Abstract
In relativistic collisions between nuclei, the creation of a strongly interacting medium, called the Quark Gluon Plasma (QGP), is expected. It is expected that such a medium also existed in the early universe just after the Big Bang. The phase transition of interest is where the dense medium of free
... read more
and unbound quarks and gluons cools down to form the ordinary matter consisting of hadrons. The phase transition from ordinary matter to a QGP is predicted in lattice formulations of Quantum Chromo Dynamics at energy densities well above a critical value of 1 GeV/fm3. At present a QGP might also exist in the dense cores of neutron stars. Properties of the QGP can be probed by high-energy partons interacting with the constituents of the QGP. In this interaction the high-energy parton exchanges momentum with the medium. As a consequence, the parton loses energy by radiating gluons. This process is called jet quenching. High-energy partons are created in the early phase of a heavy-ion collision in hard scattering processes of the constituents of the colliding nucleons. They are a well calibrated probe since the initial parton production rates can be calculated with perturbative QCD. The scattered partons propagate through the medium and fragment into jets of hadrons. This process is expected to be modified in heavy-ion collisions compared to proton-proton collisions due to jet quenching. In this thesis a phenomenological study as well as an experimental observable of jet quenching is presented. A selection of parton energy loss models is studied in a simplified and a realistic medium geometry. Due to different approximations in the parton energy loss implementations of the models, the predictions of the medium energy density in heavy-ion collisions differs significantly. High-pT measurements at RHIC and LHC are compared to model calculations. All models have difficulties describing multiple observables simultaneously. The production of jets in heavy-ion collisions is measured with data collected by ALICE at the LHC at √sNN=2.76 TeV. For this analysis jets are reconstructed from charged particles detected in the central tracking detectors of ALICE. The charged particles are the constituents of the jets and are measured with a high efficiency down to very low transverse momenta (150 MeV/c). The challenge in heavy-ion collisions is to separate the particles originating from the hard parton and from the soft underlying event. The background from soft particle production is determined for each event and subtracted. The remaining influence of underlying event fluctuations is quantified by embedding different probes into heavy-ion data. A strong suppression of inclusive jet production is observed in central events when the energy density of the created plasma is highest. For more peripheral events the jet spectrum is less suppressed. The observed suppression for jets is found to be similar to charged hadrons, which suggests that the radiated energy captured by the jet is limited. The ratio of jets with resolution parameters R=0.2 and R=0.3 is found to be similar in pp and Pb-Pb events, indicating no strong broadening of the radial jet structure.
show less
