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4.2.5 The High-Energy Frontier
The much higher centre-of-mass energies reached by
hadron colliders, notably LHC at CERN, provide entirely
new opportunities for studying the phases of QCD matter.
They provide, for the first time, access to the previously
unexplored ultra-dense region of the QCD phase diagram
far above the QGP transition temperature. In particular,
progress is expected on the following subjects:
1. Collective phenomena above the QGP transition.
Generally, increasing the centre-of-mass energy in
nucleus-nucleus collisions implies that the matter
produced in the collision is initially denser, equilibrates
faster and at a higher initial temperature, maintains
equilibrium for a longer time, and fills a larger volume
of space-time. All these features play a crucial role in
the development of collective phenomena and thus
help in determining macroscopic properties of hot
and dense QCD matter.
2. Unprecedented access to hard probes of dense
matter.
Scatterings with high momentum transfers, so called
‘hard probes’, lead to the production of jets, quarkonia
and high-transverse momentum hadrons. The dramatic
increase of the production cross-section makes
them abundantly available at the TeV scale. The strong
medium-modification of high-transverse momentum
hadrons and jets, first seen at RHIC, allows properties
of dense QCD matter to be characterised, while the
study of the entire quarkonia families is expected to
provide observables directly related to the deconfinement
transition.
3. Saturated initial conditions.
In general, the characterisation of properties of hot
QCD matter in heavy ion collisions relies on a complete
understanding of the initial conditions from which
this matter is produced, including the structure of
the colliding nuclei. At collider energies, bulk hadron
production is dominated by nuclear parton distributions
at very small momentum fractions x. The parton
distributions can be studied in proton-nucleus collisions
at high energies. They are expected to reflect
a qualitatively novel, maximally saturated state of
cold QCD matter, which is solely accessible at ultrarelativistic
energies.
Within the last decade, experiments at RHIC have
started to substantiate the above-mentioned opportunities
with data on A-A and d-A collisions at centre-of-mass
energies of up to 200 GeV. The Large Hadron Collider,
LHC, at CERN, has just begun physics operations in
2010 and is expected to be the world’s most powerful
accelerator for several decades to come. Its baseline
programme foresees the study of Pb-Pb collisions at
centre-of-mass energies up to 30 times higher than
possible at RHIC. The dedicated heavy ion experiment
ALICE, and smaller communities in ATLAS and CMS,
have approved programmes for heavy ion physics. In
addition, the LHC allows for proton-ion collisions and for
the collision of lighter ions. Such an increase in centreof-mass
energy is unprecedented in the field of heavy
ion physics. In the history of physics, order-of-magnitude
increases in energy have always led to unforeseen discoveries
which opened future new directions of research.
Therefore, planning must remain flexible to cope with
unforeseen and new findings emerging from first LHC
data. Here, we exclusively focus on those opportunities
which are strongly motivated by the current status of
theory and experiment.
Characterising QCD thermodynamics and QCD
hydrodynamics – The dense matter produced in ultrarelativistic
heavy ion collisions locally exhibits random
thermal motion. Yet, at the same time, it flows globally
following pressure gradients determined by the global
geometry of the collision. This picture is supported by the
measured abundance of hadrons and their momentum
spectra over a wide range of collision energies at the
CERN SPS and RHIC. As a consequence, the study of
collective flow has become a major tool for the characterisation
of hot QCD matter. Collective flow effects
increase with collision energy, consistent with the idea
that an increased initial density results in larger pressure
gradients driving the collective motion (see Figure 4).
The elliptic flow measured at RHIC is close to the
predictions of ideal hydrodynamical models for momenta
below 2 GeV (see Figure 10). This finding is remarkable,
since ideal fluid dynamics describes the limiting case,
in which matter is in perfect local thermal equilibrium.
Collective motion is then maximal, leading to the most
efficient response to pressure gradients. In this limit,
collective dynamics depend entirely on the QCD equation
of state, which may then be determined.
Characterising possible deviations of the elliptic flow
signal from ideal hydrodynamics provides avenues for
studying the properties of the QCD high temperature
phase. Generally, these deviations arise from dissipative
phenomena, which can be described by transport
coefficients and relaxation times, as long as a fluid
dynamic picture is valid. Transport coefficients, such
as the shear viscosity, are of as fundamental importance
as the Equation of State, in the sense that they
can be calculated from first principles in QCD. Figure 10
illustrates how data on elliptic flow make it possible to
experimentally constrain the shear viscosity, a particularly
important transport coefficient. These experimental
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