KEK is proud to announce that two muon-related research papers involving its scientists have been published simultaneously in the June 20, 2025 issue (Vol. 24, 2025) of Physical Review Letters (PRL), one of the world’s most prestigious physics journals. Remarkably, both papers were selected as Editors’ Suggestions — a rare distinction that highlights their exceptional scientific significance.
Rare Double Recognition in a Single Issue
PRL attracts global attention as one of the leading journals in physics. Only research deemed particularly important and innovative is selected as an Editors’ Suggestion. It is uncommon for two papers from the same research institution to appear in the same issue and both receive this honor. These achievements were made possible through the close collaboration of KEK’s Institute of Particle and Nuclear Studies, Institute of Materials Structure Science, and Accelerator Laboratory. By combining their specialized expertise, the teams produced groundbreaking results in muon science.
About the Research
First Paper-Precision X-ray Spectroscopy of Muonic Argon Ions
The research team successfully carried out precise measurements of X-rays emitted when electrons transition from higher to lower energy levels in highly charged muonic argon ions (μAr^16+, μAr^15+, μAr^14+) formed when negatively charged muons (μ⁻) are captured by argon (Ar) atoms. This achievement was made possible through high-resolution X-ray spectroscopy using a state-of-the-art Transition-Edge Sensor (TES) microcalorimeter — one of the most advanced X-ray detectors available today. The results enable a deeper understanding of electron behavior within atoms. The TES technology has also demonstrated outstanding performance in hadron experiments, underscoring its versatility and reliability as a tool for high-precision measurements.
Second Paper—Cooling and Acceleration of Positive Muons
The team achieved a world-first by cooling positively charged muons to near room-temperature energies (0.03 eV) and subsequently accelerating them using radio-frequency acceleration up to 100 keV. This breakthrough marks an important step toward the realization of next-generation muon accelerators. In particular, for the upcoming muon g-2/EDM experiment being prepared at the J-PARC MLF, the production of a high-brightness muon beam through cooling and acceleration is essential, and this result provides a key enabling technology. Looking ahead, the technique is also expected to contribute to the development of a “muon microscope” capable of visualizing the internal structure of materials with unprecedented precision. In addition to being selected as an Editors’ Suggestion, this paper was also featured in a “Viewpoint” article aimed at readers outside the immediate field, and its imagery was highlighted on the journal’s cover.
About Muons at KEK
Muons are elementary particles produced naturally by cosmic rays or artificially in particle accelerators. At KEK, large numbers of muons are generated at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). These muons are used both to explore the fundamental laws of particle physics and, when irradiated onto samples, to analyze the properties and composition of matter.
Japanese research is helping to open new frontiers in world-leading physics. We encourage you to follow the exciting future developments in KEK’s muon research.
About KEK
The High Energy Accelerator Research Organization (KEK) is a world-leading research institute in the field of particle and nuclear physics, materials science, and accelerator science. KEK operates advanced accelerator facilities and fosters international collaborations to advance fundamental science and its applications.