In this paper we present the construction of the equilibrium states at positive temperature in the presence of a condensation phase for a Gas of non relativistic Bose particles on an infinite space interacting through a localised two body interaction. We use methods of quantum field theory in the algebraic formulation to obtain this result and in order to prove convergence of the partition function and of the generating function of the correlation functions, we introduce an auxiliary stochastic Gaussian field which mediates the interaction of the Bose particles (Hubbard-Stratonovich transformation). The construction of the equilibrium state and of the partition function in the presence of the condensate, treating the auxiliary stochastic field as external potential, can be achieved using and adapting ideas and methods of Araki. Explicit formulas for the relative entropy of the equilibrium state with the external potential with respect to the equilibrium state of the free theory are obtained adapting known Feynman-Kac formulas for the propagators of the theory. If the two-body interaction is sufficiently weak, the proof of the convergence of the partition function after evaluation of the external stochastic field on a suitable Gaussian state can be given utilizing the properties of the relative entropy mentioned above. Limits where the localisation of the two-body interaction is removed are eventually discussed in combination of the limits of vanishing temperature and or in the weakly interacting regime.
Galanda et al. (Mon,) studied this question.