An international group of researchers has achieved the world's first direct observation of muonic molecules in resonance states using a high-resolution x-ray detector, reports a new study in Science Advances.

Resonance states are critical in determining the reaction rate of muon catalyzed fusion (µCF), a process that utilizes elementary particles known as muons. Within muonic molecules, the nuclei are confined in extremely close proximity, enabling nuclear fusion to occur even at room temperature without the need for plasma.

Currently, research aimed at the practical application of nuclear fusion is underway worldwide. In principle, fusion offers highly safe energy with no risk of runaway accidents. It utilizes fuel easily extracted from seawater and produces clean energy without carbon dioxide emissions.

To initiate conventional fusion, methods are used that involve generating plasma at extremely high temperatures and confining it with magnetic fields or instantaneously compressing fuel with lasers to achieve high-temperature, high-density plasma. In contrast, in µCF, the electrons in hydrogen molecules are replaced by muons, compressing the internuclear distance of the molecules by 1/200. Within these muonic molecules, the nuclei are confined in proximity, allowing fusion to occur even at room temperature without the need for plasma. For µCF to occur efficiently, the rapid formation of muonic molecules is crucial.

However, muon atomic and molecular processes leading to the formation of these muonic molecules were the subject of a long-standing discrepancy between theory and experiment, and the role of the resonance states of muonic molecules had remained unresolved.

Now an international group of researchers, including The University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) Professor Tadayuki Takahashi, and led by Assistant Professor Yuichi Toyama and Professor Shinji Okada of the Center for Muon Science and Technology at Chubu University, and Associate Professor Takuma Yamashita and Professor Yasushi Kino of the Department of Chemistry at Tohoku University, developed on the theoretical studies by Kino, Yamashita, and their colleagues to used a superconducting transition-edge sensor (TES) microcalorimeter (TES detector) developed by the U.S. National Institute of Standards and Technology (NIST), to separate and identify complex, overlapping x-ray spectral features with high resolution, and spectroscopically distinguished and detected x-ray components originating from muonic molecules and muonic atoms (Figure 2).

By comparing the observed spectra with high-precision theoretical calculations, the vibrational quantum states of muonic molecules, consisting of two deuterium nuclei (d) and a muon (µ) in a resonance state, were successfully identified, and their population ratios were quantitatively evaluated.

The team’s achievement establishes a scientific foundation that will guide future research toward realizing highly efficient muon catalyzed fusion, promoted under the Japanese Cabinet Office's Moonshot Research and Development Program (Goal 10), managed by the Japan Science and Technology Agency (JST). The high-resolution x-ray spectroscopy techniques established in this study, along with the insights uncovering the physical role of resonance states, provide clear guiding principles for research strategies targeting µCF efficiency enhancement. Ultimately, this is expected to further accelerate research and development toward the "Innovative muon catalyzed fusion technology for practical applications."

Details of this study were published in Science Advances on 15 April.