Velocity Fluctuations in a Homogeneous 2D Granular Gas in Steady State

We have measured the spectrum of velocity fluctuations in a granular system confined to a vertical plane and driven into a homogeneous, steady state by strong vertical vibration. The distribution of horizontal velocities is not Maxwell-Boltzmann and is given by $P\left(v\right)=C\exp [-\beta (|v|/\sigma {)}^{\alpha }]$ where $\alpha =1.55\mathrm{}\pm 0.1$ at all frequencies and amplitudes investigated, and also for varying boundary conditions. The deviation from Maxwell-Boltzmann statistics occurs in the absence of spatial clustering and does not result from an inhomogeneous average over regions of varying local density. Surprisingly, $P\left(v\right)$ has the same shape over a wide range of densities.