Evidence of fluctuation-induced first-order phase transition in active matter

We investigate the effects of density fluctuations on the near-ordering phase of a flock by studying the Malthusian Toner–Tu theory. Because of the birth/death process, characteristic of this Malthusian model, density fluctuations are partially suppressed. We show that unlike its incompressible counterpart, where the absence of the density fluctuations renders the ordering phase transition similar to a second-order phase transition, in the Malthusian theory density fluctuations may turn the phase from continuous to first-order. We study the model using a perturbative renormalization group approach. At one loop, we find that the renormalization group flow drives the system in an unstable region, suggesting a fluctuation-induced first-order phase transition.