  When analyzing the particle-like excitations in quantum field theory it is
natural to regard the field mode corresponding to the particle momentum as an
open quantum system, together with the opposite momentum mode. Provided that
the state of the field is stationary, homogeneous and isotropic, this scalar
two-mode system can be equivalently represented in terms of a pair of quantum
Brownian oscillators under a Gaussian approximation. In other words, the
two-mode system behaves as if it were interacting linearly with some effective
environment. In this paper we build the details of the effective linear
coupling and the effective environment, and argue that this quantum Brownian
representation provides a simple, universal and non-perturbative
characterization of any single particle-like excitation. As immediate
applications of the equivalence, we reanalyse the interpretation of the
self-energy in terms of decay rates in a general background state, and present
the master equation for the field mode corresponding to the particle momentum.
