The Changes in Properties of Hadrons in Nuclei and Nuclear Matter

The changes of hadron properties in dense and/or hot matter are discussed in terms of effective chiral Lagrangians with the parameters of the theory scaled in a simple way. The phenomenologically successful Walecka model is identified as a mean field chiral Lagrangian with the scaled parameters. Kaon condensation and chiral restoration transitions can be described within the same mean field framework. Invited talk given at “International Nuclear Physics Conference,” August 21-26, 1995, Beijing, China. Quantum chromodynamics (QCD) tells us that most, if not all, of light hadron masses are generated spontaneously by the breaking of chiral symmetry from SU(Nf ) × SU(Nf ) to diagonal SU(Nf ) where Nf is the number of flavors, equal to 2 without strangeness and 3 with. It is also widely believed that as a hadronic system is heated to high temperature or compressed to high density, the broken symmetry will get restored in a way paralleling what happens in condensed matter physics. A natural consequence of the restoration of the chiral symmetry must then be that the spontaneously generated masses disappear as density (and perhaps also temperature) is increased. The question we are raising is how does this “shedding of mass” occur? This question is at the core of the fundamental theory of matter: How is the mass generated, starting with the lightest object like neutrinos to the heaviest detected particle like the top quark? The aim of this talk is to describe how hadron properties get modified in medium as the system is heated or compressed. That is, immerse a hadron in medium and compress the system or heat it. What does one expect to see happening? To answer this question, let me start with the simplest nuclear system, namely the deuterium. Let us look at what happens when a soft photon is sent in to probe the system. Consider therefore the well-known inverse process