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can be described with statistical hadronisation models
using only two global parameters: the chemical freezeout
temperature and the baryon-chemical potential.
While this observation is striking, it is only necessary but
not sufficient evidence for the thermal origin of particle
production. However, the indication of early equilibration
from elliptic flow measurements makes a thermal
description the most plausible one, in particular at high
energies beyond SPS. However, measured yields at
lower energies are also consistent with a thermal picture.
The thermal freeze-out parameters obtained from
statistical model fits to data at different energies are
shown in Figure 3. All data points approximately follow a
curve (solid line) corresponding to a value of the energy
per particle in the system of about 1 GeV. In addition,
the chemical freeze-out temperatures, for values of the
baryon-chemical potential < 400 MeV, agree well with
the phase boundary predicted by LQCD. This observation
suggests that the particles detected in heavy ion
collisions at high energies originate from the hadronising
QGP. At centre-of-mass energies near 10 GeV, where
the freeze-out approximately decouples from the LQCD
transition line, interesting properties of different observables
have been found.
Similarly, pronounced maxima are observed in the
individual yield ratios of K + /π + , Λ/π and Ξ/π. In addition,
data show that near this energy the volume obtained
from Hanbury-Brown and Twiss correlations exhibits a
minimum and the system passes from baryon to meson
dominance.
Recently, it was conjectured that the above features
of hadron production observed in nuclear collisions can
be explained by the existence of three forms of matter:
Hadronic Matter, Quarkyonic Matter and a Quark-Gluon
Plasma which meet at a ‘triple point’ in the QCD phase
diagram located at the centre-of-mass energy near
10 GeV.
Figure 4 shows the excitation function of the ratio
of strange to non-strange hadrons, as a function of
centre-of-mass energy. This excitation function shows
a maximum around 10 GeV.
Hadron spectra – Hadron spectra provide complementary
information on the medium evolution. The
shape of the spectra of most hadrons at low transverse
momentum is consistent with thermal emission of a
collectively expanding source. While the shape alone
does not demand a thermal description, the evidence
from elliptic flow and the consistency with the hadron
abundances make an interpretation of spectra in terms
of models inspired by hydrodynamics meaningful.
The particle yield as a function of transverse momentum
reveals the properties of the system at the kinetic
freeze-out, where interactions of hadrons cease. In the
simplified version of such models hadron spectra can be
Figure 3. Value of temperature and baryon chemical potential
obtained from statistical model fits to yields of different hadrons
produced in heavy ion collisions over a broad span of beam
energies. Note that for low baryon chemical potential values
the experimental values are close to the phase transition region
predicted by Lattice QCD.
Figure 4. Energy dependence of the strangeness to entropy
ratio quantified by measured yields of all strange to non-strange
hadrons in heavy ion collisions. (Courtesy of A. Andronic et al.)
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 85