Dynamical resummation and damping in the O(N) model

A general real-time formalism is developed to resum the self-energy operator of broken symmetry scalar field theories in form of self-consistent gap equations for the spectral function. The solution of the equations is approximated with finite lifetime quasi-particles. In the Landau damping rates viscosity terms, analogous to gauge theories, appear, what leads to a finite damping rate for the long wavelength Goldstone modes.