  We perform numerical simulations to examine particle diffusion at steady
shear in a model granular material in two dimensions at the jamming density and
zero temperature. We confirm findings by others that the diffusion constant
depends on shear rate as $D\sim\dot\gamma^{q_D}$ with $q_D<1$, and set out to
determine a relation between $q_D$ and other exponents that characterize the
jamming transition. We then examine the the velocity auto-correlation function,
note that it is governed by two processes with different time scales, and
identify a new fundamental exponent, $\lambda$, that characterizes an algebraic
decay of correlations with time.
