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elevant experimental and analysis resources, including
the significant computing resources needed for triggering
on hard probes and analysing the data.
Figure 12. Expected yield of several classes of hard processes as
a function of transverse energy E T , resp. transverse momentum p T
calculated for one month of Pb-running with design luminosity at
5.5 TeV. (Courtesy of P. Jacobs and M. van Leeuwen)
The first experiments with heavy ions at the LHC will
mark the start of a discovery era, exploring a vast, as
yet uncharted kinematic regime. In the above, we have
highlighted fundamental open questions in the investigation
of hard probes, which can be firmly motivated
and can be addressed by experiments at the LHC. It is
conceivable that, in addition, the much higher precision
and much wider kinematic range of the LHC brings further
fundamental questions into experimental focus, or that it
reveals profound new phenomena. Since the availability of
ion-beams at dedicated proton-proton colliders depends
on many factors, it will be mandatory to react swiftly and
to support strongly any experimental follow-up which may
emerge during the LHC discovery phase.
According to our current understanding, the full exploitation
of the novel physics opportunities for hard probes
at the high-energy frontier requires, first of all, a detailed
exploration of the novel kinematic regime with statistical
and systematic precision. This calls for sufficient beam
time close to nominal luminosity, as well as the availability
of proton-proton, proton-Pb, and possibly lighter ion
beams to be able to discriminate plasma properties from
other nuclear effects. These require a long-term experimental
programme. It also calls for full support of all
Finally, we emphasise the important role of nuclear
theory in analysing the vast amount of data on hard
probes at hadron colliders. In the study of hard probes,
nuclear theory faces the challenge to interface a highly
sophisticated and experimentally tested understanding
of hard QCD processes in the vacuum with an a
priori unknown interaction of these processes with the
QCD plasma, into which they will be embedded for the
first time. The role of theory is not limited to providing
firm first-principle calculations of the sensitivity of hard
probes to QCD thermodynamic and transport properties.
It also includes the development and further improvement
of complex phenomenological modelling tools,
which are indispensable for relating measured mediummodifications
of hard processes to characteristic plasma
properties. And it starts to include essential theoretical
contributions to data analysis techniques, such as the
recent developments of fast jet finding algorithms, which
can perform within the high-multiplicity environment of
heavy ion collisions. This multi-faceted work is needed
to identify new opportunities and to draw firm conclusions
in a timely fashion. All work towards an improved
interplay between experiment and theory should be
strongly supported.
Saturated gluon matter – The knowledge of the density
of quarks and gluons (partons in general) in a proton or
a nucleus is crucial information for the understanding
of high-energy scattering. While the parton distribution
functions (PDFs) are relatively well-known for the proton,
nuclei cannot be treated as simple superpositions of
protons and neutrons. Their PDFs are subject to large
uncertainties in kinematic regions of interest to current
experiments.
Even more interestingly, the parton density seen in a
proton or nucleus is known to increase at large momentum
transfer Q 2 (i.e., high spatial resolution) when the
momentum fraction x they carry decreases. At low parton
density, this density increase is linear and can successfully
be described within perturbative QCD. This increase
cannot, however, continue indefinitely. At some point
the large number density of gluons would violate fundamental
unitarity bounds and, in fact, for large densities
non-linear effects, become important and compensate
the increase with a corresponding decrease due to gluon
fusion processes. This balance of creation and annihilation
leads to the so-called gluon saturation.
Gluon saturation is a small x phenomenon that sets in
below a certain characteristic scale in Q, the saturation
scale Q s . This scale, and with it the momentum range
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 97