Sohtaro Kanda

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.

At the U-line in J-PARC MLF MUSE, ultra-slow muons (USM) are generated by laser ionization of thermal muonium (Mu) in vacuum. Mu drifts in vacuum with a Maxwellian velocity distribution reflecting the production target temperature; consequently, the USM obtained from Mu ionization also possess correspondingly low energies. At the U-line, a high-temperature tungsten foil (2000 K, corresponding to 0.2 eV) and a room-temperature silica aerogel disk (300 K, 25 meV) are used. These USM are focused and accelerated by electrostatic lenses and transported to the experimental areas. In the U1A experimental area, a muon spin spectrometer is installed for USM- μ SR measurements. The implantation energy of muons into the sample can be controlled in the range of sub-keV to 30 keV by adjusting the voltage of a high-voltage platform on which the entire spectrometer is mounted. Whereas surface muons with an energy of 4 MeV, commonly used in materials science research, are suitable probes for bulk samples, USM are well-suited for measurements on thin film samples or interfaces within materials. Recently, we successfully demonstrated the implantation of muons selectively into a platinum thin layer sandwiched by silica layers. In this study, depth-resolved measurements were performed by scanning the beam implantation energy. This contribution presents demonstration results of USM- μ SR measurements on multilayer thin film samples.

Precision measurements of muonium, a purely leptonic atom, provide a unique platform for stringent tests of the Standard Model and searches for new physics beyond it. The sensitivity of these tests, however, is often limited by the precision of the muon mass. To overcome this limitation, we propose a Ramsey-Bordé atom interferometer with a muonium beam to determine the muon mass with unprecedented precision. A critical requirement for such an experiment is the availability of a bright, low-energy muonium beam. This paper reports on the development of a multi-stage muon cooling system designed to produce such a beam. The system combines a solid rare-gas moderator for producing epithermal muons with a subsequent laser ionization stage for generating an ultra-slow muon beam. We present the conceptual design of the cooling system and discuss the status of simulation studies for optimizing the generation and transport of slow muons.

At the J-PARC Muon Science Facility (MUSE), the MuSEUM collaboration is now performing new precision measurements of the ground state hyperfine structure (HFS) of both muonium and muonic helium atoms. High-precision measurements of the muonium ground-state HFS are recognized as one of the most sensitive tools for testing bound-state quantum electrodynamics theory to precisely probe the standard model and determine fundamental constants of the positive muon magnetic moment and mass. The same technique can also be employed to measure muonic helium HFS, obtain the negative muon magnetic moment and mass, and test and improve the theory of the three-body atomic system. Measurements at zero magnetic field have already yielded more accurate results than previous experiments for both muonium and muonic helium atoms. High-field measurements are now ready to start collecting data using the world’s most intense pulsed muon beam at the MUSE H-line. We aim to improve the precision of previous measurements ten times for muonium and a hundred times or more for muonic helium. We review all the key developments for these new measurements, focusing on the high-field experiment, and report the latest results and prospects.

The Super-Omega beamline at J-PARC Materials and Life Science Experimental Facility provides an intense pulsed slow positive muon beam. Combined with a muonium production target and laser light for muonium ionization, the pulsed ultra-slow muon facility has been developed. At the facility, a spectrometer for muon spin rotation measurements using ultra-slow muons is under commissioning. In this paper, we will report on the current status of the beam optics optimization of slow muon transport and ultra-slow muon extraction to improve the intensity and quality of the ultra-slow muon beam.

An exotic atom consisting of a negative muon and a nucleus is a muonic atom. Atomic parity violation in muonic atoms provides a unique opportunity to determine the Weinberg angle and search for physics beyond the standard model at the low energy scale. We have proposed a new experiment to perform X-ray spectroscopy of muonic atoms using a high-intensity pulsed muon beam at J-PARC. A scintillator-based calorimeter and a lowdensity gaseous target were developed for the experiment. As a feasibility study, we measured the yield of muon atoms and searched for muonic atom’s metastability using low-density methane gas. An overview of the experiment and the test experiment results are presented.