Chiral restoration phase transition in hot and/or dense hadronic matter is
discussed in terms of the BR scaling based on chiral symmetry and scale anomaly
of QCD. The precise connection between the scalar field that figures in the
trace anomaly and the sigma field that figures in the linear σ model is
established. It is suggested that in hot and/or dense medium, the nonlinear
σ model linearizes with the help of a dilaton to a linear σ model
with medium-renormalized constants. The relevance of Georgi's vector symmetry
and/or Weinberg's ``mended symmetry" in chiral restoration is pointed out. Some
striking consequences for relativistic heavy-ion collisions and dense matter in
compact stars following stellar collapse are discussed.Comment: 66 pages, Latex, 19 figures (not included, available on request),
Prepared for Physics Report