Improved Hartree--Fock resummations and spontaneous symmetry breaking

The standard Hartree--Fock approximation of the $O(N)-$invariant $\phi^4$ model suffers from serious renormalization problems. In addition, when the symmetry is spontaneously broken, another shortcoming appears in relation to the Goldstone bosons: they fail to be massless in the intermediate states. In this work, within the framework of out--of--equilibrium Quantum Field Theory, we propose a class of systematic improvements of the Hartree--Fock resummation which overcomes all the above mentioned difficulties while ensuring also exact Renormalization--Group invariance to one loop.