arXiv subjects

M. Kobayashi

Publications and source records attributed to M. Kobayashi.

At least 91 records · Page 5Linked to original sources

arXiv2020

Search for Light Dark Matter Interactions Enhanced by the Migdal effect or Bremsstrahlung in XENON1T

Direct dark matter detection experiments based on a liquid xenon target are leading the search for dark matter particles with masses above $\sim$ 5 GeV/c$^2$, but have limited sensitivity to lighter masses because of the small momentum transfer in dark matter-nucleus elastic scattering. However, there is an irreducible contribution from inelastic processes accompanying the elastic scattering, which leads to the excitation and ionization of the recoiling atom (the Migdal effect) or the emission of a Bremsstrahlung photon. In this letter, we report on a probe of low-mass dark matter with masses down to about 85 MeV/c$^2$ by looking for electronic recoils induced by the Migdal effect and Bremsstrahlung, using data from the XENON1T experiment. Besides the approach of detecting both scintillation and ionization signals, we exploit an approach that uses ionization signals only, which allows for a lower detection threshold. This analysis significantly enhances the sensitivity of XENON1T to light dark matter previously beyond its reach.

hep-ex↗

arXiv2020

Search for exotic neutrino-electron interactions using solar neutrinos in XMASS-I

We have searched for exotic neutrino-electron interactions that could be produced by a neutrino millicharge, by a neutrino magnetic moment, or by dark photons using solar neutrinos in the XMASS-I liquid xenon detector. We observed no significant signals in 711 days of data. We obtain an upper limit for neutrino millicharge of 5.4$\times$10$^{-12} e$ at 90\% confidence level assuming all three species of neutrino have common millicharge. We also set flavor dependent limits assuming the respective neutrino flavor is the only one carrying a millicharge, $7.3 \times 10^{-12} e$ for $ν_e$, $1.1 \times 10^{-11} e$ for $ν_μ$, and $1.1 \times 10^{-11} e$ for $ν_τ$. These limits are the most stringent yet obtained from direct measurements. We also obtain an upper limit for the neutrino magnetic moment of 1.8$\times$10$^{-10}$ Bohr magnetons. In addition, we obtain upper limits for the coupling constant of dark photons in the $U(1)_{B-L}$ model of 1.3$\times$10$^{-6}$ if the dark photon mass is 1$\times 10^{-3}$ MeV$/c^{2}$, and 8.8$\times$10$^{-5}$ if it is 10 MeV$/c^{2}$.

hep-ex↗

arXiv2020

Performance evaluation of the aerogel RICH counter for the Belle II spectrometer using early beam collision data

The Aerogel Ring Imaging Cherenkov (ARICH) counter serves as a particle identification device in the forward end-cap region of the Belle II spectrometer. It is capable of identifying pions and kaons with momenta up to $4 \, {\rm GeV}/c$ by detecting Cherenkov photons emitted in the silica aerogel radiator. After the detector alignment and calibration of the probability density function, we evaluate the performance of the ARICH counter using early beam collision data. Event samples of $D^{\ast +} \to D^0 π^+ (D^0 \to K^-π^+)$ were used to determine the $π(K)$ efficiency and the $K(π)$ misidentification probability. We found that the ARICH counter is capable of separating kaons from pions with an identification efficiency of $93.5 \pm 0.6 \, \%$ at a pion misidentification probability of $10.9 \pm 0.9 \, \%$. This paper describes the identification method of the counter and the evaluation of the performance during its early operation.

physics.ins-det↗

arXiv2020

Tailoring Magnetism in Self-intercalated Cr1+δTe2 Epitaxial Films

Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates to host novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability, characterized by Curie temperature (Tc) remains to be achieved in such films. Here we present self-intercalated epitaxial Cr1+δTe2 films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular beam epitaxy (MBE) based technique, we have realized epitaxial Cr1+δTe2 films with smoothly tunable over a wide range (0.33-0.82), while maintaining NiAs-type crystal structure. With increasing δ, we found monotonic enhancement of Tc from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with δ. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with δ, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. To our knowledge, this constitutes the first demonstration of tunable Tc and magnetic anisotropy across room temperature in TMD films, and paves the way for engineering novel magnetic phases for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Light Dark Matter Search with Ionization Signals in XENON1T

