arXiv subjects

Y. Ishikawa

Publications and source records attributed to Y. Ishikawa.

18 recordsLinked to original sources

arXiv2026

A cosmic collision in the making: JWST/NIRISS reveals a merging and maturing protocluster core at z~3 around a red quasar

We report the discovery of ``The Step'', a dense protocluster at z=3 revealed by JWST/NIRISS Wide-Field Slitless Spectroscopy (WFSS) of the field around the luminous quasar and starburst SDSSJ165202.64+172852.3. The protocluster contains at least 18 member galaxies and exhibits two substructures in the 3D position-velocity space - notably a distinctive step-like distribution, from which the nickname derives. We propose that the Step may be undergoing a merger of two haloes, which could also explain the large velocity span (700-850km/s) between galaxies along the densest line of sight. This sightline contains seven galaxies within 70 projected kpc from the quasar, consistent with a remarkably dense protocluster core. We derive a galaxy overdensity of a factor >100 in the core and >15 within 1 Mpc$^2$, making it one of the densest structures known at this redshift, and a very massive halo of $13.1 < log M_{\rm halo}/ M_\odot < 13.8$, which suggests the structure could potentially evolve into a massive, Coma-like cluster at z=0. Compared to the field, the star-forming galaxies in the Step show slightly elevated levels of star formation. Yet, we find two quiescent galaxies with strong D4000 breaks and no evidence for ongoing star formation. Compared to other z=3 protoclusters, the Step exhibits similar or slightly suppressed star-forming activity. We also identify two new AGN candidates and find tentative evidence for an enhanced AGN fraction (10-20%) compared to the field. The protocluster constitutes one of the emerging handful of maturing structures at 2 < z < 4 that host both star-forming and quenched galaxies. This redshift regime may thus be a key transitional epoch for cluster studies, where overdensities transition from being the most active sites of star formation in the early Universe towards shaping the massive passive ellipticals seen in the cores of today's clusters.

astro-ph.GA↗

arXiv2025

Measurement of $Λ$ Polarization in the $π^{-}p \to K^{0} Λ$ Reaction at $p_{π^{-}}=1.33$ GeV/$c$ toward a New $Λp$ Scattering Experiment

This paper presents high-precision experimental data of the polarization of the $Λ$ hyperon in the $π^{-}p \to K^{0} Λ$ reaction, measured in the angular range $0.6<\cos θ^{CM}_{K0}<1.0$ with a fine bin width of $d\cos θ^{CM}_{K0}=0.05$. The data were obtained from the J-PARC E40 experiment at the K1.8 beamline in the J-PARC Hadron Experimental Facility. The observed average polarization of $Λ$ in the range $0.60<\cos θ^{CM}_{K0}<0.85$ was $0.932 \pm 0.058 \,(\text{stat}) \pm 0.028 \,(\text{syst})$, demonstrating the successful extraction of precise polarization observables. This result provides essential experimental input for partial wave analysis (PWA) of dynamical coupled-channel (DCC) models, which aim to uncover the underlying mechanisms of $N^{*}$ resonances that emerge in intermediate states of $πN$ and $γN$ interactions. Besides, it indicates the feasibility of a strongly polarized $Λ$ beam suitable for future $Λp$ scattering experiments (e.g., J-PARC E86).

nucl-ex↗

arXiv2023

Anti-deuteron beam study at J-PARC HEF K1.8 beam line

We performed a $\overline{d}$ beam study at the K1.8 beam line of J-PARC Hadron Experimental Facility. 1.8 GeV/$c$ $\overline{d}$ beam yield was measured to be 0.30 $\pm$ 0.04 counts/spill for 30 GeV 70 $\times 10^{12}$ protons/spill irradiated on a 66 mm thick of gold target with the vertical slit opening widths of 2.2 mm, 5 mm and 5 mm for intermediate focus (IFV), mass slit 1 (MS1) and 2 (MS2), respectively. Corresponding $\overline{p}$ beam yield is roughly estimated to be $\sim$ 0.3 Mcounts/spill for the same slit condition. Then, the $\overline{d}/\overline{p}$ production ratio at extraction angle of 6 degrees is estimated to be $\sim 10^{-6}$. This is the first time measurement of the $\overline{d}$ beam yield and $\overline{d}/\overline{p}$ production ratio at J-PARC. Further beam line tuning may increase the $\overline{d}$ beam yield.

