Aging in ferromagnetic systems at criticality near four dimensions

We study the off-equilibrium response and correlation functions and the corresponding fluctuation-dissipation ratio for a purely dissipative relaxation of an O(N) symmetric vector model (model A) below its upper critical dimension. The scaling behavior of these quantities is analyzed and the associated universal functions are determined at first order in epsilon=4-d in the high-temperature phase and at criticality. A nontrivial limit of the fluctuation-dissipation ratio is found in the aging regime X( infinity )=1/2(1-(epsilon/4)(N+2)/(N+8))+O(epsilon(2)).