In a recent paper published in Physical Review Letters, the international team, led by the WPI-QUP deputy director, Prof. Toshiyuki Azuma, has successfully observed a new exotic atomic system, the multicharged muonic ion, using a state-of-the-art superconducting transition-edge sensor (TES) microcalorimeter. A multicharged muonic ion is an atomic system in which a single nucleus simultaneously binds a small number of electrons and a negative muon. Although the existence of such ions had been theoretically predicted, this is the first time they have been directly observed in an experiment.

By combining the high-intensity muon beam at J-PARC with TES detectors, the team succeeded in performing X-ray spectroscopy experiments using low-pressure gaseous atomic targets. This setup enabled measurements under conditions where charge transfer reactions are suppressed. In addition, thanks to the outstanding energy resolution of the TES detectors, the number of electrons bound to the multicharged muonic ions, as well as their specific quantum states, could be successfully distinguished and observed.

Multicharged muonic ions represent a unique system in which a positively charged nucleus simultaneously binds different types of negatively charged particles. This makes them of great interest not only as a new type of quantum few-body system but also as a promising probe for investigating the interactions between negative muons and atoms or molecules.

Published details
Journal: Physical Review Letters 134, 243001
Title: Few-electron highly charged muonic Ar atoms verified by electronic K x ray
Authors: T. Okumura, T. Azuma, R. Hayakawa et al. (43 authors in total)
DOI: https://doi.org/10.1103/PhysRevLett.134.243001

Related link
Press release from Tokyo Metropolitan University (in Japanese)
https://www.tmu.ac.jp/news/topics/37784.html
Press release from KEK (in Japanese)
https://www.kek.jp/wp-content/uploads/2025/06/pr202506191400muar.pdf
KEK News (in Japanese)
https://www.kek.jp/ja/press/202506191400muar