  We show that low density homogeneous phases of self propelled hard disks
exhibit a transition from isotropic to polar collective motion, albeit of a
qualitatively distinct class from the Vicsek one. In the absence of noise, an
abrupt discontinuous transition takes place between the isotropic phase and a
fully polar absorbing state. Increasing the noise, the transition becomes
continuous at a tri-critical point. We explain all our numerical findings in
the framework of Boltzmann theory, on the basis of the binary scattering
properties. We show that the qualitative differences observed between the
present and the Vicsek model at the level of their phase behavior, take their
origin in the complete opposite physics taking place during scattering events.
We argue that such differences will generically hold for systems of
self-propelled particles with repulsive short range interactions.
