On the nature of different types of absorbing states
We present a comparison of three different types of Langevin equation exhibiting absorbing states: the Langevin equation defining the Reggeon field theory, one with multiplicative noise, and a third type in which the noise is complex. Each one is found to describe a different underlying physical mechanism; in particular, the nature of the different absorbing states depends on the type of noise considered. By studying the stationary single-site effective potential, we analyze the impossibility of finding a reaction-diffusion model in the multiplicative noise universality class. We also discuss some theoretical questions related to the nature of complex noise, as for example, whether it is necessary or not to consider a complex equation in order to describe processes as the annihilation reaction, $A +A \to 0$.