Gaussian distributions of rotational velocities in a granular medium

We investigate experimentally a homogeneously driven granular medium. it consists ot spheres, each one containing a magnetic dipole, rolling on a horizontally placed dish and subject to a magnetic field which is sinusoidal in time and spatially homogeneous. A gaslike state is obtained. Except for extremely low amplitudes of the magnetic field, non-Gaussian distributions are found for the translatory velocities, as well as for the horizontal components of the rotational velocities. However, the vertical component of the rotational velocity obeys an almost invariant Gaussian distribution within a large range of parameters, namely frequencies and amplitudes of the magnetic field and friction coefficients on the dish's bottom. The results are close to predictions from simulations by Cafiero et al.