  We apply the decoherence formalism to an interacting scalar field theory. In
the spirit of the decoherence literature, we consider a "system field" and an
"environment field" that interact via a cubic coupling. We solve for the
propagator of the system field, where we include the self-energy corrections
due to the interaction with the environment field. In this paper, we consider
an environment in the vacuum state (T=0). We show that neglecting inaccessible
non-Gaussian correlators increases the entropy of the system as perceived by
the observer. Moreover, we consider the effect of a changing mass of the system
field in the adiabatic regime, and we find that at late times no additional
entropy has been generated.