nucl-ex↗

arXiv2023

First results from the JWST Early Release Science Program Q3D: Benchmark Comparison of Optical and Mid-IR Tracers of a Dusty, Ionized Red Quasar Wind at z=0.435

The [OIII] 5007 A emission line is the most common tracer of warm, ionized outflows in active galactic nuclei across cosmic time. JWST newly allows us to use mid-infrared spectral features at both high spatial and spectral resolution to probe these same winds. Here we present a comparison of ground-based, seeing-limited [OIII] and space-based, diffraction-limited [SIV] 10.51 micron maps of the powerful, kiloparsec-scale outflow in the Type 1 red quasar SDSS J110648.32+480712.3. The JWST data are from the Mid-InfraRed Instrument (MIRI). There is a close match in resolution between the datasets (0."6), in ionization potential of the O$^{+2}$ and S$^{+3}$ ions (35 eV), and in line sensitivity (1e-17 to 2e-17 erg/s/cm$^2$/arcsec$^2$). The [OIII] and [SIV] line shapes match in velocity and linewidth over much of the 20 kpc outflowing nebula, and [SIV] is the brightest line in the rest-frame 3.5-19.5 micron range, demonstrating its usefulness as a mid-IR probe of quasar outflows. [OIII] is nevertheless intriniscally brighter and provides better contrast with the point-source continuum, which is strong in the mid-IR. There is a strong anticorrelation of [OIII]/[SIV] with average velocity, which is consistent with a scenario of differential obscuration between the approaching (blueshifted) and receding (redshifted) sides of the flow. The dust in the wind may also obscure the central quasar, consistent with models that attribute red quasar extinction to dusty winds.

astro-ph.GA↗

arXiv2023

Electrical detection of antiferromagnetic dynamics in Gd-Co thin films by using a 154-GHz gyrotron irradiation

THz magnetization dynamics is a key property of antiferromagnets as well as ferrimagnets that could harness the THz forefront and spintronics. While most of the present THz measurement techniques are for bulk materials whose sensitivities rely on the volume of the material, measurement techniques suitable for thin films are quite limited. In this study, we explored and demonstrated electrical detection of the antiferromagnetic dynamics in ferrimagnetic Gd-Co thin films by using a 154 GHz gyrotron, a high-power electromagnetic wave source. Captured resonant modes allow us to characterize the peculiar magnetization dynamics of the Gd-Co around the net angular momentum compensation. As the gyrotron frequency is scalable up to THz, our demonstration can be an important milestone toward the THz measurements for antiferro- and ferri- magnetic thin films.

cond-mat.mtrl-sci↗

arXiv2022

Measurement of differential cross sections for $Σ^+p$ elastic scattering in the momentum range 0.44-0.80 GeV/c

We performed a novel $Σ^+ p$ scattering experiment at the J-PARC Hadron Experimental Facility. Approximately 2400 $Σ^+ p$ elastic scattering events were identified from $4.9 \times 10^7$ tagged $Σ^+$ particles in the $Σ^+$ momentum range 0.44 -- 0.80 GeV/$c$. The differential cross sections of the $Σ^+ p$ elastic scattering were derived with much better precision than in previous experiments. The obtained differential cross sections were approximately 2 mb/sr or less, which were not as large as those predicted by the fss2 and FSS models based on the quark cluster model in the short-range region. By performing phase-shift analyses for the obtained differential cross sections, we experimentally derived the phase shifts of the $^3 S_1$ and $^1 P_1$ channels for the first time. The phase shift of the $^3 S_1$ channel, where a large repulsive core was predicted owing to the Pauli effect between quarks, was evaluated as $20^\circ<|δ_{^3S_1}|<35^\circ$. If the sign of $δ_{^3S_1}$ is assumed to be negative, the interaction in this channel is moderately repulsive, as the Nijmegen extended-sort-core models predicted.

nucl-ex↗

arXiv2022

Precise measurement of differential cross sections of the Σ-p --> Λ n reaction in momentum range 470-650 MeV/c

