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.
Several aspects of hard and semihard QCD jets in relativistic heavy ion collisions are discussed, including multiproduction of minijets and the interaction of a jet with dense nuclear matter. The reduction of jet quenching effect in deconfined phase of nuclear matter is speculated to provide a signature of the formation of quark gluon plasma. HIJING Monte Carlo program which can simulate events of jets production and quenching in heavy ion collisions is briefly described. 35 refs., 13 figs.
We apply HYDJET++ model, which contains the treatment of both soft and hard processes, to study the heavy-ion collisions at LHC energies. The interplay of parametrised hydrodynamics and jets describes many features of the development of particle anisotropic flow including the break-up of mass hierarchy of elliptic and triangular flow, the falloff of the flow at certain transverse momentum and violation of the number-ofconstituent- quark (NCQ) scaling at LHC energies compared to the lower ones. Other signals, such as long-range dihadron correlations (ridge) and event-by-event (EbyE) fluctuations of the flow are also discussed. Model calculations demonstrate a good agreement with the available experimental data.
Jets have been studied in high energy heavy ion collisions by measuring the angular correlation between particles at high transverse momentum. Differences in the yield and shape of the angular correlations as a function of system size give information on the medium produced in the collision. Such modifications can be used to infer the presence of a Quark-Gluon Plasma phase, wherein parton degrees of freedom are manifest over nuclear rather than nucleonic scales. In the present work, two-particle correlations were studied in \(d+Au\) and \(Au+Au\) collisions at \(\sqrt{s_{NN}}\) = 200 GeV measured by the STAR experiment at RHIC. The technique was extended to include pseudo-rapidity, permitting jets to be characterised in two-dimensions, and enabling the jet shape to be studied in greater detail. Corrections were developed for the incomplete detector acceptance and finite two-track resolution. Both unidentified and identified particle correlations were studied, using charged tracks and neutral strange particles \(\Lambda, \overline{\Lambda}\), and \(K^0_{Short}\) reconstructed from their characteristic \(V\)0 decay topology. The focus of the analysis was the correlation peak centred at zero azimuthal separation, which is significantly enhanced in central \(Au+Au\) collisions compared to lighter systems. The modified peak was found to comprise a jet-like peak broadened in the pseudo-rapidity direction, sitting atop a long range pseudo-rapidity correlation. The former is suggestive of jet modification by the medium, and the latter may indicate a medium response to jets. Correlations with identified particles indicated the modified same side peak may in part be formed from particles originating from the underlying event.
This work has studied the particle composition of jets by determining the ratios p±/p± and K±/p± in Au + Au and d + Au collisions at vsNN = 200 GeV measured by the STAR experiment at RHIC. Jets were found by measuring the angular azimuthal correlation between particles at high transverse momentum. Jets were then identified by their back-to-back correlation. A technique was developed to identify charged particles using their specific ionisation measured in the STAR Time Projection Chamber (TPC). This thesis contains one of the first implementations of using direct photons to tag jets in heavy ion collisions. An attempt was made to extract a trigger sample rich in direct photons from neutral triggers. The hadron ratios were calculated from the jet yields as a function of transverse momentum in each collision system. Although the away side yield is suppressed in central Au + Au, there is no evidence that the relative particle yields are changed. The hadron ratios for the three systems were found to be consistent with simulated p + p events generated using the Pythia Monte Carlo event generator. This reinforces the conclusion that the fragmentation process is unchanged by interactions with the medium.
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.
Papers of the June 1989 meeting in Beijing by the China Center of Advanced Science and Technology. This small book covers nucleus- nucleus collisions, states of the vacuum, and highly relativistic heavy ions in the experimental realm. Theoretical papers deal with quark-gluon plasma, and relativistic heavy ion collisions. Annotation copyrighted by Book News, Inc., Portland, OR
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.
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.
Mini-jet and particle production are studied in the framework of HIJING Monte Carlo model which can describe pp and p{bar p} collisions well from ISR to Fermilab Tevatron energies. Mini-jets are shown to have eminent contributions to particle production in ultra-relativistic heavy ion collisions. However, parton shadowing and jet quenching also have important effects and can be studied by single particle distributions. 19 refs., 5 figs.
