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Figure 10. Elliptic flow parameter (v 2 ) versus transverse momentum
(p T ) for charged hadrons measured at RHIC and compared to
fluid dynamic simulations. The data come close to the predicted
limit of ideal hydrodynamics (green line), and favour fluid dynamic
evolutions assuming a small but finite ratio of shear viscosity over
entropy density. (Courtesy of M. Luzum)
advances have been paralleled by first calculations in the
strong coupling regime, based on novel string-theoretical
techniques and lattice QCD calculations. It is one of
the most remarkable findings of heavy ion physics in
recent years that these calculations predict values of
shear viscosity over entropy density which are much
smaller than the corresponding values for any known
substance and, at the same time, are consistent with values
extracted from RHIC data using phenomenological
models (see Figure 10). This lends support to a picture of
the deconfined QCD high-temperature phase, in which
particle-like constituents play no role for the observed
transport phenomena. This is distinctly different from
the picture of a gas of free or perturbatively interacting
quarks and gluons.
Heavy ion collisions at the LHC will determine whether
the above picture of a strongly coupled plasma characterises
the generic features of the QCD high-temperature
phase, or whether its range of validity remains limited to
the neighbourhood of the QGP transition temperature
to which RHIC has experimental access. In particular,
calculations of lattice-regularised QCD provide
indications that characteristic properties of the QCD
high-temperature phase, such as its interaction measure
(ε – 3p), or its bulk viscosity, undergo qualitative changes
if the temperature is raised well above the QGP transition
temperature, as should be possible at the LHC.
These changes are regarded as signalling the onset of
a gradual transition of the quark gluon plasma to more
and more gas-like properties at higher temperatures.
Experiments at the LHC will reach into this yet unexplored
high-temperature region. They can thus provide
insight into the question how Nature realises the transition
from a minimally viscous, strongly interacting fluid
to a gas-like state at extreme temperature. In the first
discovery phase of the LHC heavy ion programme, such
perspectives call for a multi-pronged approach.
LHC experiments are well set to constrain our understanding
of QCD thermodynamics and transport theory
in the QCD high temperature phase. Within the LHC
baseline programme of Pb+Pb collisions, this includes
the measurement of abundances, spectra and collective
flow in terms of their dependence on particle species,
transverse momentum, rapidity, collision centrality, etc.
In addition, it is necessary to better constrain the initial
conditions of the collective phenomena, since they are
currently a major source of uncertainty in determining
properties of hot matter. Experimental avenues to
this end are provided, in particular, by proton-nucleus
collisions, that provide an opportunity for identifying separated
from that of a complex collective expansion. Also,
the dependence of collective flow on the centre-of-mass
energy of the collision provides important constraints,
since it allows one to scan the dependence of properties
of matter on the initial temperature and density attained
in the collision.
On the theoretical side, the analysis of LHC data
requires phenomenological modelling of the collision
dynamics to relate experimental data to first principles
calculations in QCD. The development and further
improvement of essential modelling tools such as hydrodynamic
simulation programmes is of crucial importance.
Moreover, first principles calculations of important properties
of hot QCD, such as transport properties at strong
coupling, have only started. Numerical estimates indicate
that reliable LQCD simulations of transport coefficients
like shear viscosity require petaflops machines. Finally,
the recent calculation of shear viscosity by use of the
so-called AdS/CFT correspondence marks historically
the very first time that a string-theory based calculation
has triggered a field of experimental analysis and
has provided guidance for a class of challenging QCD
calculations. In recent years, a fruitful interdisciplinary
discussion between string theory and nuclear physics
has developed, which promises conceptually novel
approaches to long-standing questions in our understanding
of the QCD high-temperature phase. These
efforts should be further encouraged and supported.
Characterising the QCD plasma with hard probes
Experiments at RHIC have established that in nucleusnucleus
collisions, the production of hadrons at high
transverse momentum (above about p T = 5 GeV/c) is
strongly suppressed. This is evidenced by the so-called
nuclear modification factor R AA . Single inclusive hadron
spectra show a suppression of the high-p T particle yield
by a factor ~5 in the most central collisions (see Figure 11)
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 95