Chiral Symmetry in the Strong Interactions: its Restoration at High Temperature and its Role in the Extended Skyrme Model.

In this dissertation several topics concerning phenomenological aspects of quantum chromodynamics (QCD) are investigated, by means of effective Lagrangian methods. (i) A phenomenological model of lattice QCD at high temperatures is proposed, which is the first to incorporate both the Wilson line and chiral fields. The model makes it possible to study the interaction of the chiral and deconfinement phase transitions in a larger parameter space than would be accessible in ordinary QCD. The phase structure of this model is explored by mean field analysis, which may provide some qualitative understanding of phase transitions of QCD at high temperatures. (ii) A new argument for the occurrence of hadronic modes in the high temperature quark gluon plasma is given and a crude model of the plasma is introduced and used to make some qualitative estimations of properties of the plasma. Possible experimental signatures for the low-lying modes are discussed. (iii) A new method to couple the rho and the A _1 mesons to the Skyrme model is proposed. It may offer a systematic way to incorporate more mesons in the effective Lagrangian of QCD at low energy and in the large color number limit. The static properties of baryons as solitons in this model are calculated. The results obtained are improved compared with the previous models containing only the rho mesons.