At zero temperature and small baryon number density, quarks and gluons are confined inside hadrons, and chiral symmetry is spontaneously broken. Lattice simulations of quantum chromodynamics (QCD) indicate that, at temperatures of the order of 150 MeV (at zero baryon number density), quarks and gluons become deconfined, and chiral symmetry is restored. This novel thermodynamic phase of nuclear matter is commonly termed "quark-gluon plasma" (QGP). In nature, temperatures of the order of 150 MeV existed only shortly after the Big Bang. However, for more than two decades, attempts have been made to recreate similar conditions within the collision of heavy nuclei at ultrarelativistic energies. The comparison is not perfect, since a nuclear collision creates an environment that, while highly excited, is (at least initially) far from thermodynamic equilibrium, while lattice QCD describes exclusively matter in thermodynamic equilibrium. Hence, far beyond equilibrium thermodynamics, the phenomenology of relativistic heavy ion collisions has become an interdisciplinary field which involves concepts of elementary particle physics, nuclear physics, and the thermo- and hydrodynamics of hot and dense mesoscopic systems.

After fixed-target experimental programs at BNL's AGS and CERN's SPS, RHIC has become the new high-energy frontier in the study of hot and dense matter. Since heavy-ion physics is mainly driven by phenomenology, this INT program will encourage a close interaction between theorists and heavy-ion experimentalists. Detailed presentations of the data accumulated in the first 3 years of RHIC will be followed by intensive discussions of their theoretical interpretation. The program will focus on the following main topics:

• Hadronic observables: single inclusive hadronic particle spectra and moments thereof (radial flow, directed flow, elliptic flow); rapidity distribution of multiplicity, total multiplicity, particle ratios, 2-particle correlations, multiplicity fluctuations.
• Electromagnetic observables: low-, intermediate-, and high-mass dileptons; direct photons.
• Hard probes: jet production and quenching seen through high-energy hadron fragments; modified fragmentation functions; back-to-back correlations; heavy-flavor quark production.

Each observable is characteristic of a certain stage in a heavy-ion collision. The above ordering reflects roughly an anti-time ordering: hadrons emerge only in the final stage of the collision (due to their short mean free path), while electromagnetic observables emerge predominantly from the earlier, hot stage of the collision, and "hard" particles probe the very early, initial stage of the collision. The discussion will focus on a critical assessment of the proposed signatures for the creation of the QGP at RHIC. Beyond that, we also hope to gain understanding of the physics of parton saturation, the detailed mechanisms of initial and secondary particle production, the subsequent approach towards equilibration, and collective expansion of matter until freeze-out.

We aim to discuss these topics in the following order