Selected Publications

The MuSEUM collaboration conducted a precise measurement of the hyperfine structure of muonium, an atom consisting of a positive muon and an electron. We observed the hyperfine transition resonance at near-zero magnetic fields and determined the muonium hyperfine structure interval to be 4.463302(4) GHz with a relative precision of 0.9 ppm. This result was consistent with previous measurements and theoretical calculations, demonstrating the potential for future experiments using microwave spectroscopy of muonium to achieve the highest precision.

At J-PARC MLF, the MUSE facility provides the world's highest flux of pulsed muon beams. One of the facility's beamlines, U-Line, delivers an intense surface muon beam for generation of ultra-slow muons generated via laser ionization of thermal muonium in vacuo. The ultra-slow muon beam has tunable low-energy and exceptional time resolution, enabling the study of surfaces, interfaces, and fast dynamics. Commissioning of the beamline and instruments is in progress for upcoming user programs. This paper provides an overview of the facility's current status and future prospects.

Muonic atoms, composed of a negative muon and a nucleus, offer a unique opportunity to investigate atomic parity violation and explore physics beyond the standard model at low energies. A new experiment has been proposed for X-ray spectroscopy of muonic atoms using a high-intensity pulsed muon beam at J-PARC. This experiment involves the development of a scintillator-based calorimeter and a low-density gaseous target. As a feasibility study, muon atom yield and muonic atom metastability were assessed using low-density methane gas.

Muonic hydrogen, composed of a proton and a negative muon, has a significantly smaller Bohr radius compared to electronic hydrogen. This makes spectroscopy of muonic hydrogen highly sensitive to the proton's finite size effect. Laser spectroscopy of the Lamb shift in muonic hydrogen revealed a proton charge radius much smaller than previous measurements. To address this "proton radius puzzle", a new experiment proposes measuring the ground-state hyperfine splitting in muonic hydrogen. The goal is to determine the proton Zemach radius with 1% precision through measurements of the decay electron angular asymmetry. A preliminary experiment to measure the hyperfine quenching rate has been proposed to test the feasibility of laser spectroscopy.

Muons and muonium serve as unique tools in materials science, functioning as tiny magnetometers and mimicking hydrogen behavior in matter. However, their application as matter waves is limited due to issues with surface muons losing coherence during slowing down. Low-energy muons generated through laser ionization of muonium can address this problem by providing coherent slow muonium with minimal temporal and spatial dispersion. Muonium interferometers, similar to atomic interferometers, offer numerous potential applications, including muonium spectroscopy, quantum interference studies such as Berry phase measurements, and precise measurements of fundamental constants. This contribution discusses in-flight muonium spectroscopy and the potential of muonium interferometry.