Abstract To develop a dedicated muon experimental area for the study of energy materials and cells using surface muons, we plan to construct the S3 experimental area on the S-line, Muon Science Establishment (MUSE), Materials and Life Science Experimental Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC). To transport the beam to the sample position in the S3 area, two quadrupole triplets and a bend magnet will be used after the S1 and S2 branches. A dedicated kicker system will be used to share the beam among the branches of the S-line. Before starting construction of the S3 area, we perform the Monte Carlo simulation for the shield study and beam transportation. For the safe operation of the beamline, the required thickness and height of the iron shield of the S3 area are estimated to be 5 cm and 5 m, respectively.

At the J-PARC muon g-2/EDM experiment, a silicon strip detector will be used to detect positrons from muon decay. The detector consists of planes of detector modules arranged radially. The expected maximum hit rate reaches 1.4 MHz per sensor strip, and achieving high detection efficiency even under such hit-rate conditions is a key performance requirement. We have developed the smallest unit of the detector module, and its performance was evaluated using a muon beam at the J-PARC MLF H-line. The specifications of the detector module and the evaluated hit-rate capability are described in this article.

Muonium (Mu) is a hydrogenlike exotic atom composed of a positive muon and an electron ( μ ^+ e ^- ), and is a suitable probe with its hyperfine structure (HFS) for a precise determination of the magnetic moment and thus the mass of the muon, a lepton particle, which leads to a rigorous test of bound-state quantum electrodynamics (QED) and weak interactions as well as a search for any possible new physics beyond the Standard Model of particle physics. MuSEUM (Muonium Spectroscopy Experiment Using Microwave) Collaboration has so far succeeded in measuring the ground-state HFS of the Mu atom under the zero magnetic field, and is now aiming at a precision of 2 ppb ( 2 × 10^-9 ) under strong magnetic fields to surpass the last world record, using a newly operational high-intensity muon beam line at J-PARC in Japan. In contrast to conventional spectroscopy which determines the resonance center frequency by fitting the resonance curve, our new technique named Rabi-oscillation spectroscopy does not require any frequency scanning, eliminating systematic uncertainties due to possible power fluctuations. The resonance frequency can be obtained directly from the time evolution of the Rabi oscillation at a fixed frequency of the applied electromagnetic wave. We are also studying the muonic helium atom ( μ ^- α e ^- ), which is also hydrogenlike with a pseudo-proton composed of a negative muon and a helium nucleus ( μ ^- α ), using the same method of hyperfine spectroscopy. We have very recently started our experiments for Mu under a strong and highly uniform magnetic field of 1.7 T produced by a superconducting solenoid, with satisfying spectroscopic results. Current achievements and near-future strategies of our experiments are presented.

The surface muon beamline at J-PARC provides high-intensity muon beams that are essential for advanced materials science research, particularly in techniques such as muon spin rotation/relaxation (μSR). However, positron contamination in the beam poses a significant challenge by introducing background noise that affects the measurement precision. Therefore, achieving high-purity muon beams is critical for improving experimental reliability and accuracy. In this study, the G4beamline Monte Carlo simulation toolkit was employed to model the transport of muons and positrons from the production target through the beamline. The system includes a momentum and charge-based separator followed by a collimating slit. While the current slit configuration effectively suppresses positrons, it also causes substantial muon loss of approximately 76%, which significantly reduces the usable muon flux for downstream applications. To address this issue, a detailed investigation into slit size was performed. The results indicate that modest adjustments to the slit aperture size can improve the muon-to-positron ratio while retaining a greater fraction of the muon beam. These results provide valuable guidance for optimizing beamline performance and improving the quality of muon-based experiments at J-PARC.

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.

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 (mu Ar) 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 mu Ar. 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 mu Ar.

Muonic helium is a hydrogenlike atom composed of a helium atom with one of its two electrons replaced by a negative muon. Its ground-state hyperfine structure is a sensitive tool for testing the theory of three-body atomic systems and bound-state quantum electrodynamics and determining fundamental constants of the negative muon magnetic moment and mass. New precise measurements are now in progress at J-PARC Muon Science Facility (MUSE). Zero-field measurements have already been carried out, and the results are more precise than previous measurements 40 years ago. High-field measurements are now in preparation. Furthermore, a new experimental approach to recover the negative muon polarization lost during the muon cascade process in helium is being investigated, which could drastically improve the measurement accuracy. The first laser repolarization experiments have recently been performed. The status of these new muonic helium HFS measurements and the latest results are presented.

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.

Background: The nuclear charge radius and distribution are the most fundamental quantities of the atomic nucleus. From the muonic transition energies, the absolute charge radius has been experimentally obtained, while there have been no established methods to discuss the distribution. Purpose: The muonic transition energies for five palladium isotopes with the mass number $A = 104$, $105$, $106$, $108$ and $110$ were measured. The procedure to deduce the charge radii and the method to discuss the charge distribution from the muonic transition energies are proposed. Method: The experiment was performed at the MuSIC-M1 beamline at Research Center for Nuclear Physics, Osaka University. A continuous muon beam impinged on the enriched palladium targets. Muonic X rays were measured by high-purity germanium detectors. Results: The muonic transition energies up to $4f$-$3d$ transitions were determined for five palladium isotopes. Discussion and conclusion: The root-mean-square charge radii are deduced assuming the two-parameter Fermi distribution. The charge distribution of the nucleus is discussed employing the Barrett model. The muonic transition energies of the $3d$-$2p$ transitions are crucial to discuss both the charge radius and the charge distribution.

