Novel Type of Phase Transition in a System of Self-Driven Particles

A simple model with a novel type of dynamics is introduced in order to investigate the emergence of self-ordered motion in systems of particles with biologically motivated interaction. In our model point particles with the same absolute velocity have a tendency to align their velocity with that of the neighbors. We show that this model results in a kinetic phase transition from no transport (zero average velocity) to finite net transport through spontaneous symmetry breaking of the rotational symmetry. The transition is continuous and the order parameter obeys scaling. This is in contrast to the usually observed first-order phase transitions in similar models.