Two-loop Critical Fluctuation-Dissipation Ratio for the Relaxational Dynamics of the O(N) Landau-Ginzburg Hamiltonian

The off-equilibrium purely dissipative dynamics (Model A) of the O(N) vector model is considered at criticality in an $\epsilon = 4- d > 0$ up to O($\epsilon^2$). The scaling behavior of two-time response and correlation functions at zero momentum, the associated universal scaling functions, and the nontrivial limit of the fluctuation-dissipation ratio are determined in the aging regime.