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4.2 Phases of Strongly Interacting Matter
the finite T and baryon density as well as the fluctuations
and correlations of relevant physical observables.
First results on transport properties of the QGP are also
available from recent LQCD studies. However, in order
to explore properties of nuclear matter over a broad
parameter range at finite density and temperature it is
necessary to further develop analytical methods. Studies
within the perturbative and non-perturbative approaches
based on the Renormalisation Group techniques and the
Dyson-Schwinger equations have been very successful
in describing collective effects and critical phenomena
in dense nuclear matter related with chiral dynamics
and deconfinement.
LQCD is the central numerical method, derived from
fi rst principles, to describe the properties of hot and
dense nuclear matter over a broad parameter range.
However, to extract physically relevant predictions from
the calculations, extrapolations to the continuum limit
have to be made. This requires large-scale computing
and access to dedicated supercomputers with petaflop
performance.
Probing the QCD phase diagram in Heavy Ion
Collisions – Experimentally, different regions of the QCD
phase diagram can be probed in heavy ion collisions by
varying the beam energy of the colliding nuclei. At very
high energies, such as those reachable at RHIC and at
the LHC, the region of small µ B and large T is explored, for
which reliable LQCD predictions are available. At lower
energies, the regime of high µ B is probed at moderate
T, which can only be described with phenomenological
models. The first principle LQCD studies and effective
models, as well as heavy ion experiments, are essential
to characterise the phase structure and the EoS of hot
and dense nuclear matter.
The experimental exploration of the phase diagram
relies heavily on the applicability of thermodynamics to
the system created in heavy ion collisions. Once this is
established, phenomenological studies of the bulk properties
yield important information on thermal parameters
relevant for this exploration. Three important observables
to study global properties using hadron distributions in
the final state have been established: particle correlations,
particle yields and particle spectra.
Hadron correlations – Strong evidence for collective
expansion in heavy ion collisions is derived from the
observation of the anisotropy in particle momentum
distributions correlated with the reaction plane. One
of the most striking manifestations of anisotropic flow
and strong collective expansion is the so-called elliptic
flow. The strength of this elliptic flow is characterised
by the second Fourier coefficient (v 2 ) of the azimuthal
momentum-space anisotropy.
√⎺s NN (GeV)
Figure 2. Elliptic flow v 2 at mid-rapidity and integrated over
transverse momentum. Experimental data are extrapolated
to LHC energies. (Courtesy of N. Borghini et al.)
Figure 2 shows the measured dependence of v 2 on
the centre-of-mass energy. At low energies (E CM < 1.5
GeV) v 2 is positive refl ecting the angular momentum
conservation of di-nuclear systems, which leads to a
preferential emission in plane. With increasing energy
the sign changes to negative and v 2 reaches its lowest
value at an energy of about 2 GeV refl ecting particle
emission from the strongly compressed matter in the
centre of the collision that is shadowed by the passing
spectator nucleons. This causes the produced particles
to emerge perpendicularly to the reaction plane
leading to a negative value of v 2 (squeeze-out). At these
energies the elliptic flow is very sensitive to the nuclear
compressibility, i.e., the EoS. Above this, energy v 2 rises,
eventually becoming positive again. At AGS, SPS and
RHIC energies the timescale for spectator nucleons to
pass the created hot and dense system becomes much
shorter than the characteristic time for the build-up of
the transverse flow. At these energies the elliptic flow
becomes in plane again (positive v 2 ). The magnitude of
v 2 , above (E CM = 10 GeV), is directly proportional to the
initial spatial anisotropy and the interactions among the
constituents. The large elliptic flow observed indicates a
high level of equilibration at a relatively early stage of the
collision. Comparison of RHIC data to hydrodynamical
models suggests that equilibration occurs early in the
collision history and at the partonic level. Extrapolation
to LHC energies suggests very large values of the flow
and correspondingly large sensitivity to the initial conditions.
Hadron yields – Integrated yields of different hadrons
provide information on the medium properties. A detailed
analysis of heavy ion data from SIS(GSI) to RHIC(BNL)
energies has shown that relative yields of most hadrons
84 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010