Coefficient of Restitution as a Fluctuating Quantity
  The coefficient of restitution of a spherical particle in contact with a flat plate is investigated as a function of the impact velocity. As an experimental observation we notice non-trivial (non-Gaussian) fluctuations of the measured values. For a fixed impact velocity, the probability density of the coefficient of restitution, $p(\epsilon)$, is formed by two exponential functions (one increasing, one decreasing) of different slope. This behavior may be explained by a certain roughness of the particle which leads to energy transfer between the linear and rotational degrees of freedom. 
Marina Montaine, Michael Heckel, Christof Kruelle, Thomas Schwager,   Thorsten Poeschel
cond-mat.stat-mech