Primer of Strong-Field Quantum Electrodynamics for Experimentalists

TL;DR

This paper tackles how to study QED in ultra-strong electromagnetic fields where perturbation theory breaks down. It develops an experimentalist-friendly primer around the Furry picture, introducing the central SFQED parameters $ξ$, $χ$, and $η$, and detailing nonlinear processes such as nonlinear Compton scattering and nonlinear Breit–Wheeler pair production. It surveys environments—laser–particle setups, crystals and high-Z fields, ultra-peripheral collisions, and astrophysical contexts—and discusses representative experiments (e.g., E-144, E-320, LUXE) that probe nonlinear and potential nonperturbative QED effects. The work provides actionable guidance for designing and interpreting strong-field QED experiments, enabling tests of nonperturbative regimes and vacuum phenomena with practical experimental platforms.

Abstract

This document serves as a conceptual and practical introduction to Strong-Field Quantum Electrodynamics (SFQED), written from the standpoint of experimental physicists. It was developed in the context of the "Strong-Field QED Workshop 2024" at DESY, aimed at supporting early-career researchers entering the field from the experimental side. Rather than providing a comprehensive theoretical review, the document focuses on the core ideas, terminology, and challenges in SFQED that are most relevant to experimental design and interpretation. Our goal is to offer a first point of contact with the subject, bridging the gap between foundational theory and hands-on experimental work, and complementing more formal literature in the field.

Figures (9)

• Figure 1: Nonperturbativity arises in two ways: from the coupling between the electron and the background field, depicted in red, and the coupling between the charge and the quantised radiation field, depicted in orange. Nonperturbativity can arise via interaction with the background field, or via interaction with the quantised radiation field. The former can be side-stepped via the Furry formalism, where the background is treated exactly. Figure from Gonoskov:2021hwf.
• Figure 2: Diagrammatic expansion of the dressed Furry picture propagator, represented with a double line, regarding position-space Feynman diagrams, from Fedotov:2022ely. The dressed propagator can be viewed as a resummation of an infinite series of diagrams in which the electron interacts with $n$ effective background photons (represented by a wavy line ending with an X).
• Figure 3: Regimes and related rates for pair-production defined by $\chi_\gamma$ and $\xi$Abramowicz:2021zja.
• Figure 4: Feynman diagrams for linear, nonlinear perturbative, and nonlinear nonperturbative Compton Scattering. Dressed electrons for the nonperturbative case are represented by a double line.
• Figure 5: Lowest order virtual one-loop box diagrams for elastic $\gamma\gamma \to \gamma \gamma$ scattering involving fermions, with each cross denoting an external field leg. Despite it being challenging to observe, given that it is an $\alpha^4 \approx 3 \times 10^{-9}$ process, it has been achieved in various experiments. Figure from ATLAS:2017fur.