QCD phenomenology based on a chiral effective Lagrangian

Abstract

We review the Nambu-Jona-Lasinio (NJL) approach to the dynamical breaking of chiral symmetry in Quantum Chromodynamics (QCD). After a general overview of the nonperturbative aspects of OCD, we introduce the NJL model as a low-energy effective theory of QCD. The collective nature of hadrons and the constituent quark model are treated in a unified way. Various aspects of QCD related to the dynamical and explicit breaking of chiral symmetry and the axial anomaly can be well described. The subjects treated in part I include the vacuum structure of QCD, mass spectra and coupling constants of hadrons, flavor mixing in mesons, the violation of the OZI rule in baryons, and the validity of the chiral perturbation in QCD. It is shown that a subtle interplay between the axial anomaly and the current-quark masses plays important roles, and a realistic evaluation of the strangeness and heavy quark contents of hadrons is given. Also the problem of elusive scalar mesons is studied in detail. For a pedagogical reason, we first present an account of basic ingredients and detailed technical aspects of the NJL model using simple versions of it. In part II, the NJL model is applied to the system at finite temperature ( T) and density (ϱ) relevant to the early universe, interior of the neutron stars and the ultrarelativistic heavy ion collisions. After a brief introduction of the field theory at finite temperature, phenomena associated with the restoration of chiral symmetry in the medium are examined. The subjects treated here include the quark condensates in the medium, meson properties at finite T (ϱ) and their experimental implications. A special attemtion is paid to fluctuation phenomena near the critical temperature, i.e., possible existence of soft modes in the scalar channel and a jump of the quark-number susceptibility in the vector channel.