  We study the dynamics of a viscoelastic medium driven through quenched
disorder by expanding about mean field theory in $6-\epsilon$ dimensions. The
model exhibits a critical point separating a region where the dynamics is
hysteretic with a macroscopic jump between strongly pinned and weakly pinned
states, from a region where the sliding state is unique and no jump occurs. The
disappearance of the jump at the critical point is described by universal
exponents. As suggested in \onlinecite{MMP00}, the model appears to be in the
same universality class as the zero-temperature random field Ising model of
hysteresis in magnets.