The differential cross sections of the Σ-p --> Λ n reaction were measured accurately for the Σ- momentum (p_{Σ}) ranging from 470 to 650 MeV/c at the J-PARC Hadron Experimental Facility. Precise angular information about the Σ-p --> Λ n reaction was obtained for the first time by detecting approximately 100 reaction events at each angular step of Δcosθ = 0.1. The obtained differential cross sections show slightly forward-peaking structure in the measured momentum regions.The cross sections integrated for -0.7 < cosθ < 1.0 were obtained as 22.5 +- 0.68 (stat.) +- 0.65 (syst.) mb and 15.8 +-0.83(stat.) +- 0.52 (syst.) mb for 470<p_{Σ}(MeV/c)<550 and 550<p_{Σ}(MeV/c)<650, respectively. These results show a drastic improvement compared to past measurements of the hyperon-proton scattering experiments. They will play essential roles in updating the theoretical models of the baryon-baryon interactions.

nucl-ex↗

arXiv2021

Measurement of the differential cross sections of the Sigma-p elastic scattering in momentum range of 470 to 850 MeV/c

A high statistics $Σp$ scattering experiment has been performed at the J-PARC Hadron Experimental Facility. Data for momentum-tagged $Σ^{-}$ running in a liquid hydrogen target were accumulated by detecting the $π^{-}p \to K^{+}Σ^{-}$ reaction with a high intensity $π^{-}$ beam of 20 M/spill. Differential cross sections of the $Σ^{-}p$ elastic scattering were derived with a drastically improved accuracy by identifying the largest statistics of about 4,500 events from 1.72 $\times$ $10^{7}$ $Σ^{-}$. The derived differential cross section shows a clear forward-peaking angular distribution for a $Σ^{-}$ momentum range from 470 to 850 MeV/$c$. The accurate data will impose a strong constraint on the theoretical models of the baryon-baryon interactions.

nucl-ex↗

arXiv2021

First observation of a nuclear $s$-state of $Ξ$ hypernucleus, $^{15}_Ξ{\rm C}$

Bound-systems of $Ξ^-$--$^{14}_{}{\rm N}$ are studied via $Ξ^-$ capture at rest followed by emission of a twin single-$Λ$ hypernucleus in the emulsion detectors. Two events forming extremely deep $Ξ^-$ bound states were obtained by analysis of a hybrid method in the E07 experiment at J-PARC and reanalysis of the E373 experiment at KEK-PS. The decay mode of one event was assigned as $Ξ^-+^{14}_{}{\rm N}\to^{5}_Λ{\rm He}$+$^{5}_Λ{\rm He}$+$^{4}_{}{\rm He}$+n. Since there are no excited states for daughter particles, the binding energy of the $Ξ^-$ hyperon, $B_{Ξ^-}$, in $^{14}_{}{\rm N}$ nucleus was uniquely determined to be 6.27 $\pm$ 0.27 MeV. Another $Ξ^-$--$^{14}_{}{\rm N}$ system via the decay $^{9}_Λ{\rm Be}$ + $^{5}_Λ{\rm He}$ + n brings a $B_{Ξ^-}$ value, 8.00 $\pm$ 0.77 MeV or 4.96 $\pm$ 0.77 MeV, where the two possible values of $B_{Ξ^-}$ correspond to the ground and the excited states of the daughter $^{9}_Λ{\rm Be}$ nucleus, respectively. Because the $B_{Ξ^-}$ values are larger than those of the previously reported events (KISO and IBUKI), which are both interpreted as the nuclear $1p$ state of the $Ξ^-$--$^{14}_{}{\rm N}$ system, these new events give the first indication of the nuclear $1s$ state of the $Ξ$ hypernucleus, $^{15}_Ξ{\rm C}$.

nucl-ex↗

arXiv2021

Observation of Coulomb-assisted nuclear bound state of $Ξ^-$-$^{14}$N system

In an emulsion-counter hybrid experiment performed at J-PARC, a $Ξ^-$ absorption event was observed which decayed into twin single-$Λ$ hypernuclei. Kinematic calculations enabled a unique identification of the reaction process as $Ξ^{-} + ^{14}$N$\ \rightarrow\ ^{10}_Λ$Be + $^5_Λ$He. For the binding energy of the $Ξ^{-}$ hyperon in the $Ξ^-$-$^{14}$N system a value of $1.27 \pm 0.21$ MeV was deduced. The energy level of $Ξ^-$ is likely a nuclear $1p$ state which indicates a weak $ΞN$-$ΛΛ$ coupling.

