  We report experimental measurements of particle dynamics on slowly sheared
granular matter in a three-dimensional (3D) Couette cell. A closely-packed
ensemble of transparent spherical beads is confined by an external pressure and
filled with fluid to match both the density and refractive index of the beads.
This allows us to track tracer particles embedded in the system and obtain
three-dimensional trajectories as a function of time. We study the PDF of the
vertical and radial displacements, finding Gaussian and exponential
distributions, respectively. For slow shear rates, the mean-square fluctuations
in all three directions are found to be dependent only on the angular
displacement of the Couette cell. Both the diffusivity and mobility of tracer
particles are proportional to the shear rate, giving rise to a constant
effective temperature, characteristic of the jammed system.
