Auxiliary Field Loop expansion for the Effective Action for Stochastic   Partial Differential equations II
  We extend our discussion of effective actions for stochastic partial differential equations to systems that give rise to a Martin-Siggia-Rose (MSR) type of action. This type of action naturally arises when one uses the many-body formalism of Doi and Peliti to describe reaction-diffusion models which undergo transitions into the absorbing state and which are described by a Master equation. These models include predator prey models, and directed percolation models as well as chemical kinetic models. For classical dynamical systems with external noise it is always possible to construct an MSR action.   Using a path integral representation for the generator of the correlation functions, we show how, by introducing a composite auxiliary field, one can generate an auxiliary field loop expansion for the effective action for both types of systems. As a specific example of the Doi-Peliti formalism we determine the effective action for the chemical reaction annihilation and diffusion process $A+A \rightarrow 0$. For the external noise problem we evaluate the effective action for the Cole-Hopf form of the Kardar-Parisi Zhang (KPZ) equation as well as for the Ginzburg Landau model of spin relaxation. We determine for arbitrary spatial dimension $d$, the renormalized effective potential in leading order in the auxiliary field loop expansion (LOAF) and also determine the renormalization group equation for the running of the reaction rate (coupling constant) for arbitrary $d$. We compare our results with known perturbative and non-perturbative results for the renormalization group equations. 
Fred Cooper
cond-mat.stat-mech physics.chem-ph