Absorbing state phase transitions with a non-accessible vacuum

We analyse from the renormalization group perspective a universality class of reaction.diffusion systems with absorbing states. In this class, models where the vacuum state is not accessible are represented as the set of reactions 2A → A together with creation processes of the form A → nA with n ≥ 2. This class includes the (exactly solvable in one dimension) reversible model 2A ↔ A as a particular example, as well as many other non-reversible sets of reactions, proving that reversibility is not the main feature of this class as previously thought. By using field theoretical techniques we show that the critical point appears at zero creation rate (in accordance with known results for the reversible case) and it is controlled by the well known pair-coagulation renormalization group fixed point, with non-trivial exactly computable critical exponents in any dimension. Finally, we report on Monte Carlo simulations, confirming the field theoretical predictions in one and two dimensions for various reversible and non-reversible sets of reactions.