  We study the collective dynamics of repulsive self-propelled particles. The
particles are governed by coupled equations of motion that include polar
self-propulsion, damping of velocity and of polarity, repulsive
particle-particle interaction, and deterministic dynamics. Particle dynamics
simulations show that the collective coherent motion with large density
fluctuations spontaneously emerges from a disordered, isotropic state. In the
parameter region where the rotational damping of polarity is strong, the
systems undergoes an abrupt shift to the absorbing ordered state after a
waiting period in the metastable disordered state. In order to obtain a simple
understanding of the mechanism underlying the collective behavior, we analyze
binary particle scattering process. We show that this approach correctly
predicts the order-disorder transition at dilute limit. The same approach is
expanded for finite densities, although it disagrees with the result from
many-particle simulations due to many-body correlations and density
fluctuations.
