Mapping the QCD phase diagram

The QCD vacuum in which we live, which has the familiar hadrons as its excitations, is but one phase of QCD, and far from the simplest one at that. One way to better understand this phase and the nonperturbative dynamics of QCD more generally is to study other phases and the transitions between phases. We are engaged in a voyage of exploration, mapping the QCD phase diagram as a function of temperature T and baryon number chemical potential μ. Because of asymptotic freedom, the high temperature and high baryon density phases of QCD are more simply and more appropriately described in terms of quarks and gluons as degrees of freedom, rather than hadrons. The chiral symmetry breaking condensate which characterizes the vacuum phase melts away. At high densities, quarks form Cooper pairs and new condensates develop. The formation of such superconducting phases [1-4] requires only weak attractive interactions; these phases may nevertheless break chiral symmetry [5] and have excitations which are indistinguishable from those in a confined phase [5-8]. These cold dense quark matter phases may arise in the centers of neutron stars; mapping this region of the phase diagram will require an interplay between theory and neutron star phenomenology. The goal of the experimental heavy ion physics program is to explore and map the higher temperature regions of the diagram. Recent theoretical developments suggest that a key qualitative feature, namely a critical point which in a sense defines the landscape to be mapped, may be within reach of discovery and analysis by the CERN SPS, if data is taken at several different energies [9,10]. The discovery of the critical point would in a stroke transform the map of the QCD phase diagram which we sketch below from one based only on reasonable inference from universality, lattice gauge theory and models into one with a solid experimental basis.