  We use multi-speckle diffusive wave spectroscopy (MSDWS) to probe the
micron-scale dynamics of a granular pile submitted to discrete gentle taps. The
typical time-scale between plastic events is found to increase dramatically
with the number of applied taps. Furthermore, this microscopic dynamics weakly
depends on the solid fraction of the sample. This process is strongly analogous
to the aging phenomenon observed in thermal glassy systems. We propose a
heuristic model where this slowing down mechanism is associated with a slow
evolution of the distribution of the contact forces between particles. This
model accounts for the main features of the observed dynamics.