We report constraints on light dark matter (DM) models using ionization signals in the XENON1T experiment. We mitigate backgrounds with strong event selections, rather than requiring a scintillation signal, leaving an effective exposure of $(22 \pm 3)$ tonne-days. Above $\sim\!0.4\,\mathrm{keV}_\mathrm{ee}$, we observe $<1 \, \text{event}/(\text{tonne} \times \text{day} \times \text{keV}_\text{ee})$, which is more than one thousand times lower than in similar searches with other detectors. Despite observing a higher rate at lower energies, no DM or CEvNS detection may be claimed because we cannot model all of our backgrounds. We thus exclude new regions in the parameter spaces for DM-nucleus scattering for DM masses $m_χ$ within $3-6\,\mathrm{GeV}/\mathrm{c}^2$, DM-electron scattering for $m_χ> 30\,\mathrm{MeV}/\mathrm{c}^2$, and absorption of dark photons and axion-like particles for $m_χ$ within $0.186 - 1 \, \mathrm{keV}/\mathrm{c}^2$.

hep-ex↗

arXiv2019

Structure of 13Be probed via quasi-free scattering

We present an investigation of the structure of 13Be obtained via a kinematically complete measurement of the (p; pn) reaction in inverse kinematics at 265 MeV/nucleon. The relative energy spectrum of 13Be is compared to Transfer-to-the-Continuum calculations which use as structure inputs the overlaps of the 14Be ground-state wave function, computed in a three-body model, with the unbound states of the 13Be residual nucleus. The key role of neutron p-wave orbital in the interpretation of the low-relative-energy part of the spectrum is discussed.

nucl-ex↗

arXiv2019

The XENON1T Data Acquisition System

The XENON1T liquid xenon time projection chamber is the most sensitive detector built to date for the measurement of direct interactions of weakly interacting massive particles with normal matter. The data acquisition system (DAQ) is constructed from commercial, open source, and custom components to digitize signals from the detector and store them for later analysis. The system achieves an extremely low signal threshold below a tenth of a photoelectron using a parallelized readout with the global trigger deferred to a later, software stage. The event identification is based on MongoDB database queries and has over 97% efficiency at recognizing interactions at the analysis energy threshold. A readout bandwidth over 300 MB/s is reached in calibration modes and is further expandable via parallelization. This DAQ system was successfully used during three years of operation of XENON1T.

physics.ins-det↗

arXiv2019

k-Resolved electronic structure of buried heterostructure and impurity systems by soft-X-ray ARPES

Angle-resolved photoelectron spectroscopy (ARPES) is the main experimental tool to explore electronic structure of solids resolved in the electron momentum k . Soft-X-ray ARPES (SX-ARPES), operating in a photon energy range around 1 keV, benefits from enhanced probing depth compared to the conventional VUV-range ARPES, and elemental/chemical state specificity achieved with resonant photoemission. These advantages make SX-ARPES ideally suited for buried heterostructure and impurity systems, which are at the heart of current and future electronics. These applications are illustrated here with a few pioneering results, including buried quantum-well states in semiconductor and oxide heterostructures, their bosonic coupling critically affecting electron transport, magnetic impurities in diluted magnetic semiconductors and topological materials, etc. High photon flux and detection efficiency are crucial for pushing the SX-ARPES experiment to these most photon-hungry cases.

cond-mat.mtrl-sci↗

arXiv2019

Measurement of the electron transmission rate of the gating foil for the TPC of the ILC experiment

We have developed a gating foil for the time projection chamber envisaged as a central tracker for the international linear collider experiment. It has a structure similar to the Gas Electron Multiplier (GEM) with a higher optical aperture ratio and functions as an ion gate without gas amplification. The transmission rate for electrons was measured in a counting mode for a wide range of the voltages applied across the foil using an $^{55}$Fe source and a laser in the absence of a magnetic field. The blocking power of the foil against positive ions was estimated from the electron transmissions.

physics.ins-det↗

arXiv2019

Search for WIMP-$^{129}$Xe inelastic scattering with particle identification in XMASS-I

A search for Weakly Interacting Massive Particles (WIMPs) was conducted with the single-phase liquid-xenon detector XMASS through inelastic scattering in which $^{129}$Xe nuclei were excited, using an exposure ($\rm 327\; kg \times 800.0 \; days$) 48 times larger than that of our previous study. The inelastic excitation sensitivity was improved by detailed evaluation of background, event classification based on scintillation timing that distinguished $γ$-rays and $β$-rays, and simultaneous fitting of the energy spectra of $γ$-like and $β$-like samples. No evidence of a WIMP signal was found. Thus, we set the upper limits of the inelastic channel cross section at 90\% confidence level, for example, $4.1\times 10^{-39} \;{\rm cm^2}$ for a $200\; {\rm GeV}/c^2$ WIMP. This result provides the most stringent limits on the SD WIMP-neutron interaction and is better by a factor of 7.7 at $200\;{\rm GeV}/c^2$ than the existing experimental limit.

