  Using a path integral formulation for correlation functions of stochastic
partial differential equations based on the Onsager-Machlup approach, we show
how, by introducing a composite auxiliary field one can generate an auxiliary
field loop expansion for the correlation functions which is similar to the one
used in the $1/N$ expansion for an $O(N)$ scalar quantum field theory. We apply
this formalism to the Kardar Parisi Zhang (KPZ) equation, and introduce the
composite field $\sigma = \frac{\lambda}{2} \nabla \phi \cdot \nabla \phi$ by
inserting a representation of the unit operator into the path integral which
enforces this constraint. In leading order we obtain a self-consistent mean
field approximation for the effective action similar to that used for the
Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein Condensate (BEC) theories of
dilute Fermi and Bose gases. This approximation, though related to a
self-consistent Gaussian approximation, preserves all symmetries and broken
symmetries. We derive the leading order in the auxiliary field (LOAF) effective
potential and compare our results to the one loop in the fluctuation strength
${\cal A}$ approximation. We find, contrary to what is found in the one loop
and self-consistent Gaussian approximation schemes that in the LOAF
approximation there is no fluctuation induced symmetry breaking as a function
of the coupling constant in any dimension $d$.
