Pushing the limits: Testing Quantum Electrodynamics with heavy, highly charged ions in a storage ring

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Quantum Electrodynamics (QED) stands as the cornerstone theoretical framework that elucidates the intricate interplay between charged particles, particularly electrons, and electromagnetic fields. By quantizing the electromagnetic field and portraying particles as excitations within these fields, QED has successfully unraveled various physics phenomena, including the Lamb shift [1]. Named after Willis Lamb, the Lamb shift refers to an anomalous difference in energy between two electron orbitals in an atom, arising from the interaction of the bound electron with the quantum fluctuations of the QED vacuum. Whilst, in a classical perspective, an electron orbiting a nucleus would not interact with its self-generated field, quantum field theory introduces virtual particles in the vacuum, contributing significantly to the Lamb shift.

To date, the most rigorous tests of strong-field QED have primarily focused on measuring transition energies, hyperfine structure, bound electron g-factors [2], and the Lamb shift [3]. However, these tests have predominantly been confined to relatively light few-electron atoms, leaving unexplored territories in the realm of heavy (high atomic number) nuclei systems. In our recent groundbreaking precision experiment [4] conducted at the Experimental Storage Ring (ESR) [4] at the GSI/FAIR laboratory in Germany, QED underwent rigorous testing in strong electromagnetic fields by exploring intrashell transitions in the heaviest two-electron system that can be found in nature. Achieving an unprecedented precision of 37 parts per million makes this experiment highly sensitive to higher-order QED contributions, specifically one-electron two-loop and two-electron QED effects. This marks a noteworthy progression in our comprehension of QED in an extremely strong Coulomb field. The experiment focused on helium-like uranium ions (with two electrons) created in excited states through electron capture from a gas jet target on stored hydrogen-like uranium ions (with one electron) in a storage ring - a circular device capable of extended retention of ions. The subsequent decays from the excited states were precisely measured using two high-resolution crystal spectrometers positioned at perpendicular observation angles. For a more in-depth exploration, refer to the paper.

In the era of precision experimental physics, endeavours such as these not only validate existing theories but also set the foundation for developing advanced detection systems. Looking ahead, the next step involves delving into an already planned, more precise measurement at the ESR storage ring to establish more precise benchmarks for QED and, in the future, to explore physics beyond the Standard Model.


[1] W. E. Lamb, R. C. Retherford, Phys. Rev. 72, 241 (1947).
[2] J. Morgner et al., Nature 622, 53–57 (2023).
[3] A. Gumberidze et al., Phys. Rev. Lett. 94, 223001 (2005).
[4] R. Loetzsch et al., Nature 625, 673-678 (2024).
[5] M. Steck, Yu. A. Litvinov, Prog. Part. Nucl. Phys. 115, 103811 (2020).

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