonic’ matter
As mentioned in the previous chapter, the observation
of a limiting chemical freeze-out temperature of about
160 MeV indicates a change in the degrees-of-freedom
of the fireball. Such temperatures may be reached using
heavy ion collisions between beams with energies of
about 30 A GeV on fixed targets. At the same energy,
maxima in the excitation functions of the ratio of strangeto-nonstrange
particles have been found (see previous
chapter). This observation has been interpreted as a
signature for a transition from baryon to meson dominated
matter, but is still controversial. In particular, the
strangeness-to-entropy ratio measured by NA49 at
SPS energies exhibits a sharp structure, which cannot
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