  We experimentally study a monolayer of vibrated disks with a built-in polar
asymmetry which enables them to move quasi-balistically on a large persistence
length. Alignment occurs during collisions as a result of self-propulsion and
hard core repulsion. Varying the amplitude of the vibration, we observe the
onset of large-scale collective motion and the existence of giant number
fluctuations with a scaling exponent in agreement with the predicted
theoretical value.
