The J/[psi] suppression was originally proposed as a signature of the quark-gluon plasma. Strong suppression of J/[psi] production was indeed observed recently by the NA50 Collaboration at CERN SPS. Is it the first signature of a long-awaited quark-gluon matter, or just a peculiar combination of ''conventional'' effects acting together to produce the puzzling pattern observed experimentally? In this lecture, I am trying to summarize the existing theoretical explanations.

The authors review the search for the quark-gluon plasma using the signal of the suppression of J/[psi] production in high-energy heavy-ion collisions. Recent anomalous J/[psi] suppression in high-energy Pb-Pb collisions observed by the NA50 Collaboration are examined and compared with earlier results from pA and nucleus-nucleus collisions with heavy ions of smaller mass numbers. The anomalous suppression of J/[psi] production in Pb-Pb collisions can be explained as due to the occurrence of a new phase of strong J/[psi] absorption, which sets in when the number of nucleon-nucleon collisions at a spatial point exceeds about 4 and corresponds to a local energy density of about 3.4 GeV/fm3.

Two decades ago Matsui and Satz suggested that Debye screening in the quark-gluon plasma would result in J/{psi} suppression in heavy ion collisions. Much has happened in the subsequent years, and the picture of quark-gluon plasma at present is rapidly evolving - what does it imply for the J/{psi} suppression? What are the recent RHIC and SPS results trying to tell us? What else has to be done? This talk is an attempt to address these questions.

In order to understand the J/[psi] suppression arising from the possible occurrence of the quark-gluon plasma in high-energy heavy-ion collisions, it is necessary to have a comprehensive picture how the J/[psi] and its precursors are produced, what their properties after production are, and how the J/[psi] and its precursors propagate inside nuclear matter. There are unresolved questions in the descriptions of J/[psi] production and propagation. The author outlines some of these questions and discusses the approaches for their resolution.

This thesis covers several important topics relevant to our understanding of quark-gluon plasma. It describes measurement of the third-order harmonic flow using two-particle correlations and isolation of flow and non-flow contributions to particle correlations in gold-gold collisions. The work also investigates long-range longitudinal correlations in small systems of deuteron-gold collisions. The former is related to the hydrodynamic transport properties of the quark-gluon plasma created in gold-gold collisions. The latter pertains to the question whether hydrodynamics is applicable to small systems, such as deuteron-gold collisions, and whether the quark-gluon plasma can be formed in those small-system collisions. The work presented in this thesis was conducted with the STAR experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, where the center-of-mass energy of both collision systems was a factor of 100 larger than the rest mass of the colliding nuclei. The results contained in this thesis are highly relevant to our quest for deeper understanding of quantum chromodynamics. The results obtained challenge the interpretation of previous works from several other experiments on small systems, and provoke a fresh look at the physics of hydrodynamics and particle correlations pertinent to high energy nuclear collisions.

Quark-Gluon Plasma introduces the primordial matter, composed of two types of elementary particles, created at the time of the Big Bang. During the evolution of the universe, Quark-Gluon Plasma (QGP) undergoes a transition to hadronic matter governed by quantum chromodynamics, the law of strong interactions. After an introduction to gauge theories, various aspects of quantum chromodynamic phase transitions are illustrated in a self-contained manner. The cosmological approach and renormalization group are discussed, as well as the cosmological and astrophysical implications of QGP, on the basis of Einstein's equations. Recent developments towards the formation of QGP in ultrarelativistic heavy ion collisions are also presented in detail. This text is suitable as an introduction for graduate students, as well as providing a valuable reference for researchers already working in this and related fields. It includes eight appendices and over a hundred exercises.

This book offers the unique possibility of tackling the problem of hadronic deconfinement from different perspectives. After general introductions to the physical issues, from both the theoretical and the experimental point of view, the book presents the most recent expertise on field theory approaches to the QCD phase diagram, many-body techniques and applications, the dynamics of phase transitions, and phenomenological analysis of relativistic heavy ion collisions.One of the major goals of this book is to promote interchange among those fields of research, which have traditionally been cultivated by different communities of physicists. The contributions in the book help in obtaining deep comprehension of this new state of matter, a system of deconfined quarks and gluons. At the same time the book offers a few examples of how the seeds of the deconfined state are looked for in the phenomenological analysis of the observables measured in relativistic heavy ion collisions. The main topics are dealt with in a pedagogical style, suitable for beginners as well as experienced researchers.