Onset of collective and cohesive motion

TL;DR

This paper investigates the phase transition to collective motion in groups of self-propelled particles, revealing it is always discontinuous and that cohesion is lost near the onset due to fluctuations.

Contribution

It demonstrates that the transition to collective motion is inherently discontinuous in two dimensions, challenging previous claims of a critical point in minimal models.

Findings

Transition is always discontinuous in 2D

Cohesion is lost near the onset of motion

Fluctuations in density, velocity, and shape drive cohesion loss

Abstract

We study the onset of collective motion, with and without cohesion, of groups of noisy self-propelled particles interacting locally. We find that this phase transition, in two space dimensions, is always discontinuous, including for the minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for which a non-trivial critical point was previously advocated. We also show that cohesion is always lost near onset, as a result of the interplay of density, velocity, and shape fluctuations.