  We show by using a Discrete Element Method that a bilayer of vibrated
granular bidisperse spheres exhibits the striking feature that the horizontal
velocity distribution of the top layer particles has a quasi-Gaussian shape,
whereas that of the bottom layer is far from Gaussian. We examine in detail the
relevance of all physical parameters (acceleration of the bottom plate, mass
ratio, layer coverage). Moreover, a microscopic analysis of the trajectories
and the collision statistics reveal how the mechanism of randomization.
