Abstract
This paper introduces a new approach to measure the muon magnetic moment anomaly $a_{\mu} = (g-2)/2$ and the muon electric dipole moment (EDM) $d_{\mu}$ at the J-PARC muon facility. The goal ...of our experiment is to measure $a_{\mu}$ and $d_{\mu}$ using an independent method with a factor of 10 lower muon momentum, and a factor of 20 smaller diameter storage-ring solenoid compared with previous and ongoing muon $g-2$ experiments with unprecedented quality of the storage magnetic field. Additional significant differences from the present experimental method include a factor of 1000 smaller transverse emittance of the muon beam (reaccelerated thermal muon beam), its efficient vertical injection into the solenoid, and tracking each decay positron from muon decay to obtain its momentum vector. The precision goal for $a_{\mu}$ is a statistical uncertainty of 450 parts per billion (ppb), similar to the present experimental uncertainty, and a systematic uncertainty less than 70 ppb. The goal for EDM is a sensitivity of $1.5\times 10^{-21}~e\cdot\mbox{cm}$.
A hydrogen-like atom consisting of a positive muon and an electron is known as muonium. It is a near-ideal two-body system for a precision test of bound-state theory and fundamental symmetries. The ...MuSEUM collaboration performed a new precision measurement of the muonium ground-state hyperfine structure at J-PARC using a high-intensity pulsed muon beam and a high-rate capable positron counter. The resonance of hyperfine transition was successfully observed at a near-zero magnetic field, and the muonium hyperfine structure interval of νHFS=4.463302(4)GHz was obtained with a relative precision of 0.9 ppm. The result was consistent with the previous ones obtained at Los Alamos National Laboratory and the current theoretical calculation. We present a demonstration of the microwave spectroscopy of muonium for future experiments to achieve the highest precision.
High precision measurements of the ground state hyperfine structure (HFS) of muonium is a stringent tool for testing bound-state quantum electrodynamics (QED) theory, determining fundamental ...constants of the muon magnetic moment and mass, and searches for new physics. Muonium is the most suitable system to test QED because both theoretical and experimental values can be precisely determined. Previous measurements were performed decades ago at LAMPF with uncertainties mostly dominated by statistical errors. At the J-PARC Muon Science Facility (MUSE), the MuSEUM collaboration is planning complementary measurements of muonium HFS both at zero and high magnetic field. The new high-intensity muon beam that will soon be available at H-Line will provide an opportunity to improve the precision of these measurements by one order of magnitude. An overview of the different aspects of these new muonium HFS measurements, the current status of the preparation for high-field measurements, and the latest results at zero field are presented.
Recently a few groups reported existence of Earth's background free oscillations even on seismically quiet days. Observed features suggest that they are persistently excited on or just above the ...Earth's surface owing most likely to atmospheric and/or oceanic disturbances. To constrain their excitation mechanisms we developed a new method for estimation of spatial distribution of their excitation sources by modeling cross spectra between pairs of stations. The method is to calculate synthetic cross spectra for spatially homogeneous distribution of random sources and invert with them the observed cross spectra to the heterogeneous source distribution. We applied this method to the IRIS records at 54 stations during 1988–2000. The result showed clear temporal variations of spatial patterns. From November to April the spatial distribution shows a degree 1 pattern with the maximum in the North Pacific Ocean. From May to October strong excitation sources are located along the eastern and western Pacific rims through the Indian Ocean. In all the time periods, excitation sources on continents are weaker than in oceanic areas. This temporal variation of the spatial pattern is qualitatively consistent with that reported by Rhie and Romanowicz (2004). However, the excitation sources are not localized in shallow seas, as might be expected from the hypothesis of excitation by ocean infragravity waves, but must be distributed on the whole sea surface.
The South Pacific region is characterized by a broadly elevated seafloor known as the South Pacific superswell. This region has a concentration of midplate volcanoes that experienced massive ...eruptions in the mid‐Cretaceous period (90–120 Ma). These characteristics suggest the presence of a large‐scale mantle plume beneath the South Pacific, called the South Pacific superplume. The geometry, origin depth, temperature, and composition of the superplume remain controversial, however, mainly due to the lack of seismological data that documents the mantle structure beneath the South Pacific. Seismic stations are sparse in the area due to its remote ocean environment. To obtain a better seismic image of the superplume, we deployed temporary broadband seismographs on oceanic islands and the seafloor in the South Pacific, which made possible the highest spatial resolution that has ever been achieved for the mantle structure beneath the region. The seismic image obtained from this new seismic data indicates that large‐scale low‐velocity anomalies (on the order of 1000 km in diameter), indicative of the superplume, are located from the bottom of the mantle to a depth of 1000 km, and small‐scale low‐velocity anomalies (on the order of 100 km in diameter) are present above it. A comparison of the seismic image with recent mantle convection studies based upon laboratory and numerical experiments suggests that the superplume may be a hot and chemically distinct mantle dome, and that the small‐scale anomalies may be narrow plumes generated from the top of the dome. This model may explain various characteristics of hot spots in the South Pacific, such as the seafloor swell, short‐lived hot spot chains, and the periodicity of massive eruptions.
