Naritoshi Kawamura

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.

A highly charged muonic ion is a unique few-body atomic system where a negatively charged muon and a few electrons are simultaneously bound to a single nucleus. We report the first state-selective observation of highly charged muonic Ar (μAr) by electronic K x-ray spectroscopy using an array of transition-edge sensor microcalorimeters. The high-precision K x-ray spectra provide a clear signature of the presence of muonic atoms with one, two, and three electrons, i.e., H-like, He-like, and Li-like μAr. With the aid of theoretical calculations, we confirmed that the peak positions are consistent with the x-ray energies from highly charged Cl ions, and the intensities reflect deexcitation dynamics of highly charged μAr.

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

The X-ray spectroscopy of the muonic atom has attracted atomic, nuclear, and particle physicists since its discovery. The properties of a muonic atom, such as its binding energy or atomic radius, are different from an ordinary atom because of the difference in the mass between the muon and electron. Our collaboration has employed superconductor transition-edge sensor (TES) microcalorimeters for the x-ray spectroscopy of the muonic atom. Thanks to the recent detector development, the 44-keV lines from muonic Ar, which is important for the precision test of bound-state quantum electrodynamics, and the 76-keV lines from muonic Si, which is of interest from the viewpoint of the measurement of nuclear radii, have been reached by the dynamic range of the state-of-art TES microcalorimeters. An accelerator facility that can produce a high-intensity muon beam is necessary for such spectroscopic experiments. We performed a commissioning experiment of the hard x-ray and gamma-ray TES microcalorimeter at the J-PARC MLF MUSE muon beam line. The energy resolution, gain stability, and performance of timing selection of the pulses were evaluated in the environment of a large-scale accelerator facility.