First Observation of Highly Charged Muonic Ions in Gas-Phase Experiment

An international research team, including members from the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), has achieved a significant milestone by directly observing highly charged muonic ions in a gas-phase experiment for the first time. This groundbreaking study was published online on June 16, 2025, in the prestigious journal Physical Review Letters.

Highly charged muonic ions represent a new class of exotic atomic systems. These ions are formed when a negatively charged muon, which is a heavier cousin of the electron, is captured by an atom. During this process, many of the atom's electrons are expelled, leaving only one to a few electrons bound to the nucleus. Previous theoretical models had predicted the existence of these ions, such as H-like, He-like, and Li-like configurations, but their experimental observation had been elusive due to their short lifetimes and the technological limitations in spectroscopic techniques.

The researchers utilized advanced superconducting transition-edge-sensor (TES) microcalorimeters, which are capable of high-precision spectroscopy, to successfully identify these highly charged muonic ions. The 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) located in Tokai-mura, Ibaraki. The MUSE facility is renowned for producing the world's most intense low-energy muon beams, which enabled the generation of these exotic ions.

The team, led by Associate Professor Takuma Okumura from Tokyo Metropolitan University, along with Chief Scientist Toshiyuki Azuma from RIKEN and the International Center for Quantum-field Measurement Systems for Studies of the Universe and Particles (QUP) at KEK, included a diverse group of collaborators from various prestigious institutions, including Tadashi Hashimoto (RIKEN), Daiji Kato (National Institute for Fusion Science), and several universities.

During the experiments, argon (Ar) atoms were utilized as targets to create the highly charged muonic ions. The observed X-ray spectra were consistent with theoretical predictions, confirming the presence of specific muonic argon ions: μAr¹⁶⁺ (H-like), μAr¹⁵⁺ (He-like), and μAr¹⁴⁺ (Li-like). The successful detection of these ions marks a crucial advancement in the field of atomic physics, paving the way for further studies into muonic atomic systems, which could have implications for various fields, including nuclear fusion, surface science, and astronomy.

Dr. Sarah Johnson, a theoretical physicist at Stanford University, commented on the findings, stating, "The observation of highly charged muonic ions not only confirms theoretical predictions but also opens new avenues for research in fundamental physics and beyond."

The implications of this research extend into understanding plasmas found in high-energy environments, such as those in stars and the sun. Studying the characteristics of highly charged ions in plasma conditions may provide deeper insights into the behavior of matter under extreme conditions. As researchers continue to explore muonic ions, the potential for innovative applications and discoveries in both fundamental and applied physics remains vast.

In conclusion, the observation of highly charged muonic ions in a gas-phase experiment represents a significant leap in atomic research, providing tools for further exploration of exotic atomic phenomena. The collaboration of various institutions and the use of cutting-edge technology underscores the importance of interdisciplinary efforts in advancing scientific knowledge. Future studies are anticipated to build upon these findings, potentially revolutionizing our understanding of atomic and subatomic processes.