  We use confocal microscopy and time-resolved light scattering to investigate
plasticity in a col- loidal polycrystal, following the evolution of the network
of grain boundaries as the sample is submitted to thousands of shear
deformation cycles. The grain boundary motion is found to be ballistic, with a
velocity distribution function exhibiting non-trivial power law tails. The
shear- induced dynamics initially slow down, similarly to the aging of the
spontaneous dynamics in glassy materials, but eventually reach a steady state.
Surprisingly, the cross-over time between the ini- tial aging regime and the
steady state decreases with increasing probed length scale, hinting at a
hierarchical organization of the grain boundary dynamics.
