MATTER UNDER EXTREME CONDITIONS

We study scenarios for chiral symmetry restoration and deconfinement at finite temperature and/or density, based on assuming universal scaling relations for some hadron masses. We explore and discuss consequences of this scaling assumption in nuclear structure and show that most experiments support the scaling. The relevance of soft and hard scales as pertaining to chiral symmetry restoration and deconfinement is emphasized, and scaling relations for nonstrange and strange Goldstone bosons are presented. Theoretical support for the scaling relations is found from the analysis of effective Lagrangians in hot and dense matter, as well as finite-temperature and finite-density QCD sum rules. Both approaches suggest the validity of approximate scaling relations and identify sources of violation of the latter. Finite-temperature hadron masses can be compared to the results of QCD lattice gauge calculations. We show that large screening masses above T(c) merging into chiral multiplets are consistent with thermal, rather then dynamically generated, quark masses on the lattice. The results are consistent with vanishing dynamically generated masses above T(c), where T(c) is interpreted as the chiral symmetry restoration temperature. We review the ''dynamical confinement'' scenario of hot quarks and show that the hadronic wavefunctions obtained above T(c) are consistent with those obtained from recent measurements of the latter in lattice QCD. This suggests that there are strong quark/antiquark correlations in the vector meson channels. We conclude that for finite-temperature QCD, chiral symmetry above T(c) is realized in terms of essentially massless multiplets of chiral partners, and that chiral symmetry restoration is the only phase transition.