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

TL;DR

This paper explores whether high-energy partons in a quark-gluon plasma behave as independent particles or as strongly coupled entities, using large-Nf QED as a simplified model to test different theoretical approaches to jet quenching.

Contribution

It introduces a framework for testing strong versus weak coupling pictures of jet quenching using first-principles quantum field theory calculations in large-Nf QED.

Findings

Numerical results as a function of Nfα show the impact of coupling strength on in-medium shower physics.

The study highlights the importance of next-to-leading order LPM effects in understanding energy loss.

Large-Nf QED serves as a useful proxy for exploring jet quenching mechanisms.

Abstract

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.