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2 Theoretical background
Cold dense medium absorbs some of the created J/ψ. This effect is called normal suppression.
But even if one takes this normal suppression into account, an additional suppression
was seen by the NA50 collaboration (see figure 2.3). This additional suppression
is likely caused by the presence of a QGP [NA500].
For LHC energies the expectation is different. At these collision energies a large number
of charm and anti-charm quarks will be created. At hadronization there is a certain
chance that charm and anti-charm quarks will be close together just for statistical reasons
and form a J/ψ. Therefore an enhancement rather than a suppression is expected for the
LHC data [BMS07].
2.3.2 Jet-Quenching
During the collision of nuclei in an accelerator, hard parton-parton collisions take place.
Depending on the collision parameters the two partons can get large transverse momenta.
Without the presence of a QGP the scattered partons fragment directly into
hadrons which are moving in the direction of the primordial parton. This is called a
jet. Since both scattered partons form a jet one sees two jets in an ϕangle of 180 °.
In a QGP the situation is different. The parton scattering takes place in a very early
stage of the collision. In most cases the scattering does not happen just in the center of
the collision zone. Therefore one of the partons has to travel a longer distance in the hot
and dense QGP where it will loose energy. As a result, the correlation between the two
jets should be distorted [YHM05].
2.3.3 Strangeness Enhancement
Strange particles are created always in pairs due to strangeness conservation. Therefore
in collisions without a QGP two mesons have to be created. The creation of a kaon pair
has a threshold energy of about 987 MeV. Since the kaon is the lightest meson with a
strange quark the mentioned threshold energy is the general threshold energy for the
creation of particle pairs with strange quarks. In the quark gluon plasma the ss can be
produced directly. Therefore only the energy for the two quarks is needed. The threshold
energy for the creation of a ss pair is about 300 MeV, much lower than for a kaon pair.
Consequently, the number of produced strange particles will be higher in case of a QGP
[YHM05].
2.3.4 Direct Photons
Direct photons are all photons not originating from hadronic decays. They can be subdivided
in prompt photons emitted during hard scattering and thermal photons emitted
from a thermally equilibrated phase. Since photons are not affected by the strong interaction
and hadronization processes, they are a tool to study the different stages of a heavy
ion collision.
Due to a large background from decay photons, direct photon signals are difficult to
extract from the measured data. Uncertainties in the theoretical calculations complicate
the interpretation of the data [KB04, ST01].
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