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4.2 Phases of Strongly Interacting Matter
compared to elementary p+p collisions. This suppression
persists for all measured hadrons up to the highest transverse
momenta measured so far. The suppression has a
characteristic centrality dependence: it vanishes in very
peripheral collisions and is not observed for photons.
These findings support a picture in which highly energetic
quarks and gluons, produced in the first partonic
collisions, lose energy in the surrounding dense QCD
matter prior to fragmenting into the observed high-p T
hadrons. This interpretation is also supported by the
measurement of jet-like particle correlations.
With the discovery of the ‘jet quenching’ phenomenon
at RHIC, the study of high transverse momentum
processes has become one of the major new fields of
research in high-energy nucleus-nucleus collisions. This
is so, since hard processes promise to provide qualitatively
novel tools for the study of hot QCD matter. On the
one hand, they can be calibrated both experimentally
and theoretically with unprecedented accuracy in the
absence of medium effects. On the other hand, they
show on top of this well-controlled baseline a strong sensitivity
to properties of the hot and dense QCD matter.
For the useful exploitation of hard processes as
probes of the medium, an unambiguous interpretation
of jet quenching in terms of specific medium properties
is needed. This requires experimental constraints on the
parton dynamics underlying jet quenching. At present,
the conclusions about medium properties drawn from
jet quenching are consistent with estimates of the matter
density obtained by other means. However, they show
significant model dependences. This currently limits
the practical use for characterising properties of the
produced plasma. These uncertainties can be largely
removed by additional measurements, including for
instance the modification of characteristic internal jet
structures such as jet multiplicity, jet broadening, and
jet hadrochemistry, or the determination of the hierarchy
in the suppression pattern of light-flavoured and
heavy-flavoured hadrons. Such refined measurements
have the potential to provide detailed information about
the parton composition of the produced hot matter and
about its transport properties. To date, data from such
refined measurements are either not yet available, or their
precision allows only for rather qualitative conclusions
about the properties of the medium.
As may be seen from Figure 12, the higher centreof-mass
energy at the LHC will make it possible to
extend the kinematic reach for the characterisation of
hard processes in dense QCD matter by, typically, one
order of magnitude in transverse momentum. The much
larger production rates at higher centre-of-mass energy
drastically improve the statistical precision over that of
previous measurements. The much wider kinematic reach
also facilitates the identification and analysis of hard
Figure 11. Nuclear modification factor of neutral pions as a
function of p T , measured in central √s = 200 GeV Au-Au collision
at RHIC. R AA is the ratio of the particle yield in nucleus-nucleus
collisions, compared to the yield in an equivalent number of protonproton
collisions.
probes, since they stand out more prominently above
the background, even in the high-multiplicity environment
of heavy ion collisions. In addition, qualitatively novel
measurements of hard probes, such as ‘true’ jets above
50 GeV, and their internal structure become experimentally
accessible. They will clarify the above-mentioned
questions, and will significantly enhance the use of hard
probes for characterising plasma properties.
Beyond clarifying central open questions in the current
interpretation of jet quenching, the study of hard probes
at the LHC is also expected to open up qualitatively new
directions in the investigation of extreme QCD matter. A
prominent example is the measurement of confinementrelated
observables by a characterisation of the entire
charmonium and bottomonium family in hot QCD matter.
The radii of the tightly bound heavy quark-antiquark
systems provide a unique set of decreasing length scales
in strong interaction physics. On general grounds, it is
expected that the attraction between a heavy quark
and an antiquark is sensitive to the medium in which the
bound state is embedded. This attraction weakens with
increasing temperature, when the medium screens the
quark colour charges from each other. Some quarkonia
states are through their radii sensitive to the screening
length, the natural length scale displayed by the medium
produced in heavy ion collisions, which is directly related
to the inverse temperature 1/T. Thus, a characterisation
of the yield of bound states of both families provides a
unique opportunity for characterising the temperature
and screening of the QGP. Lattice QCD results provide
predictions of the mass-hierarchy of these charmonium
suppression patterns. At the LHC, such measurements
of sufficiently abundant yields of charmonium and bottomonium
states will become experimentally accessible
for the first time.
96 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010