  Systems of self-propelled particles (SPP) interacting by a velocity alignment
mechanism in the presence of noise exhibit a rich clustering dynamics. It can
be argued that clusters are responsible for the distribution of (local)
information in these systems. Here, we investigate the statistical properties
of single clusters in SPP systems, like the asymmetric spreading of clusters
with respect to their moving direction. In addition, we formulate a
Smoluchowski-type kinetic model to describe the evolution of the cluster size
distribution (CSD). This model predicts the emergence of steady-state CSDs in
SPP systems. We test our theoretical predictions in simulations of SPP with
nematic interactions and find that our simple kinetic model reproduces
qualitatively the transition to aggregation observed in simulations.