astro-ph.CO↗

arXiv2019

Development of low radioactivity photomultiplier tubes for the XMASS-I detector

XMASS-I is a single-phase liquid xenon detector whose purpose is direct detection of dark matter. To achieve the low background requirements necessary in the detector, a new model of photomultiplier tubes (PMTs), R10789, with a hexagonal window was developed based on the R8778 PMT used in the XMASS prototype detector. We screened the numerous component materials for their radioactivity. During development, the largest contributions to the reduction of radioactivity came from the stem and the dynode support. The glass stem was exchanged to the Kovar alloy one and the ceramic support were changed to the quartz one. R10789 is the first model of Hamamatsu Photonics K. K. that adopted these materials for low background purposes and provided a groundbreaking step for further reductions of radioactivity in PMTs. Measurements with germanium detectors showed 1.2$\pm$0.3 mBq/PMT of $^{226}$Ra, less than 0.78 mBq/PMT of $^{228}$Ra, 9.1$\pm$2.2 mBq/PMT of $^{40}$K, and 2.8$\pm$0.2 mBq/PMT of $^{60}$Co. In this paper, the radioactive details of the developed R10789 are described together with our screening methods and the components of the PMT.

physics.ins-det↗

arXiv2019

Effect of Coulomb interaction on the two-dimensional electronic structure of the van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$

In order to investigate the electronic properties of the semiconducting van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$ (CGT), where ferromagnetic layers are bonded through van der Waals forces, we have performed angle-resolved photoemission spectroscopy (ARPES) measurements and density-functional-theory (DFT+U) calculations. The valence-band maximum at the Γ point is located $\sim$ 0.2 eV below the Fermi level, consistent with the semiconducting property of CGT. Comparison of the experimental density of states with the DFT calculation has suggested that Coulomb interaction between the Cr 3d electrons U$_{\rm eff}$ $\sim$ 1.1 eV. The DFT+U calculation indicates that magnetic coupling between Cr atoms within the layer is ferromagnetic if Coulomb U $_{\rm eff}$ is smaller than 3.0 eV and that the inter-layer coupling is ferromagnetic below U$_{\rm eff}$ $\sim$ 1.0 eV. We therefore conclude that, for U$_{\rm eff}$ deduced by the experiment, the intra-layer Cr-Cr coupling is ferromagnetic and the inter-layer coupling is near the boundary between ferromagnetic and antiferromagnetic, which means experimentally deduced U$_{\rm eff}$ is consistent with theoretical ferromagnetic condition.

cond-mat.mtrl-sci↗

arXiv2018

A direct dark matter search in XMASS-I

A search for dark matter using an underground single-phase liquid xenon detector was conducted at the Kamioka Observatory in Japan, particularly for Weakly Interacting Massive Particles (WIMPs). We have used 705.9 live days of data in a fiducial volume containing 97 kg of liquid xenon at the center of the detector. The event rate in the fiducial volume after the data reduction was ${\rm (4.2 \pm 0.2) \times 10^{-3} \, day^{-1}kg^{-1} keV_{ee}^{-1}}$ at ${\rm 5 \, keV_{ee}}$, with a signal efficiency of ${\rm 20\%}$. All the remaining events are consistent with our background evaluation, mostly of the "mis-reconstructed events" originated from $^{210}$Pb in the copper plates lining the detector's inner surface. The obtained upper limit on a spin-independent WIMP-nucleon cross section was ${\rm 2.2 \times 10^{-44} \, cm^{2}}$ for a WIMP mass of ${\rm 60 \, GeV/c^{2}}$ at the $90\%$ confidence level, which was the most stringent limit among results from single-phase liquid xenon detectors.

astro-ph.CO↗

arXiv2018

A measurement of the scintillation decay time constant of nuclear recoils in liquid xenon with the XMASS-I detector

