- The nuclear charge radius of the lead isotope 208Pb is calculated with unprecedented accuracy and is found to be significantly larger than previously thought.
- The new value essentially resolves a long-standing discrepancy in muonic spectroscopy data.
- The work provides a new benchmark for extracting nuclear charge radii in heavy nuclei.
Nuclear radii are fundamental benchmarks in physics. They test nuclear theories, serve as crucial input for atomic and molecular calculations, and are essential for precision tests of quantum electrodynamics — and even for searches for new physics beyond the Standard Model. A state-of-the art measure is the “root-mean-square” charge radius which can be experimentally accessed via the interaction with electrons or muons. The muon can be considered as the “heavier brother” of the electron.
The lead isotope 208Pb is especially important as the heaviest stable nucleus and it is usually considered as one of the best understood in nuclear physics. Its charge radius has been measured with two independent methods: electron scattering and spectroscopy of muonic atoms, each supported by multiple experiments. But on closer inspection, both methods reveal shortcomings: scattering data are inconsistent with each other, while the traditional fit to muonic-atom data shows a poor quality — about 200 times worse than normally accepted.
A research team led by Natalia S. Oreshkina in the theory department of Christoph H. Keitel at MPIK has now tackled this long-standing puzzle. Using a rigorous theoretical framework combined with state-of-the-art numerical methods, they achieved a twenty-fold improvement in the fit quality and proposed a new value for the charge radius of the lead nucleus [1]. Strikingly, their result is 3 to 4 standard deviations larger than the value commonly used today, underscoring the need for a critical reanalysis. Further model independent approach from Li-like ions [2] suggest an even larger though less accurate value.
Most importantly, this work provides a new benchmark for extracting nuclear charge radii in heavy nuclei. It also opens the path toward systematic reevaluations across the entire nuclear chart based on muonic spectroscopy, as recently demonstrated for 90Zr and 120Sn [3].
[1] 208Pb nuclear charge radius revisited: closing the fine-structure-anomaly gap
Zewen Sun, Konstantin A. Beyer, Zoia A. Mandrykina, Igor A. Valuev, Christoph H. Keitel and Natalia S. Oreshkina
Physical Review Letters 135, 163002 (2025). DOI: 10.1103/h3xz-xdxr
[2] Model-independent determination of nuclear charge radii from Li-like ions
V. A. Yerokhin and B. Ohayon
arXiv
[3] Relativistic recoil as a key to the fine-structure puzzle in muonic 90Zr
Konstantin A. Beyer, Igor A. Valuev, Zoia A. Mandrykina, Zewen Sun and Natalia S. Oreshkina
arXiv
Weblinks:
Group ‘Exotic Quantum Systems’ at MPIK
Division ‘Theoretical Quantum Dynamics and Quantum Electrodynamics’ at MPIK
