Katsuhiko Ishida

Muon catalyzed fusion (μCF) is a plasma-free process in which the formation of muonic hydrogen molecules precedes and enables fusion between their constituent nuclei. Despite decades of study, the reaction dynamics of μCF remain elusive. Recent theories predict that resonance states of the muonic molecules play a key role and that these states can be probed using x-ray techniques. Using an array of transition-edge sensor microcalorimeters with 10-fold improved energy resolution compared to conventional silicon detectors, we observed x-rays from resonance states of muonic deuterium molecules despite an intense background. The spectrum is well explained by high-precision calculations incorporating the vibrational states. This work identifies the long-overlooked resonance state pathway as crucial in μCF and provides the direct evidence of the efficient muonic molecular formation.

Acceleration of positive muons from thermal energy to 100 keV has been demonstrated. Thermal muons were generated by resonant multiphoton ionization of muonium atoms emitted from a sheet of laser-ablated aerogel. The thermal muons were first electrostatically accelerated to 5.7 keV, followed by further acceleration to 100 keV using a radio-frequency quadrupole with an intensity of 2 × 10 − 3 μ + / pulse . The transverse normalized rms emittance of the accelerated muons in the horizontal and vertical planes were 0.85 ± 0.25 ( s t a t ) − 0.13 + 0.22 ( syst ) π mm mrad and 0.32 ± 0.03 ( stat ) − 0.02 + 0.05 ( syst ) π mm mrad , respectively. The measured emittance values demonstrated phase-space reduction by a factor of 2.0 × 10 2 (horizontal) and 4.1 × 10 2 (vertical) allowing good acceleration efficiency. These results pave the way to realize the first-ever muon accelerator for a variety of applications in particle physics, material science, and other fields. Published by the American Physical Society 2025

Background: The energy distribution of excited states populated by the nuclear muon capture reaction can facilitate an understanding of the reaction mechanism; however, experimental data are fairly sparse.