Onset of Collective and Cohesive Motion

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. [<article>Phys. Rev. Lett.<coden>PRLTAO</coden><issn>0031-9007</issn> 75,1226 (1995)<doi>10.1103/PhysRevLett.75.1226</doi></article>] for which a nontrivial 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.