Spectral Change of Hadrons and Chiral Symmetry

Abstract

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 sigma-meson which is a fluctuation of the chiral order parameter \bar{q}q. Although sigma is at best a broad resonance in the vacuum, it may suffer a substantial red-shift and show a characteristic spectral enhancement at the 2 m_{pi} 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.