  Inspired by groups of animals and robots, we study the collective dynamics of
large numbers of active particles, each one trying to get to its own randomly
placed target, while avoiding collisions with each other. The particles we
study are repulsive homing active Brownian particles (HABPs) - self-propelled
particles whose orientation relaxes at a finite rate towards an absorbing
target in $2d$ continuous space. For a wide range of parameters, these
particles form synchronised system-wide chiral flocks, in spite of the absence
of explicit alignment interactions. We show that this dramatic behavior obtains
for different system sizes and density, that it is robust against the addition
of noise, polydispersity, and bounding walls, and that it can exhibit dynamical
topological defects. We develop an analogy to an off-lattice, ferromagnetic XY
model, which allows us to interpret the different phases, as well as the
topological defects.
