A new class of dry, dilute, active matter

Simple models such as the Vicsek model have been decisive for our current understanding of dry dilute active matter. In spite of, but also thanks to their simplicity, Vicsek-style models have helped uncover a wealth of new physics, such as the generic long-range correlations and anomalous fluctuations at play in their ordered phases. Until now, three main universality classes have emerged, characterized by the symmetries of particles’ motion (polar/apolar) and alignment (ferromagnetic/nematic).
In this work we define and study a 4^th class considering apolar particles interacting ferromagnetically. To obtain non-trivial results, we separate velocity V and polarity P, with V = +/- P where the sign changes at some finite rate. We show, using a combination of particle-based simulations and continuous descriptions, that this new class exhibits a qualitatively different analytic structure. In particular it gives rise to polar quasi long range order (QLRO) at low noise/large density but is deprived of the usual phase separation scenario. We show further that the transition to polar QLRO does not belong to the Berezinskii-Kosterlitz-Thouless class. This conclusion is strengthened at the qualitative level by a study of the dynamics of topological defects in this out of equilibrium context.