[en] The J-PARC 3 GeV Rapid Cycling Synchrotron (RCS) delivers a 1-MW, high-intensity beam to facilities downstream. In such high-intensity accelerators, the operational beam intensity is limited to keep worker exposure to the residual dose within acceptable tolerances. Therefore, we continue to pursue accelerator commissioning that reduces beam loss. In order to achieve further high-intensity operation, the J-PARC accelerator system has been drastically upgraded over the past two years. As a result, it was found that beam loss decreased, whereas output power increased; the residual doses were kept at the same level or decreased in RCS. A malfunction of a collimator occurred in April 2016, and we replaced it with a spare duct in a hurry. The broken collimator was higher activated, but exposure to workers was kept within the acceptable level. (author)

[en] The decay lifetime of free neutrons (∼ 880 s) is an important parameter of the weak interaction and for Big Bang Nucleosynthesis. However, results of measurements currently show discrepancies depending on the method used. As most experiments nowadays employ ultra cold neutrons, we have developed a new cold-beam experiment which we have performed at the Japan Proton Accelerator Research Complex. As a special feature, a polarized neutron beam is bunched by a spin flip chopper. A time projection chamber operated with He and CO₂ gas, including a well-controlled amount of ³He, is used for detection of the beta-decays and simultaneous determination of the beam intensity. Using the data between 2014 and 2016, we evaluated our first, preliminary result of the neutron lifetime as [896 ± 10(stat.)_-10⁺¹⁴ (syst)] seconds. We plan several upgrades to achieve our precision goal of 1 s. (authors)

[en] We searched for the $CP$-violating rare decay of the neutral kaon, $K_L\to {\pi }^0\nu \overline{\nu }$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm 0.16$ background events were expected. We set an upper limit of $5.1\times {10}^{-8}$ for the branching fraction at the 90% confidence level (C.L.). An upper limit of $3.7\times {10}^{-8}$ at the 90% C.L. for the $K_L\to {\pi }^0{X}^0$ decay was also set for the first time, where $X^0$ is an invisible particle with a mass of $135\text{MeV}/c^2$.

[en] A theoretical investigation is done to clarify the origin of the peak structure observed near the $K^{-}pp$ threshold in the in-flight ${}^3\text{He}(K^{-},\Lambda p)n$ reaction of the J-PARC E15 experiment, which could be a signal of the lightest kaonic nuclei, i.e., the $\overline{K}NN(I=1/2)$ state. For the investigation, we evaluate the $\Lambda p$ invariant mass spectrum assuming two possible scenarios to interpret the experimental peak. One assumes that the $\Lambda (1405)$ resonance is generated after the emission of an energetic neutron from the absorption of the initial $K^{-}$, not forming a bound state with the remaining proton. This uncorrelated $\Lambda (1405)p$ system subsequently decays into the final $\Lambda p$. The other scenario implies that, after the emission of the energetic neutron, a $\overline{K}NN$ bound state is formed, decaying eventually into a $\Lambda p$ pair. Our results show that the experimental signal observed in the in-flight ${}^3\text{He}(K^{-},\Lambda p)n$ reaction at J-PARC is qualitatively well reproduced by the assumption that a $\overline{K}NN$ bound state is generated in the reaction, definitely discarding the interpretation in terms of an uncorrelated $\Lambda (1405)p$ state.

[en] In J-PARC MR, beam collimator system is prepared in the injection straight in order to localize the beam loss. The collimator system started with three collimator units of scraper-catcher method. However, the loss capacity and beam instability problem required the system change to the one-pass collimation method. The response of collimators to the beam loss distribution is complicated because the scattered particles circulate in the ring. The responses on the beam loss distribution by each collimator are summarized in this report. (author)

