Speaker
Description
Atomic physics with negatively charged muons provides a powerful approach to probing fundamental physics because their mass is about 200 times heavier than that of ordinary electrons. When a negative muon is captured by a light nucleus, it rapidly strips off all bound electrons through the muon-induced Auger process, creating an isolated muonic atom consisting only of the nucleus and the bound muon. Because the muon orbits so close to the nucleus, it experiences an extremely strong electromagnetic field—far beyond what can be artificially generated—which makes muonic atoms highly sensitive probes for testing quantum electrodynamics (QED) in regimes where perturbative approaches fail. Our initial goal was to explore such untested regions of QED [1].
Until recently, producing isolated exotic atoms/ions in a vacuum and measuring their transition energies with high precision had not been feasible. This situation changed with the advent of the intense, slow negative muon beam at J-PARC, combined with high-resolution multi-pixel TES (Transition Edge Sensor) microcalorimeters, achieving energy resolutions of a few eV with high efficiency. Using this system, we realized a proof-of-principle experiment by preparing a two-body quantum system—muonic neon (μNe)—and precisely measuring its muonic x rays with a superconducting calorimeter [2].
During this campaign, we also discovered that electronic characteristic x rays emitted from muonic atoms can be measured with similar precision. For example, spectra from muonic iron (μFe) revealed a characteristic structure with significant broadening, which, when compared with theoretical simulations, uncovered ultrafast (tens of femtoseconds) dynamics involving the muon and bound electrons processes that can be extended to condensed matter physics [3,4]. Likewise, measurements of electronic x rays from muonic argon (μAr) demonstrated the existence of few-body exotic ions (H-like, He-like, and Li-like), in which a muon shields the nuclear charge while a small number of electrons remain bound. Theoretical analysis showed that these K x rays are emitted following slow charge transfer processes occurring on the order of hundreds of nanoseconds [5].
Thus, by combining intense slow muon beams with state-of-the-art superconducting x-ray calorimeters, we have established a new capability to precisely measure both muonic and electronic x rays from muonic atoms. This interdisciplinary approach opens novel pathways in fundamental physics, atomic physics, condensed matter, and even astrophysics.
References
[1] N. Paul, T. Azuma, et al, Phys. Rev. Lett. 126, 1730018 (2021).
[2] T. Okumura, T. Azuma, et al., Phys. Rev. Lett. 130, 173001 (2023).
[3] T. Okumura, T. Azuma, et al., Phys. Rev. Lett. 127, 053001 (2021).
[4] X. M. Tong, T. Azuma, et al., Phys. Rev. A 107, 012804 (2023).
[5] T. Okumura, T. Azuma, et al., Phys. Rev. Lett. 134, 243001 (2025).
