Strong- vs. weak-coupling pictures of jet quenching: a dry run using QED

High-energy partons ($E \gg T$) traveling through a quark-gluon plasma lose energy by splitting via bremsstrahlung and pair production. Regardless of whether or not the quark-gluon plasma itself is strongly coupled, an important question lying at the heart of philosophically different approaches to energy loss is whether the high-energy partons of an in-medium shower can be thought of as a collection of individual particles, or whether their coupling to each other is also so strong that a description as high-energy `particles' is inappropriate. We discuss some possible theorists' tests of this question for simple situations (e.g. an infinite, non-expanding plasma) using thought experiments and first-principles quantum field theory calculations (with some simplifying approximations). The physics of in-medium showers is substantially affected by the Landau-Pomeranchuk-Midgal (LPM) effect, and our proposed tests require use of what might be called `next-to-leading order' LPM results, which account for quantum interference between consecutive splittings. The complete set of such results is not yet available for QCD but is already available for the theory of large-$N_f$ QED. We therefore use large-$N_f$ QED as an example, presenting numerical results as a function of $N_f\alpha$, where $\alpha$ is the strength of the coupling at the relevant high-energy scale characterizing splittings of the high-energy particles.