  We analyse the collective dynamics of self-propelled particles in the large
density regime where passive particles undergo a kinetic arrest to an amorphous
glassy state. We capture the competition between self-propulsion and crowding
effects using a two-dimensional model of self-propelled hard disks, which we
study using Monte-Carlo simulations. Although the activity drives the system
far from equilibrium, self-propelled particles undergo a kinetic arrest, which
we characterize in detail and compare with its equilibrium counterpart. In
particular, the critical density for dynamic arrest continuously shifts to
larger density with increasing activity, and the relaxation time is
surprisingly well described by an algebraic divergence resulting from the
emergence of highly collective dynamics. These results show that dense
assemblies of active particles undergo a nonequilibrium glass transition which
is profoundly affected by self-propulsion mechanisms.
