align their direction of motion in a shorter time scale than the one associated to the reorientation of their self-propulsion velocity. As a result, particles are kinetically trapped in the interaction area, at a typical distance of the order of $R_{\theta}$, and move along (on average) parallel trajectories for long periods of times. As the density increases the collision rate grows, such that subsequent particles accumulate in the neighborhood more easily, growing a cluster which, in turn, can capture more particles. Then, only a rare random fluctuation of the orientation of a particle in the cluster’s boundary can make it escape. In the absence of alignment ($\text{g} \ll 1$), self-propelled disks slow down as they collide. They hinder their relative motion for a period of time of the order of $\tau$, until their orientation changes by rotational diffusion. At high enough densities, this self-trapping mechanism, illustrated in Fig. \[aline\], is at the origin of MIPS in ABP systems. While this mechanism is present in our model, as we show, it is not the leading one in the parameter regimes we explored. The system of polar ABPs exhibits a transition from a fluid made of a distribution of finite-size clusters to 