Role of mesons in chiral symmetry restoration and associated signals (Q5949214)

Chiral symmetry is partly restored in the nuclear medium, with a decrease of the order parameter, i.e., of the magnitude of the quark condensate, by about 40\% at normal density. The virtual nuclear pions are an important component of this restoration, acting in the same way as the real ones of a heat bath. Among the other mesons the scalar-isoscalar one, the \(\sigma\) meson, is expected to play a special role since it has the same quantum numbers as the condensate. It is thus the only meson which can directly dissolve into the condensate. We give the expression of its contribution to the restoration process in the framework of the linear sigma model, displaying the difference with the pionic contribution. We also discuss the various signals associated with these two components of the restoration. The signal associated with the pion is the `mixing of the axial and vector correlators', taken not literally but in a sense that will be defined. For the scalar meson, it is the reduction of the hadron masses in the nuclear medium. This perspective leads naturally to the description of the mean scalar field of QHD, the quantum hadrodynamics theory of nuclei. We discuss, in particular, how it appears in the linear sigma model. This demonstrates how QHD is incorporated in chiral theories.