Matter & More in Nuclear Collisions

The aim of high energy nuclear collisions is to study the transition from hadronic matter to a plasma of deconfined quarks and gluons. I review the basic questions of this search and summarize recent theoretical developments in the field. 1. New States of Matter Statistical QCD predicts that high temperatures and baryon densities will lead to new states of strongly interacting matter. Increasing T at low baryon density transforms a meson gas into a deconfined plasma of quarks and gluons (QGP); this transition has been studied extensively in computer simulations of finite temperature lattice QCD. High baryon densities at low T are expected to produce a condensate of colored diquarks. The resulting phase diagram in terms of temperature and baryochemical potential µ is schematically illustrated in Fig. 1, with hadronic matter as color insulator, the QGP as color conductor, and the diquark condensate as color superconductor. T c T µ Quark-Gluon-Plasma c Diquark Condensate Matter Hadronic µ Figure 1: Phase diagram of strongly interacting matter. With high energy nuclear collisions, we want to study in the laboratory the deconfine-ment transition and the properties of the QGP. Hard probes, such as the production of quarkonia, open charm and beauty, jets and photons are expected to provide information