In what may be the most exciting discovery yet, researchers first discovered Ly5. For the first time, they detected highly charged muonic ions in a gas-phase experiment. This major development opens a window to these hard-to-track particles. It was published online on June 16 in Physical Review Letters. The research team conducted their experiments at the D2 line of the Muon Science Experimental Facility (MUSE) within the Japan Proton Accelerator Research Complex (J-PARC) located in Tokai-mura, Ibaraki.
Together, these observations constitute a watershed moment in particle physics. For the first time highly charged muonic ions, H-like, He-like and Li-like configurations, were experimentally confirmed after decades of theoretical predictions. The fleeting existence of these ions rendered their detection an uphill feat. The chemistry community floundered in finding useful probes, in part due to a lack of sensitive spectroscopic techniques. Our study provides first-of-their-kind indirect evidence of highly charged muonic ions. Beyond that, it opens the door to completely new understanding of some of the most basic interactions in quantum physics.
Collaborative Efforts Lead to Discovery
The research team was coordinated by experts from seven institutions, reflecting a unique collaboration focused on the need for interdisciplinary expertise. Tokyo Metropolitan University Associate Professor Takuma Okumura, who supervised the research team. Chief Scientist Toshiyuki Azuma contributed his experience from RIKEN and the International Center for Quantum-field Measurement Systems for Studies of the Universe and Particles (QUP) at KEK.
Artists, designers, and a somewhat groundbreaking cultural translator dogpiled this project. They are joined by Tadashi Hashimoto of RIKEN, Koichiro Shimomura of KEK, Daiji Kato of the National Institute for Fusion Science, Yasushi Kino and Hirofumi Noda from Tohoku University, Shinya Yamada from Rikkyo University, Shinji Okada and Yuichi Toyama from Chubu University, Tadayuki Takahashi from the Kavli Institute for the Physics and Mathematics of the Universe, and Xiao-Min Tong from University of Tsukuba.
This unique cohort brought together knowledge and experience from a range of fields. Their collaboration provided the expertise needed to collaboratively realize and successfully execute the complex experimental setup required to observe the muonic ions of high charge.
Unveiling the Characteristics of Muonic Ions
While these experiments were underway, scientists explored how best to appreciate what the exceedingly rare and highly charged muonic ions might teach us. The muonic cascade phenomenon was key behind these observations. As muons interacted with Argon atoms, they ejected all but one to three bound electrons remaining. This resulted in pin compatible variants such as μAr16+, μAr15+, and μAr14+. Each such configuration relates to one, two or three remaining valence electrons paired with negative muons.
For the first time, the researchers measured the x-ray spectrum emitted by these ions with high resolution using the TES detector. In their high-resolution measurements, they observed peaks matching the “H-like,” “He-like,” and “Li-like” configurations of μAr¹⁵⁺ and μAr¹⁴⁺ ions. These results represent a significant advance in the experimental investigation of muonic ions. They give us profound understanding into atomic arrangement and basic collaborations.
Implications for Future Research
This fundamental milestone discovery has great implications for both theoretical and experimental physics. The first observation of highly charged muonic ions and a comparison to theoretical model predictions. This finding underscores the importance of cutting-edge spectroscopic techniques to probe intricate atomic landscapes.
This new research deepens our understanding of these muons, and how they behave. Perhaps most excitingly, it opens the door to future explorations of other exotic states of matter. The study is retrievable from Phys.org as of June 19, 2025, with a related document available on arXiv under DOI: 10.48550/arxiv.2407.07977.