Vacuum structure of QCD in an effective Lagrangian approach

The vacuum structure of QCD is studied using an anomalous effective Lagrangian approach. This approach makes it possible to determine how physical observables depend on the strong CP violation parameter, &thetas;. The &thetas;-dependence of QCD and the phenomenology of the light pseudoscalar mesons in this theory are illustrated. The vacuum structure of QCD is shown to be quite complex with the prediction of a number of different types of nontrivial vacuum states. Two specific examples of nontrivial vacuum states are analysed in more detail. The decay rate of a metastable vacuum state for the &thetas; = 0 case is nonperturbatively calculated in both the zero and high temperature limits. The formation of a nontrivial &thetas;-vacuum state in heavy ion collisions is predicted in a simplified numerical model. These results have implications for the study of the evolution of the early universe near the QCD phase transition and may be tested experimentally very soon in heavy ion collision experiments.