The use of high-pT particles as calibrated probes has proven to be an effective tool for understanding the properties of the system produced in relativistic heavy ion collisions. In this thesis, two such measurements are presented using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC): 1. The transverse momentum spectra of charged particles produced near mid rapidity in Cu+Cu collisions with center-of-mass energies of 62.4 and 200 GeV per nucleon pair 2. Two-particle correlations with a high transverse momentum trigger particle (pT> 2.5 GeV=c) in Au+Au collisions at ... 200 GeV over the broad longitudinal acceptance of the PHOBOS detector ... In central Au+Au collisions at 200 GeV, the single-particle yields are suppressed at high-pT by a factor of about five compared to p+p collisions scaled by the number of binary collisions. This is typically understood to be a consequence of energy loss by high-pT partons in the dense QCD medium, as such a suppression is absent in d+Au collisions. In Cu+Cu collisions, the nuclear modification factor, RAA, has been measured relative to p+p data as a function of collision centrality. For the same number of participating nucleons (Npart), RAA is essentially the same for the Cu+Cu and Au+Au systems over the measured range of pT, in spite of the significantly different geometries. At high-pT, the similarity between the two systems can be described by simple, geometric models of parton energy loss. Two-particle angular correlations are a more powerful tool for examining how highpT jets lose energy and how the medium is modified by the deposited energy. In central Au+Au collisions, particle production correlated with a high-pT trigger is strongly modified compared to p+p. Not only is the away-side yield much broader in, the nearside peak of jet fragments now sits atop an unmistakable 'ridge' of correlated partners extending continuously and undiminished all the way to = 4.

The production of identified charged particles in pp collisions at .sqrt.s = 63 GeV with an identified high p/sub T/ trigger particle emitted in the central region is studied. The measurements were performed at the CERN ISR using the Axial Field Spectrometer. Trigger particle ratios, sigma(.pi./sup + -/)/ sigma(all/sup + -/), sigma(K/sup + -/) and sigma(p/sup + -/)/sigma(all/sup + -/) are presented for p/sub T/ from 5 GeV/c to 8 GeV/c. In addition sigma(.pi./sup + -/)/sigma(all/sup + -/) is presented in the p/sub T/ region from 2.5 GeV/c to 4.5 GeV/c. The charge compensation in the hemisphere containing the trigger particle is shown to depend strongly on the identity of the trigger particle and on the identity of the associated particles. 13 references.

The charged particle transverse momentum (pT) spectra are presented for pp collisions at sqrt(s)=0.9 and 7 TeV. The data samples were collected with the CMS detector at the LHC and correspond to integrated luminosities of 231 inverse microbarns and 2.96 inverse picobarns, respectively. Calorimeter-based high-transverse-energy triggers are employed to enhance the statistical reach of the high-pT measurements. The results are compared with both leading-order QCD and with an empirical scaling of measurements at different collision energies using the scaling variable xT = 2 pT/sqrt(s) over the pT range up to 200 GeV/c. Using a combination of xT scaling and direct interpolation at fixed pT, a reference transverse momentum spectrum at sqrt(s)=2.76 TeV is constructed, which can be used for studying high-pT particle suppression in the dense QCD medium produced in heavy-ion collisions at that centre-of-mass energy.

The field of particle physics is developing very rapidly. During this past year, physicists added a new instrument to their arsenal for the study of quark-quark, quark-lepton, and lepton lepton interactions. This machine, the PROTON-ANTIPROTON COLLIDER, achieved the highest energy in the world. With its five detectors, it is beginning to explore hitherto inaccessible regions for new physics (Section I). Lepton-Iepto~ machines with detectors at full efficiency are producing copious data of the very highest precision. The possibility of glueballs and the detailing of the properties of the upsilon family have been of major importance this year (Section II). The particle jets which are believed to be direct manifesta tions of the quark structure of matter continue to provide valuable data against which we can test the ideas of QCD (Section III). With the advent of more and better data it is now possible to study in detail the formation evolution of hadronic states. Especially interesting are the properties of heavy quark states (Section IV). A far-seeing look into the future development of any fecund scienti fic field is rarely accurate, but is always stimulating (Section V). It is against this background of participating in the clarifi cation of the Physics in Collision that we continue this series.

One of few books to address both high-pT physics and relativistic heavy ion collisions. Essential handbook for graduates and researchers.

Aimed at graduate students and researchers in the field of high-energy nuclear physics, this book provides an overview of the basic concepts of large transverse momentum particle physics, with a focus on pQCD phenomena. It examines high-pT probes of relativistic heavy-ion collisions and will serve as a handbook for those working on RHIC and LHC data analyses. Starting with an introduction and review of the field, the authors look at basic observables and experimental techniques, concentrating on relativistic particle kinematics, before moving onto a discussion about the origins of high-pT physics. The main features of high-pT physics are placed within a historical context and the authors adopt an experimental outlook, highlighting the most important discoveries leading up to the foundation of modern QCD theory. Advanced methods are described in detail, making this book especially useful for newcomers to the field.