Muon acceleration is an important technique in exploring the new frontier of physics. A new measurement of the muon dipole moments is planned in J-PARC using the muon linear accelerator. The ...low-energy (LE) muon source using the thin metal foil target and beam diagnostic system were developed for the world's first muon acceleration. Negative muonium ions from the thin metal foil target as the LE muon source was successfully observed. Also the beam profile of the LE positive muon was measured by the LE-dedicated beam profile monitor. The muon acceleration test using a Radio-Frequency Quadrupole linac (RFQ) is being prepared as the first step of the muon accelerator development. In this paper, the latest status of the first muon acceleration test is described.
Our P-wave whole mantle tomography revealed a low velocity region oceanward of the Northern Honshu slab of the Pacific plate at depths around the 410-km seismic discontinuity. Resolution tests and ...scrutiny of the traveltime residuals for the ray paths passing through the low velocity region indicate that this anomaly is a resolvable feature and not an artifact due to the strong slab anomalies. The existence of the slow anomalies is also supported by the analysis of the P-wave records from the J-array (a large-aperture seismic array in Japan) for a Bonin earthquake. The P arrivals to Northern Honshu (at epicentral distances of 13–20°) are strongly triplicated because of the 410-km discontinuity. The later arrivals along the retrograde branch, where ray paths pass through the low velocity region, are anomalously slow. Comparison of the observed and synthetic waveforms indicates not only slow anomalies but also depression of the 410-km discontinuity. This depression represents the direct evidence for the low velocity zone of primarily thermal origin. An excess temperature of 200 K and the associated fractional melt of less than 1% can explain both the results of the tomographic and waveform analyses.
The invariant mass spectra of e+e- pairs produced in 12 GeV proton-induced nuclear reactions are measured at the KEK Proton Synchrotron. On the low-mass side of the meson peak, a significant ...enhancement over the known hadronic sources has been observed. The mass spectra, including the excess, are well reproduced by a model that takes into account the density dependence of the vector meson mass modification, as theoretically predicted.
Barotropic tidal currents over bottom topography force density surfaces to oscillate vertically and thereby to act as quasi‐stationary internal tide sources. Deploying a seafloor pressure gauge array ...with an aperture of 30 km for a year (2014–2015), we detected the low‐mode semidiurnal internal tidal waves propagating with a horizontal phase speed of ~1 m/s in the onshore and offshore directions over the array along the eastern slope of Aogashima Island, south of Japan. The amplitudes of the offshore propagating waves were greater than those of the onshore propagating waves, and both were positively correlated with the amplitudes of the local semidiurnal tide, which peaked in September and March. A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed onshore and offshore internal tidal wave propagation. The model indicated a standing wave region on the slope, where offshore propagating waves interact with standing waves locally pinned to the slope. Along the same profile over a distance of 100 km, we conducted seismic‐oceanographic analysis of the legacy multichannel seismic reflection data to retrieve vertical cross sections of the reflecting layers, which indicated sharp temperature changes in the ocean. Many of the slant reflecting layers were subparallel to the contour lines of the semidiurnal internal‐tide‐associated temperature anomalies in the JCOPE‐T model, suggesting a causal link between the fine reflection layering structure and the semidiurnal low‐mode internal tidal field.
Plain Language Summary
Internal tidal waves are generated in the ocean by interactions between surface tides and the bottom topography. Their amplitudes are less than a few centimeters at the sea surface but can be more than a few tens of meters within the ocean. These waves play important roles in tidal dissipation, ocean mixing, hydroacoustic transmission, and submersible technology. Thus far, observations of internal tides have mainly been made from above by satellite altimetry and through ocean columns by cable mooring. In this report, we show that a pressure gauge array at the bottom of the ocean is a promising tool for detecting internal tides and constraining their generation mechanisms. We incorporated a tide‐resolving ocean circulation model that could simulate bottom pressure variations at the same locations in the same period as that during which the observations were made so that the same analyses could be performed for both the observed and simulated data. Remarkable agreement was found between the observed and simulated internal tidal signals. Concurrent simulation of ocean circulation with tides is useful for interpreting not only the array records of ocean bottom pressure gauges but also vertical cross‐sectional images of ocean acoustic reflections obtained using a seismic–oceanographic technique.
Key Points
We detected partly standing semidiurnal internal tides on the eastern slope of the Izu‐Bonin ridge using a seafloor pressure gauge array
A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed nature of internal tides, including seasonal variations
The temperature anomaly contours of the modeled internal tides tended to be subparallel to the acoustically detected reflecting layers
A negative muonium ion (Mu−) source using an aluminum foil target (Al target) was developed as a low-energy muon source.Mu−ions are produced by irradiating the Al target with a 3-MeV positive muon ...(μ+) beam and observed using a microchannel plate. An experiment to produceMu−ions was conducted to evaluate the performance of thisMu−ion source. The measured event rate ofMu−ions was(1.7±0.3)×10−3Mu−/swhen the event rate of the incidentμ+beam was1.3×106/s. The experiment was conducted at the Muon Science Establishment, D-line in the Materials and Life Science Experimental Facility within the Japan Proton Accelerator Research Complex. The formation probability, defined as the ratio of theMu−ions to the incident muons on the Al target, was(1.1±0.2(stat)+0.1−0.0(syst))×10−6. This Mu−ion source was first adopted in the commissioning of the muon accelerator at the D-line, and the event rate of the acceleratedMu−ions was consistent with the expectation. ThisMu−ion source boosted the development of the muon accelerator, and the practicality of this low-energy muon source obtained using a relatively simple apparatus was demonstrated.
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