  This letter reports experimental and numerical results on particle dynamics
in an out-of-equilibrium granular medium. We observed two distinct types of
grain motion: the well known cage motion, during which a grain is always
surrounded by the same neighbors, and low probability "jumps", during which a
grain moves significantly more relative to the others. These observations are
similar to the results obtained for other out-of-equilibrium systems (glasses,
colloidal systems, etc.). Although such jumps are extremely rare, by inhibiting
them in numerical simulations we demonstrate that they play a significant role
in the relaxation of out-of-equilibrium systems
