Hadrons and nuclear physics with EM probes: From QCD point of view

Abstract

Quantum Chromodynamics (QCD) is the “theory of everything” in the physics of strong interaction. This chapter provides a brief introduction of the current problems in QCD, and discusses selected topics on the physics of in-medium hadrons. They are in-medium QCD sum rules, spectral functions in lattice QCD, and critical fluctuation of the chiral order parameter in dense medium. QCD sum rules which have great success in reproducing hadronic properties are based on the Wilson's operator product expansion; in a way it makes use of the appropriate window of energy scale where both hadronic description and the quark-gluon description are valid. Special emphasis is put on the QCD spectral function, which is the key observable in both theoretical and experimental studies. The focus is on in-medium hadrons. Some recent theoretical developments are also reviewed.