This thesis presents the first measurements of jets in relativistic heavy ion collisions as reported by the ATLAS Collaboration. These include the first direct observation of jet quenching through the observation of a centrality-dependent dijet asymmetry. Also, a series of jet suppression measurements are presented, which provide quantitative constraints on theoretical models of jet quenching. These results follow a detailed introduction to heavy ion physics with emphasis on the phenomenon of jet quenching and a comprehensive description of the ATLAS detector and its capabilities with regard to performing these measurements.

Muons originating from background sources, primarily Charm hadrons, pion and kaon decays, have been removed from the analysis using template fits to the distribution of a quantity(p T^rel) capable of statistically distinguishing between signal and background. The measured nuclear modification factor R AA has been presented in different centrality bins as a function of the b-jet transverse momentum p T.The results of R AA indicate that the yield of the most central event (0-10%) experiences more suppression compared to the most peripheral event (60-80%) by a factor of approximate 2.

The R_AA shows the strongest suppression in central collisions and the least suppression in peripheral collisions. It shows a slight increase with jet p_T and a decrease with increasing rapidity at high p_T. Finally, the dijet asymmetry for R=0.4 jets is also reported in Xe+Xe collisions at √sNN = 5.44 TeV compared to Pb+Pb and pp collisions at √sNN = 5.02 TeV. No difference is observed between Pb+Pb and Xe+Xe collisions, within the uncertainties of the measurement, as a function of the number of participants or the collision centrality.

The collision of highly relativistic nuclei can produce a volume of high energy density which can be used to learn about the behavior of quantum chromodynamics (QCD) at extreme conditions, such as those of the universe at times before and during the formation of hadrons out of quarks and gluons. This thesis presents experimental studies of jet-quenching phenomena in PbPb collisions at [square root of]sNN = 2.76 TeV, with a focus on the study of the energy-loss, namely the analysis of correlations of jet pairs as measured by the CMS detector at LHC. The implications of the results are discussed by comparison of simple models with data. It is observed that the geometrical configuration of the dijets and the nuclear collision region plays an important role in the correlations between the jets, and the set of CMS results reviewed can illustrate the dependence of the amount of lost energy on the length of the traversed medium.

Annotation. Text reviews the major topics in Quark-Gluon Plasma, including: the QCD phase diagram, the transition temperature, equation of state, heavy quark free energies, and thermal modifications of hadron properties. Includes index, references, and appendix. For researchers and practitioners.

A new jet tomographic model and numerical code, CUJET, is developed in this thesis and applied to the phenomenological study of the Quark Gluon Plasma produced in Heavy Ion Collisions.