Measurements of the muonic helium atom hyperfine structure (HFS) are a sensitive tool to test the theory of three-body atomic systems and bound-state quantum electrodynamics (QED) and to determine fundamental constants of the negative muon magnetic moment and mass. The world's most intense pulsed negative muon beam at J-PARC MUSE brings an opportunity to improve previous measurements and test further CPT invariance by comparing the magnetic moments and masses of positive and negative muons. Test measurements at D-line are now in progress utilizing MuSEUM apparatus at zero field. The first results already have better accuracy than previous measurements in the 1980s. Also, the investigation of a new experimental approach to improve HFS measurements by repolarizing muonic helium atoms using a spin-exchange optical pumping (SEOP) technique was started. If successful, this would drastically improve the measurement accuracy.

The mass mμ− of the negative muon is one of the parameters of the elementary particle Standard Model and it allows us to verify the CPT (charge–parity–time) symmetry theorem by comparing mμ− value with the mass mμ+ of the positive muon. However, the experimental determination precision of mμ− is 3.1ppm, which is an order of magnitude lower than the determination precision of mμ+ at 120ppb. The authors aim to determine mμ− and the magnetic moment μμ− with a precision of O(10ppb) through spectroscopy of the hyperfine structure (HFS) of muonic helium-4 atom (4Heμ−e−) under high magnetic fields. He4μ−e− is an exotic atom where one of the two electrons of the He4 atom is replaced by a negative muon. To achieve the goal, it is necessary to determine the HFS of He4μ−e− with a precision of O(1ppb). This paper describes the determination procedure of the HFS of He4μ−e− in weak magnetic fields reported recently, and the work towards achieving the goal of higher precision measurement.

Precision microwave spectroscopy of the ground-state hyperfine structure in muonium provides a stringent test of the Standard Model in particle physics. The MuSEUM collaboration is preparing for such a measurement, aiming for precision down to ≈1 ppb, utilizing the world’s most intense pulsed muon beam at J-PARC. The measurement of the Zeeman-split structure with an external magnetic field of 1.7T also precisely determines the muon’s magnetic moment (≈10 ppb). In the future, improved precision of the magnetic moment can be potentially obtained by measurements with different magnetic field strengths, however, it entails upgrading current cylindrical microwave cavities to rectangular ones. As the first step for the upgrade, we have developed a dual-mode rectangular cavity for the measurement with 2.9T field. The electromagnetic design and production have been established, and frequency sweeping with two desired modes has been successfully demonstrated. Moreover, the overall performance of the measurement at 2.9T field was evaluated with Monte Carlo simulations. These studies pave the way for a further extension of the MuSEUM experiment at various strengths of the magnetic fields.

J-PARC Muon Facility: MUSE (Muon Science Establishment) is responsible for the inter-university user program and the operation, maintenance, and construction of the muon beamlines, namely D-line, S-line, U-line, and H-line, along with the muon source at J-PARC Materials and Life Science Facility (MLF). In this paper, recent developments are briefly presented.

The amount of C in steel, which is critical in determining its properties, is strongly influenced by steel production technology. We propose a novel method of quantifying the bulk C content in steel non-destructively using muons. This revolutionary method may be used not only in the quality control of steel in production, but also in analyzing precious steel archaeological artifacts. A negatively charged muon forms an atomic system owing to its negative charge, and is finally absorbed into the nucleus or decays to an electron. The lifetimes of muons differ significantly, depending on whether they are trapped by Fe or C atoms, and identifying the elemental content at the muon stoppage position is possible via muon lifetime measurements. The relationship between the muon capture probabilities of C/Fe and the elemental content of C exhibits a good linearity, and the C content in the steel may be quantitatively determined via muon lifetime measurements. Furthermore, by controlling the incident energies of the muons, they may be stopped in each layer of a stacked sample consisting of three types of steel plates with thicknesses of 0.5 mm, and we successfully determined the C contents in the range 0.20–1.03 wt% depth-selectively, without sample destruction.

At the Muon Science Establishment (MUSE) in the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC), the D-line is a unique beamline capable of providing both positive and negative decay muons. To utilize these muon beams more efficiently and effectively, we upgraded the electrostatic separator, adjusted the kicker timing, conducted beam momentum measurements, and minimized the background for negative muon spin relaxation (µ-SR) measurements. These upgrades and commissioning efforts have resulted in a reduction in the amount of contaminating electrons and background noise, thereby yielding a cleaner signal. We also determined the optimal conditions for the kicker in a single bunching operation. The standard deviation of the muon beam momentum distribution, as measured in the D1 area, was estimated to be approximately 3%. These enhancements have enabled us to utilize a beam of higher quality than was previously possible.