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At J-PARC MUSE, since the µ SR2017 conference and up to FY2022, there have been several new developments at the facility, including the completion of a new experimental area S2 at the surface muon beamline S-line and the first muon beam extraction to the H1 area in the H-line, mainly to carry out high-statistics fundamental physics experiments. Several new studies are also underway, such as applying negative muon non-destructive elemental analysis to the analysis of samples returned from the asteroid Ryugu in the D2 area of the D-line. This paper reports on the latest status of MUSE.

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.

A novel device to compress the phase space of a muon beam by a factor of \\[10^{10}\\] with a \\[10^{-3}\\] efficiency is under development. A surface muon beam is stopped in a helium gas target consisting of several compression stages, wherein strong electric and magnetic fields are applied. The spatial extent of the stopped muon swarm is decreased by means of these fields until muons with eV energy are extracted into vacuum through a small orifice. It was observed that a 20 cm long muon stop distribution can be compressed in the longitudinal direction to a sub-mm extent within \\[2~\\upmu \\hbox {s}\\]. Additionally, a drift perpendicular to the magnetic field of the compressed low-energy muon swarm was successfully demonstrated, paving the way towards extraction from the gas and re-acceleration of the muons.

At J-PARC, the MuSEUM (Muonium Spectroscopy Experiment Using Microwave) collaboration aims to precisely measure the ground-state hyperfine splitting of muonium atoms arising from the muon and electron spins. The pulsed muon beam is stopped in a krypton gas cell to form muonium atoms. The transitions of spin states are induced with a microwave cavity, which are then measured by positron counters. After the previously performed successful measurements with a nearly-zero magnetic field, we are currently planning a measurement with the 2.9T magnetic field by measuring two Zeeman-split sub-levels, so that increased statistics will allow us to more precisely determine the transition frequency down to ∼1ppb. Moreover, a new microwave cavity with a unique geometry is being designed to perform the measurement at an even stronger field of 2.9T in the future.

The MEG experiment, designed to search for the μ+→e+γ decay, completed data-taking in 2013 reaching a sensitivity level of 5.3×10-13 for the branching ratio. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6×10-14, a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance.

We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, μ + → e + X , X → γ γ . This search uses a dataset resulting from 7.5 × 10 14 stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013. No significant excess is found in the mass region 20–45 MeV/c 2 for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20–40 MeV/c 2 , down to O ( 10 - 11 ) at 90% confidence level.

Several techniques for the reduced dimensionality of finite element formulations were considered as component mode reduction methods in the middle sixties. These techniques are widely used in flexible multibody simulations for solving small deformation problems. The absolute nodal coordinate formulation for solving large rotation and deformation problems has been established as a full finite element method instead of using similar kinds of reduction techniques. In this paper, a reduced order absolute nodal coordinate formulation is newly established by introducing the global beam shape function and the analytical deformation modes as a full finite element. This formulation leads to a constant and symmetric mass matrix as the conventional absolute nodal coordinate formulation, and makes it possible to reduce the number of elements and system coordinates of the beam structure which undergoes large rotations and large deformations. Numerical examples show that the excellent agreements between the present formulation and the conventional absolute nodal coordinate formulation using a large number of elements are examined. These results demonstrate that the present formulation has high accuracy in the sense that the present solutions are similar to the conventional ones with fewer system coordinates, and high efficiency in computation.

Several techniques for the reduced dimensionality of finite elementformulations were considered as component mode reduction methods in themiddle sixties. These techniques are widely used in flexiblemultibody simulations for solving small deformation problems. Theabsolute nodal coordinate formulation for solving large rotation anddeformation problems has been established as a full finite elementmethod instead of using similar kinds of reduction techniques. In thispaper, a reduced order absolute nodal coordinate formulation is newlyestablished by introducing the global beam shape function and theanalytical deformation modes as a full finite element. This formulationleads to a constant and symmetric mass matrix as the conventionalabsolute nodal coordinate formulation, and makes it possible to reducethe number of elements and system coordinates of the beam structurewhich undergoes large rotations and large deformations. Numericalexamples show that the excellent agreements between thepresent formulation and the conventional absolute nodal coordinateformulation using a large number of elements are examined. These results demonstratethat the present formulation has high accuracy in the sense that thepresent solutions are similar to the conventional ones with fewersystem coordinates, and high efficiency in computation.

We are developing a novel high-brightness atomic beam, comprised of a two-body exotic atom called muonium (M \\( = \\mu^+ + e^-\\)), for next-generation atomic physics and gravitational interaction measurements. This M source originates from a thin sheet of superfluid helium (SFHe), hence diagnostics and later measurements require a detection system which is operational in a dilution cryostat at temperatures below 1 K. In this paper, we describe the operation and characterization of silicon photomultipliers (SiPMs) at ultra-low temperatures in SFHe targets. We show the temperature dependence of the signal shape, breakdown voltage, and single photon detection efficiency, concluding that single photon detection with SiPMs below 0.85 K is feasible. Furthermore, we show the development of segmented scintillation detectors, where 16 channels at 1.7 K and one channel at 170 mK were commissioned using a muon beam.

We present the first demonstration of simultaneous phase space compression in two spatial dimensions of a positive muon beam, the first stage of the novel high-brightness muon beam under development by the muCool collaboration at the Paul Scherrer Institute. The keV-energy, sub-mm size beam would enable a factor 10\\(^5\\) improvement in brightness for precision muSR, and atomic and particle physics measurements with positive muons. This compression is achieved within a cryogenic helium gas target with a strong density gradient, placed in a homogeneous magnetic field, under the influence of a complex electric field. In the next phase, the muon beam will be extracted into vacuum.