We report an in-situ measurement of the nuclear recoil (NR) scintillation decay time constant in liquid xenon (LXe) using the XMASS-I detector at the Kamioka underground laboratory in Japan. XMASS-I is a large single-phase LXe scintillation detector whose purpose is the direct detection of dark matter via NR which can be induced by collisions between Weakly Interacting Massive Particles (WIMPs) and a xenon nucleus. The inner detector volume contains 832 kg of LXe. $^{252}$Cf was used as an external neutron source for irradiating the detector. The scintillation decay time constant of the resulting neutron induced NR was evaluated by comparing the observed photon detection times with Monte Carlo simulations. Fits to the decay time prefer two decay time components, one for each of the Xe$_{2}^{*}$ singlet and triplet states, with $τ_{S}$ = 4.3$\pm$0.6 ns taken from prior research, $τ_{T}$ was measured to be 26.9$^{+0.7}_{-1.1}$ ns with a singlet state fraction F$_{S}$ of 0.252$^{+0.027}_{-0.019}$.We also evaluated the performance of pulse shape discrimination between NR and electron recoil (ER) with the aim of reducing the electromagnetic background in WIMP searches. For a 50\% NR acceptance, the ER acceptance was 13.7${\pm}$1.0\% and 4.1${\pm}$0.7\% in the energy ranges of 5--10 keV$_{\rm ee}$ and 10--15 keV$_{\rm ee}$, respectively.

physics.ins-det↗

arXiv2018

Search for sub-GeV dark matter by annual modulation using XMASS-I detector

A search for dark matter (DM) with mass in the sub-GeV region (0.32-1 GeV) was conducted by looking for an annual modulation signal in XMASS, a single-phase liquid xenon detector. Inelastic nuclear scattering accompanied by bremsstrahlung emission was used to search down to an electron equivalent energy of 1 keV. The data used had a live time of 2.8 years (3.5 years in calendar time), resulting in a total exposure of 2.38 ton-years. No significant modulation signal was observed and 90% confidence level upper limits of $1.6 \times 10^{-33}$ cm$^2$ at 0.5 GeV was set for the DM-nucleon cross section. This is the first experimental result of a search for DM mediated by the bremsstrahlung effect. In addition, a search for DM with mass in the multi-GeV region (4-20 GeV) was conducted with a lower energy threshold than previous analysis of XMASS. Elastic nuclear scattering was used to search down to a nuclear recoil equivalent energy of 2.3 keV, and upper limits of 2.9 $\times$10$^{-42}$ cm$^2$ at 8 GeV was obtained.

astro-ph.CO↗

arXiv2018

Magnetic Phases of Frustrated Ferromagnetic Spin-Trimer System Gd_3_Ru_4_Al_12_ With a Distorted Kagome Lattice Structure

The magnetization and specific heat measurements have been performed on single-crystalline Gd_3_Ru_4_Al_12_ with a distorted Kagome lattice structure. This spin system is regarded as an antiferromagnetic triangular lattice of XY like Heisenberg model at low temperatures. The magnetic phase diagrams indicate the existence of frustration and Z_2_ degeneracy. The magnetization and specific heat imply the successive phase transitions with partial disorder and a T-shaped spin structure in the ground state.

cond-mat.str-el↗

arXiv2018

First measurement of a long-lived $π^+ π^-$ atom lifetime

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogen-like $π^+π^-$ atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96~mm and got broken up in the 2.1~\textmu{}m thick platinum foil. Analysing the observed number of atomic pairs, $n_A^L= \left.436^{+157}_{-61}\right|_\mathrm{tot}$, the lifetime of the 2$p$ state is found to be ${τ_{2p}=(\left.0.45^{+1.08}_{-0.30}\right|_\mathrm{tot}) \cdot10^{-11}}$s, not contradicting the corresponding QED $2p$ state lifetime ${τ_{2p}^\mathrm{QED}=1.17 \cdot 10^{-11}}$s. This lifetime value is three orders of magnitude larger than our previously measured value of the $π^+π^-$ atom ground state lifetime $τ=(\left.3.15^{+0.28}_{-0.26}\right|_\mathrm{tot})\cdot 10^{-15}$s. Further studies of long-lived $π^+π^-$ atoms will allow to measure energy differences between $p$ and $s$ atomic states and so to determine $ππ$ scattering lengths with the aim to check QCD predictions.

hep-ex↗

arXiv2018

Search for dark matter in the form of hidden photons and axion-like particles in the XMASS detector

Hidden photons and axion-like particles are candidates for cold dark matter if they were produced non-thermally in the early universe. We conducted a search for both of these bosons using 800 live-days of data from the XMASS detector with 327 kg of liquid xenon in the fiducial volume. No significant signal was observed, and thus we set constraints on the $α' / α$ parameter related to kinetic mixing of hidden photons and the coupling constant $g_{Ae}$ of axion-like particles in the mass range from 40 to 120 keV/$c^2$, resulting in $α' / α< 6 \times 10^{-26}$ and $g_{Ae} < 4 \times 10^{-13}$. These limits are the most stringent derived from both direct and indirect searches to date.

astro-ph.CO↗