  We study a model of interacting random walkers that proposes a simple
mechanism for the emergence of cooperation in group of individuals. Each
individual, represented by a Brownian particle, experiences an interaction
produced by the local unbalance in the spatial distribution of the other
individuals. This interaction results in a nonlinear velocity driving the
particle trajectories in the direction of the nearest more crowded regions; the
competition among different aggregating centers generates nontrivial dynamical
regimes. Our simulations show that for sufficiently low randomness, the system
evolves through a coalescence behavior characterized by clusters of particles
growing with a power law in time. In addition, the typical scaling properties
of the general theory of stochastic aggregation processes are verified.