nucl-ex↗

arXiv2020

SuperBoRG: Exploration of point sources at $z\sim8$ in HST parallel fields

To extend the search for quasars in the epoch of reionization beyond the tip of the luminosity function, we explore point source candidates at redshift $z\sim8$ in SuperBoRG, a compilation of $\sim$0.4deg$^2$ archival medium-deep ($m_{\rm F160W}\sim 26.5$ABmag, 5$σ$) parallel IR images taken with the Hubble Space Telescope (HST). Initial candidates are selected by using the Lyman-break technique. We then carefully analyze source morphology, and robustly identify 3 point sources at $z\sim8$. Photometric redshift analysis reveals that they are preferentially fit by extra-galactic templates, and we conclude that they are unlikely to be low-$z$ interlopers, including brown dwarfs. A clear IRAC ch2 flux excess is seen in one of the point sources, which is expected if the source has strong H$β$+[O III] emission with rest-frame equivalent width of $\sim3000$AA. Deep spectroscopic data taken with Keck/MOSFIRE, however, do not reveal Ly$α$ emission from the object. In combination with the estimated H$β$+[O III] equivalent width, we place an upper limit on its Ly$α$ escape fraction $f_{\rm esc, Lyα}< 2 \%$. We estimate the number density of these point sources $\sim1\times10^{-6}$Mpc$^{-3}$mag$^{-1}$ at $M_{\rm UV}\sim-23$mag. The final interpretation of our results remains inconclusive: extrapolation from low-$z$ studies of $faint$ quasars suggests that $>100\times$ survey volume may be required to find one of this luminosity. The James Webb Space Telescope will be able to conclusively determine the nature of our luminous point source candidates, while the Roman Space Telescope will probe $\sim 200$ times the area of the sky with the same observing time considered in this HST study.

astro-ph.GA↗

arXiv2019

Dynamic nuclear polarization and ESR hole burning in As doped silicon

We present an experimental study of the Dynamic Nuclear Polarization (DNP) of \si{} nuclei in silicon crystals of natural abundance doped with As in the temperature range 0.1-1 K and in strong magnetic field of 4.6 T. This ensures very high degree of electron spin polarization, extremely slow nuclear relaxation and optimal conditions for realization of Overhauser and resolved solid effects. We found that the solid effect DNP leads to an appearance of a pattern of holes and peaks in the ESR line, separated by the super-hyperfine interaction between the donor electron and \si{} nuclei closest to the donor. On the contrary, the Overhauser effect DNP mainly affects the remote \si{} nuclei having the weakest interaction with the donor electron. This leads to an appearance of a very narrow ($\approx$ 3 mG wide) hole in the ESR line. We studied relaxation of the holes after burning, which is caused by the nuclear spin diffusion. Analyzing the spin diffusion data with a simple one-dimensional spectral diffusion model leads to a value of the spectral diffusion coefficient $D=8(3)\times 10^{-3}$ mG$^2$/s. Our data indicate that the spin diffusion is not completely prevented even in the frozen core near the donors. The emergence of the narrow hole after the Overhauser DNP may be explained by a partial "softening" of the frozen core caused by Rabi oscillations of the electron spin.

cond-mat.mes-hall↗

arXiv2019

Observation of a Be double-Lambda hypernucleus in the J-PARC E07 experiment

A double-$Λ$ hypernucleus, ${}_{ΛΛ}\mathrm{Be}$, was observed by the J-PARC E07 collaboration in nuclear emulsions tagged by the $(K^{-},K^{+})$ reaction. This event was interpreted as a production and decay of $ {}_{ΛΛ}^{\;10}\mathrm{Be}$, ${}_{ΛΛ}^{\;11}\mathrm{Be}$, or ${}_{ΛΛ}^{\;12}\mathrm{Be}^{*}$ via $Ξ^{-}$ capture in ${}^{16}\mathrm{O}$. By assuming the capture in the atomic 3D state, the binding energy of two $Λ$ hyperons$\,$($B_{ΛΛ}$) of these double-$Λ$ hypernuclei are obtained to be $15.05 \pm 0.11\,\mathrm{MeV}$, $19.07 \pm 0.11\,\mathrm{MeV}$, and $13.68 \pm 0.11\,\mathrm{MeV}$, respectively. Based on the kinematic fitting, ${}_{ΛΛ}^{\;11}\mathrm{Be}$ is the most likely explanation for the observed event.

