Analysis of the evolution of curvature perturbations in preheating by using δN formalism

We study the evolution of the curvature perturbation during preheating for a simple preheating model whose potential is given by V(φ, χ)=1/4λφ4+1/2g2φ2χ2 with g2/λ = 2. In order to evaluate the final amplitude of the curvature perturbation after preheating, a method beyond linear perturbation theory is required. We make use of the δN formalism and compare our results with the previous studies which make use of the Hartree approximation. We find that the final amplitude of the curvature perturbation after preheating obtained by the δN formalism is smaller than that obtained in the previous studies