Out of Equilibrium Relativistic Quantum Field Theory --- Perturbation   Theory and Progress of Phase Transition
  This paper describes perturbative framework, on the basis of closed-time-path formalism, for studying quasiuniform relativistic quantum field systems near equilibrium and nonequilibrium quasistationary systems. At the first part, starting from first principles, we construct perturbative schemes for relativistic complex-scalar-field theory. We clarify what assumption is involved in arriving at a standard perturbative framework and to what extent the $n (\geq 4)$-point initial correlation functions that are usually discarded in the standard framework can in fact be discarded. Two calculational schemes are introduced, the one is formulated on the basis of the initial particle distribution function and the one is formulated on the basis of the `` physical'' particle distribution function. Both schemes are equivalent and lead to a generalized relativistic kinetic or Boltzmann equation. At the second part, using the perturbative loop-expansion scheme for an $O (N)$ linear $\sigma$ model, we analyze how the chiral phase transition proceeds through disoriented chiral condensates. The system of coupled equations that governs the spacetime evolution of the condensate or order-parameter fields is derived. The region where the curvature of the ``potential'' is negative is dealt with by introducing the random-force fields. Application to simple situations is made. 
A. Ni\'egawa