  The dynamics of a bi-dimensional dense granular packing under cyclic shear is
experimentally investigated close to the jamming transition. Measurement of
multi-point correlation functions are produced. The self-intermediate
scattering function, displaying slower than exponential relaxation, suggests
dynamic heterogeneity. Further analysis of four point correlation functions
reveal that the grain relaxations are strongly correlated and spatially
heterogeneous, especially at the time scale of the collective rearrangements.
Finally, a dynamical correlation length is extracted from spatio-temporal
pattern of mobility. Our experimental results open the way to a systematic
study of dynamic correlation functions in granular materials.
