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4.2 Phases of Strongly Interacting Matter
be described by hadronic models. This disagreement
between theoretical estimates and data has caused
speculation about the possible onset of deconfinement
already at low SPS energies.
A careful beam energy scan will be required to discover
structures possibly caused by the deconfinement
phase transition and/or the critical endpoint in the QCD
phase diagram. In order to obtain a consistent picture,
one has to investigate a comprehensive set of observables,
and search for a non-monotonous behaviour in their
excitation functions. The challenge is to identify signatures
of the partonic phase which survive hadronisation.
It is obvious that those observables which are generated
in the early phase of the collision, and which are not
distorted by final-state interactions during the evolution
of the fireball, are the most promising candidates in this
respect. These observables are discussed below.
Collective flow – One of the observables which is sensitive
to the initial (anisotropic) fireball shape in coordinate
space is elliptic flow. A central question is whether the
hadron elliptic flow, although expected to be significantly
smaller in magnitude than at RHIC energies, will
show features similar to those found at high energy. In
particular, whether the flow scales with the number of
constituent quarks, thereby suggesting that the effect
originates already in the partonic phase. Will this scaling
feature disappear below a certain beam energy The
answer to this question requires a beam energy scan of
the elliptic flow of pions, kaons, phi-mesons, D-mesons,
charmonium, as well as of nucleons, and (multi-) strange
hyperons (including the antiparticles). The experimental
challenge will be to measure all these particles up to high
transverse momenta.
Particles with low hadronic cross sections like Φ mesons,
Ω hyperons and J/φ mesons are expected to be
particularly sensitive probes of the partonic phase.
Indeed, in Pb+Pb collisions at 158 A GeV the inverse
slope parameter (or effective temperature) of Φ, Ω, and
Jφ and φ’ is found to be T eff = 200 – 250 MeV which is
significantly lower than T eff for protons or Lambdas. This
observation indicates that Φ, Ω, and Jφ and φ’ pick up
less radial flow, and their T eff values are dominated by
the temperature of an earlier phase of the system evolution.
A similar observation was made for lepton pairs.
The inverse slope parameters T eff of the dimuon transverse
momentum spectra measured in In+In collisions
at 158 A GeV increases with invariant mass of the muon
pair up to 1 GeV/c 2 , and then drops and stays constant
for heavier masses. A possible interpretation of this effect
is the following: the low mass muon pairs are created via
π-π collisions and, hence, are blue-shifted by the collective
radial motion of hadrons, whereas the heavy muon
pairs are created via q-q fusion in the partonic phase.
Charm production and absorption – Heavy charm
quarks are very promising diagnostic probes of hot and
dense nuclear matter. The (c,c-bar) pairs are created in
hard parton collisions in the initial stage of the nucleusnucleus
reaction, and subsequently propagate through
the dense medium. If this medium is deconfined, Debye
screening hinders the formation of the charmonium
hadronic state, and the charm quarks mostly combine
with light quarks into hadrons with open charm. A suppression
of the J/Ψ yield relative to muon pairs from
Drell-Yan processes was observed by the NA50 collaboration
for central Pb+Pb collisions at 158 A GeV.
However, absorption of J/Ψ in cold nuclear matter also
leads to significant charmonium suppression, which
is able to explain most of the experimentally observed
effect. Beyond this cold nuclear matter effect, an ‘anomalous’
suppression of J/Ψ mesons by about 25% is still
visible in very central Pb+Pb collisions. In establishing
this result, knowledge about the J/Ψ absorption cross
section obtained from measurements in p+A collisions
has been essential. In order to disentangle charmonium
absorption in cold nuclear matter and shadowing effects
from charmonium dissociation due to Debye screening
in partonic matter, high-precision multi-differential data
on charmonium and open charm production in nucleusnucleus
and proton-nucleus collisions are needed.
The suppression of charmonium can be ideally studied
by normalising the yield of J/Ψ and Ψ’ mesons to that
for charmed mesons. However, no measurement of D
mesons has been performed in heavy ion collisions at
SPS energies up to date.
Future experiments will have to perform comprehensive
and systematic measurements of open and hidden
charm in order to fully exploit the potential of charm as
a diagnostic probe of dense baryonic matter.
Critical fluctuations – The presence of a phase transition
is associated with a rapid change (with temperature
and chemical potentials) of the thermodynamic susceptibilities,
which reflect the fluctuations of the active degree
of freedom of the system. The well-known phenomenon
of critical opalescence is a result of fluctuations at all
length scales due to a second order phase transition.
First order transitions, on the other hand, give rise to bubble
formation, i.e., large density fluctuations. Therefore,
an experimental search for a possible critical point and
for a first order phase coexistence region in the QCD
phase diagram has to include the measurement of particle
number or momentum fluctuations event by event
and correlations in heavy ion collisions as function of
beam energy. Fluctuations of higher-order moments
of particle distributions are expected to be particularly
sensitive to the correlation length, which should fluctuate
at the critical point. Experiments at top SPS and RHIC
90 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010