Universality in Two Classes of Reaction-Diffusion Systems

We investigate the asymptotic properties of two reaction-diffusion systems. The first is an annihilation / scattering system with the reactions $2 A -> \emptyset$, $2 A -> 2 B$, $2 B -> 2 A$, and $2 B -> \emptyset$. The second one consists of competing annihilation and fission processes, $2 A -> \emptyset$ and $2 A -> (n+2) A$. We find that both these diffusion-limited reaction systems belong to the same universality class as the single species annihilation $2 A -> \emptyset$. We discuss the implications of our analysis for a recent study of an active / absorbing transition in a system with multiplicative noise.