An international team of researchers, including members from the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), has directly observed “highly charged muonic ions”, a completely new class of exotic atomic systems, in a gas-phase experiment for the first time. This groundbreaking study was selected as an Editors' Suggestion in Physical Review Letters, published online on June 16.

The observation highlights the capabilities of advanced superconducting transition-edge-sensor (TES) microcalorimeters in revealing previously inaccessible atomic phenomena.

Normal atoms consist of a nucleus and bound electrons and are electrically neutral. However, when many electrons are removed, the atom becomes highly charged. These charged atoms, known as highly charged ions, are valuable tools for research across various fields, including fundamental physics, nuclear fusion, surface science, and astronomy.

Researchers are particularly interested in plasmas—ionized gas consisting positive ions and electrons—found in high-energy environments such as the Sun and stars. Studying highly charged ions in plasma helps deepen our understanding of matter under extreme conditions. Although direct access to these environments is impossible, analyzing the characteristic x-rays emitted by highly charged ions provides insights into their structure and behavior.

The research team, led by Associate Professor Takuma Okumura from Tokyo Metropolitan University and Chief Scientist Toshiyuki Azuma from RIKEN and International Center for Quantum-field Measurement Systems for Studies of the Universe and Particles (QUP) at High Energy Accelerator Research Organization (KEK), focused on highly charged muonic ions, which contain negatively charged elementary particles called muons (Figure 1). The research team included collaborators from multiple institutions: Tadashi Hashimoto (RIKEN), Koichiro Shimomura (KEK), Daiji Kato (National Institute for Fusion Science), Yasushi Kino and Hirofumi Noda (Tohoku University), Shinya Yamada (Rikkyo University), Shinji Okada and Yuichi Toyama (Chubu University), Tadayuki Takahashi (Kavli IPMU), and Xiao-Min Tong (University of Tsukuba). Studying these ions could open novel research avenues.

Highly charged muonic ions are formed when a negative muon—a heavier cousin of the electron—is captured by an atom. During the muonic cascade, most of the bound electrons are ejected, leaving only one to a few in the atom. Although theoretical models predicted the existence of highly charged muonic ions, such as H-like, He-like, and Li-like configurations, they had never been experimentally observed due to their short lifetimes and the lack of sufficiently sensitive spectroscopic techniques.

Experiments were conducted at the D2 line of the Muon Science Experimental Facility (MUSE) of the Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai-mura, Ibaraki. The MUSE is capable of producing the most intense low-energy muon beams in the world, enabling the generation of highly charged muonic ions. To detect these ions, the team improved their experimental setup.

They employed a superconducting transition-edge sensor (TES) microcalorimeter, an x-ray detector developed for high-precision spectroscopy, including cosmic x-ray observations. The TES is capable of measuring x-ray energies down to several keV with high energy resolution, making it ideal for identifying rare exotic atoms like highly charged muonic ions.

Using argon (Ar) atoms as targets, the measured x-ray spectra agreed with theoretical predictions (Figure 2). The peak observed on the high-energy side was emitted from a “H-like” highly charged muonic argon ion (μAr¹⁶⁺) with one bound electron, while three peaks on the low-energy side corresponded to characteristic x-rays emitted by “He-like” μAr¹⁵⁺ and “Li-like” μAr¹⁴⁺ ions with two or three bound electrons, respectively.

The successful observation of highly charged muonic ions demonstrates the effectiveness of the team’s methods and paves the way for expanded research into muonic atomic systems.