Spectral Change of Hadrons and Chiral Symmetry

After a brief summary of the QCD phase structure with light quarks, we discuss two recent developments on in-medium hadrons. First topic is the σ-meson which is a fluctuation of the chiral order parameter q̄q. Although σ is at best a broad resonance in the vacuum, it may suffer a substantial red-shift and show a characteristic spectral enhancement at the 2mπ threshold at finite temperature and baryon density. Possible experimental signatures of this phenomenon are also discussed. Another topic is the first principle lattice QCD calculation of the hadronic spectral functions using the maximum entropy method (MEM). The basic idea and a successful example of MEM are presented. Possible applications of MEM to study the in-medium hadrons in lattice QCD simulations are discussed.