No abstract

We study baryons in an AdS/CFT model of QCD by Sakai and Sugimoto, realized as small instantons with fundamental string hairs. We introduce an effective field theory of the baryons in the five-dimensional setting, and show that the instanton interpretation implies a particular magnetic coupling. Dimensional reduction to four dimensions reproduces the usual chiral effective action, and in particular we estimate the axial coupling gA between baryons and pions and the magnetic dipole moments, both of which are proportional to Nc. We extrapolate to finite Nc and discuss subleading corrections.

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.