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4.2 Phases of Strongly Interacting Matter
effective masses determines the kinetic observables
established in the entrance channel, i.e., the transverse
momentum distributions of pre-equilibrium particles and
collective flow. They will need to be explored in detail
in the near future.
The liquid-gas phase transition – Nuclear matter is
now known to undergo a transition from its liquid ground
state to a gaseous state of nucleons at a temperature
of few MeV. Impressive progress has been achieved
in the past years towards the characterisation of this
phase transition. This progress is largely based on the
exploitation of data analysed with second-generation
4 detectors like INDRA and CHIMERA and other more
specialised detector arrays. In particular, approximate
values for the temperature, energy and density of this
phase change have been established. Studying the transition
with finite nuclei has the extra advantage of also
revealing the thermodynamic anomalies, which should be
associated with first order phase transitions of any finite
system (negative heat capacity, bimodal distributions).
A negative heat capacity from anomalously large
kinetic energy fluctuations was observed in the past,
but the signal has turned out to be inconclusive. This is
mainly attributed to uncertainties connected to the evaluation
of the effect of secondary decays. Very recently
bimodal distributions of the order parameter have been
measured for the Au+Au system using different experimental
setups (Figure 6).
The high Z 1 peak corresponds to evaporation events,
while the low Z 1 peak is associated with the expected
opening of the multi-fragmentation channel. The presence
of a minimum between the two is non-trivial, and
it is interpreted as a signature of the finite system counterpart
of a first order phase transition.
INDRA
Au+Au 60 AMeV
80 AMeV
100 AMeV
For the first time, these data have allowed a determination
of the latent heat of the transition. Experimentally,
this is revealed by the difference between the average
excitation energy associated with the two observed
peaks.
Phase transition in asymmetric matter – To identify the
transition line in the global phase diagram and explore
its isotopic dependence, it is necessary to carry out
multi-fragmentation experiments with exotic beams (i.e.,
beams of extreme N/Z ratios). First exploratory results
on the isotopic dependence of the nuclear caloric curve
have been obtained with the ALADiN experimental setup
at GSI (Figure 7). Temperatures appear to be very slightly
influenced by the isospin degree of freedom within the
valley of stability.
In the next-generation experiments, the onset of fragmentation
will be established using the techniques that
have been developed for stable beams. The change
of the fragmentation threshold with the source charge
and asymmetry will provide access to the charge and
asymmetry dependence of level densities and limiting
temperatures. Experiments exploring the asymmetry
in a yet inaccessible region through the study of scaling
violations of fragment observables may reveal new
physics. The study of multi-fragmentation with exotic
beams has important astrophysical consequences.
Indeed, multi-fragmentation is a unique laboratory for
the formation of inhomogeneous structures due to the
interplay between nuclear and Coulomb effects. Such
structures have to be correctly modelled for the supernova
explosion process and the cooling dynamics of
proto-neutron stars. Moreover, the electron capture rate
on nuclei and/or free protons in pre-supernova explosions
is especially sensitive to the symmetry energy at
MULTICS
35 AMeV
Figure 6. Distribution of the heaviest fragment (normalised to the source size in the left panel) in the fragmentation of a 197 Au nucleus, clearly
exhibiting a bimodality signal. Results from different beam energies and experimental setups and data analysis techniques are shown to be
fully compatible. (Courtesy of E. Bonnet, M. D’agostino et al.)
88 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010