  We investigate experimentally the connection between short time dynamics and
long time dynamical heterogeneities within a dense granular media under cyclic
shear. We show that dynamical heterogeneities result from a two timescales
process. Short time but already collective events consisting in clustered cage
jumps concentrate most of the non affine displacements. On larger timescales
such clusters appear aggregated both temporally and spatially in avalanches
which eventually build the large scales dynamical heterogeneities. Our results
indicate that facilitation plays an important role in the relaxation process
although it does not appear to be conserved as proposed in many models studied
in the literature.
