On Drag Forces and Jet Quenching in Strongly Coupled Plasmas

We compute the drag force experienced by a heavy quark that moves through plasma in a gauge theory whose dual description involves arbitrary metric and dilaton fields. As a concrete application, we consider the cascading gauge theory at temperatures high above the deconfining scale, where we obtain a drag force with a non-trivial velocity dependence. We compare our results with the jet-quenching parameter for the same theory, and find qualitative agreement between the two approaches. Conversely, we calculate the jet-quenching parameter for N=4 super-Yang-Mills with an R-charge density (or equivalently, a chemical potential), and compare our result with the corresponding drag force.