No abstract available

[en] This report presents the conceptual design of a new experiment, COMET (COherent Muon to Electron Transition), to search for coherent neutrino-less conversion of muons to electrons (μ^--e^- conversion), in the presence of a nucleus, μ^- + N(A, Z) → e^- + N(A, Z), with a single event sensitivity B(μ^-N → e^-N) < 10^-16. Flavor transitions between the charged leptons (Charged Lepton Flavor Violation, cLFV) have great potential to reveal new physics phenomena beyond the Standard Model. Many models of such physics, such as those based on supersymmetric grand unification (SUSY-GUT), supersymmetric seesaws (SUSY-Seesaw) and extra dimensions, require the existence of cLFV that is accessible to realizable future experiments. Muons provide the best laboratory to study cLFV as they can be produced in substantial numbers and have a sufficiently long lifetime for precise measurements of their decays. Present accelerators can produce about 10¹⁵ muons/year, and it is anticipated that it will be possible to produce 10¹⁸ -10¹⁹ muons/year with a new high intensity source that is proposed in conjunction with the main J-PARC proton synchrotron ring (MR). The main processes which are accessible for cLFV are the decays μ → eγ, μ → eee and μ^--e^- conversion. Of these, μ^--e^- conversion has the best sensitivity to new physics, as the potentially measurable branching ratio of about 10^-16 is significantly better than the limits expected with current technology for the μ → eγ, μ → eee processes. Within the Standard Model, modified to take account of neutrino oscillation, the expected rate is less than 10^-50 and so any observation of a μ^--e^- conversion would be a clear signal of physics beyond the Standard Model. A measurement ≤ 10^-16 for μ^--e^- conversion, which is the COMET goal, is a factor of 10,000 better than that of current experiments. This report present a new experiment (COMET) of searching for coherent neutrino-less μ^--e^- conversion in a muonic atom of aluminum, μ^- + Al → e^- + Al, at a sensitivity of B(μ^-Al → e^-Al) < 10^-16 at J-PARC. COMET will use a bunched proton beam, slow-extracted from the J-PARC MR. Beam bunching will be necessary to reject beam-related backgrounds. The experimental setup consists of high magnetic-field solenoids for pion capture, C-shaped curved solenoids with momentum selection, and a C-shaped curved solenoid spectrometer. The experiment will require about 2×10¹⁸ stopping muons, which, with the expected transmission of muons in our muon beamline, will require about 8.5×10²⁰ protons of 8 GeV on target. This new initiative has been taken to achieve an early and timely start of a series of searches for μ^--e^- conversion. We consider that the proposed experiment would have large opportunity for great discovery and J-PARC is the best proton facility to carry out this important experiment. (author)

[en] In the fast extraction (FX) operation of J-PARC Main Ring (MR), the delivered beam power has been increased from 240 kW to 360 kW since October, 2014. The keys of the beam commissioning were to distinguish the beam loss sources and to take steps to meet the sources. Both of the hardware and software improvements for these keys are discussed. (author)

[en] Stepper motors are used for mechanical drive in collimators and wire scanner monitors of J-PARC accelerators. Many of these drive unit hardware have been used since the beginning of J-PARC construction, and it is necessary to take measures against aging deterioration. Therefore, we started updating the motor and control system from around 2017. However, when the stepper motor was updated to the current product in the RCS H0 collimator, a malfunction occurred. This is because the drive unit control system cannot correctly recognize the state of the LS (limit switch) due to the noise generated by the motor driver, which hinders the operation. When the noise generated from the old and new stepper motors was measured in a simple test environment for confirmation, it was found that the current product was clearly larger. As a countermeasure, when the wiring of the stepper motor, which was bundled in a single multi-core cable, was separated into separate cables for the power system and LS signal system, the noise level was reduced to about 1/10 and normal operation was restored. I was able to. In this case, we report on noise countermeasures for the RCS H0 collimator drive unit. (author)

[en] The effects of pre-strain on the mechanical properties of high-strength martensitic steels were investigated using either strain tempering (ST) or quenching and tempering (QT) samples. In the tensile tests at deformation temperatures between 296 and 573 K, the ST sample exhibited an increase in both the tensile strength (TS) and uniform elongation (U.El) at 473 to 523 K, whereas the QT sample showed an increase in U.El with little change in the TS and yield strength (YS). The results of in situ neutron diffraction experiments revealed an increase in the stress partitioning to the bcc phase with an increase in the deformation temperature from 296 to 523 K. The difference in the phase stress between the bcc and cementite phases decreased with an increase in the temperature due to the decrease in the cementite strength. Pre-strain of 0.5% increased the YS at 296 K with a slight work hardening; the initial dislocation density (ρ) decreased at 523 K, but increased significantly after yielding, leading to a better combination of TS and U.El. The combination of pre-strain, tempering, and deformation temperatures caused the change in ρ before deformation and the increase in ρ after yielding of the martensitic steel. (author)