  We investigate the problem of quantum back reactions due to particle
production by the parametric resonance mechanism in an environment of
nonequilibrium chiral phase transition. We work with the linear sigma model and
employ Boyanovsky et al.'s formalism to take account of back reactions under
the Hartree-Fock approximation. We calculate the single pion momentum
distributions and two pion correlations under the initial condition of thermal
equilibrium and small amplitude sigma oscillations around a potential minimum.
We observe that the resonance peak survives under the back reaction which is
remarkable considering the strong coupling of the sigma model.
