HOT AND DENSE QCD

We give a brief review of modern theoretical understanding of physics of QCD at finite temperature and density. We concentrate on discussing “hot” systems with zero baryon number density. We consider, first, lukewarm pion gas and, in particular, dwell on kinetic properties of hadron collective excitations. Next, we discuss physics of the chiral restoration phase transition and confront theoretical expectations with available lattice data and with perspective heavy-ion experiments. We proceed then to the high-temperature QCD phase, the quark-gluon plasma. We notice that the temperatures one can expect to reach at the heavy-ion collider at RHIC are not yet high enough for perturbation theory in the QCD coupling constant to work well. Lastly, we discuss systems with nonzero baryon number density. We argue that, though the phase transition in temperature is probably absent, it can appear with vengeance when finite density effects are taken into account. An interesting color superconductivity phase is expected to show up at large densities.