New universality classes govern the critical and multicritical behavior of an active Ising model

The Ising model is one of the most well known models in statistical physics, with its critical behavior governed by the Wilson-Fisher universality class (UC). When active motility is incorporated into the Ising model by, e.g., dictating that the spins' directional movements follow their orientations, the spin number density necessarily constitutes a soft mode in the hydrodynamic description, and can therefore modify the scaling behavior of the system. Here, we show that this is indeed the case in a critical active Ising model in which density can impede the system's collective motion. Specifically, we use a perturbative dynamic renormalization group method to the one-loop level to uncover three new UCs, one of which supersedes the Wilson-Fisher UC to become the generic UC that governs the critical behavior of the active Ising model.