Noisy multistate voter model for flocking in finite dimensions

We study a model for the collective behavior of self-propelled particles subject to pairwise copying interactions and noise. Particles move at a constant speed v on a two-dimensional space and, in a single step of the dynamics, each particle adopts the direction of motion of a randomly chosen neighboring particle within a distance R=1, with the addition of a perturbation of amplitude eta (noise). We investigate how the global level of particles' alignment (order) is affected by their motion and the noise amplitude eta. In the static case scenario v=0 where particles are fixed at the sites of a square lattice and interact with their first neighbors, we find that for any noise eta > 0 the system reaches a steady state of complete disorder in the thermodynamic limit, while for eta=0 full order is eventually achieved for a system with any number of particles N. Therefore, the model displays a transition at zero noise when particles are static, and thus there are no ordered steady states for a finite noise ( eta>0). We show that the finite-size transition noise vanishes with Nas eta_c^(1D)~ N^-1 and eta_c^(2D)~ (N lnN)^-1/2 in one- and two-dimensional lattices, respectively, which is linked to known results on the behavior of a type of noisy voter model for catalytic reactions. When particles are allowed to move in the space at a finite speed v>0, an ordered phase emerges, characterized by a fraction of particles moving in a similar direction. The system exhibits an order-disorder phase transition at a noise amplitude eta_c >0 that is proportional to v, and that scales approximately as eta_c ~ v(-lnv)^-1/2 for v<