nucl-ex↗

arXiv2018

Quantum paramagnet near spin-state transition

Spin-state transition, also known as spin crossover, plays a key role in diverse systems, including minerals and biological materials. In theory, the boundary range between the low- and high-spin states is expected to enrich the transition and give rise to unusual physical states. However, no compound that realizes a nearly degenerate critical range as the ground state without requiring special external conditions has yet been experimentally identified. This study reports that, by comprehensive measurements of macroscopic physical properties, X-ray diffractometry, and neutron spectroscopy, the Sc substitution in LaCoO$_3$ destabilizes its nonmagnetic low-spin state and generates an anomalous paramagnetic state accompanied by the enhancement of transport gap and magneto-lattice-expansion as well as the contraction of Co--O distance with the increase of electron site-transfer. These phenomena are not well described by the mixture of conventional low- and high-spin states, but by their quantum superposition occurring on the verge of a spin-state transition. The present study enables us to significantly accelerate the design of new advanced materials without requiring special equipment based on the concept of quantum spin-state criticality.

cond-mat.str-el↗

arXiv2006

Recent progress of avalanche photodiodes in high-resolution X-rays and Gamma-rays detection

We have studied the performance of large area avalanche photodiodes (APDs) recently developed by Hamamatsu Photonics K.K, in high-resolution X-rays and Gamma-rays detections. We show that reach-through APD can be an excellent soft X-ray detector operating at room temperature or moderately cooled environment. We obtain the best energy resolution ever achieved with APDs, 6.4 % for 5.9 keV X-rays, and obtain the energy threshold as low as 0.5 keV measured at -20deg. Thanks to its fast timing response, signal carriers in the APD device are collected within a short time interval of 1.9 nsec (FWHM). This type of APDs can therefore be used as a low-energy, high-counting particle monitor onboard the forthcoming Pico-satellite Cute1.7. As a scintillation photon detector, reverse-type APDs have a good advantage of reducing the dark noise significantly. The best FWHM energy resolutions of 9.4+-0.3 % and 4.9+-0.2 % were obtained for 59.5 keV and 662 keV Gamma-rays, respectively, as measured with a CsI(Tl) crystal. Combination of APDs with various other scintillators (BGO, GSO, and YAP) also showed better results than that obtained with a photomultiplier tube (PMT). These results suggest that APD could be a promising device for replacing traditional PMT usage in some applications. In particular 2-dim APD array, which we present in this paper, will be a promising device for a wide-band X-ray and Gamma-ray imaging detector in future space research and nuclear medicine.

astro-ph↗

arXiv2006

An Active Gain-control System for Avalanche Photo-Diodes under Moderate Temperature Variations

Avalanche photodiodes (APDs) are promising light sensor for various fields of experimental physics. It has been argued, however, that variation of APD gain with temperature could be a serious problem preventing APDs from replacing traditional photomultiplier tubes (PMTs) in some applications. Here we develop an active gain-control system to keep the APD gain stable under moderate temperature variations. As a performance demonstration of the proposed system, we have tested the response of a scintillation photon detector consisting of a 5x5 mm^2 reverse-type APD optically coupled with a CsI(Tl) crystal. We show that the APD gain was successfully controlled under a temperature variation of DT = 20deg, within a time-cycle of 6000 sec. The best FWHM energy resolution of 6.1+-0.2 % was obtained for 662 keV gamma-rays, and the energy threshold was as low as 6.5 keV, by integrating data from +20deg - 0deg cycles. The corresponding values for -20deg - 0deg cycles were 6.9+-0.2 % and 5.2 keV, respectively. These results are comparable, or only slightly worse than that obtained at a fixed temperature. Our results suggest new potential uses for APDs in various space researches and nuclear physics. As examples, we briefly introduce the NeXT and Cute-1.7 satellite missions that will carry the APDs as scientific instruments for the first time.

astro-ph↗

arXiv1999

Orbital Magnetism and Current Distribution of Two-Dimensional Electrons under Confining Potential

The spatial distribution of electric current under magnetic field and the resultant orbital magnetism have been studied for two-dimensional electrons under a harmonic confining potential $V(\vecvar{r})=m ω_0^2 r^2/2$ in various regimes of temperature and magnetic field, and the microscopic conditions for the validity of Landau diamagnetism are clarified. Under a weak magnetic field $(ω_c\lsimω_0, ω_c$ being a cyclotron frequency) and at low temperature $(T\lsim\hbarω_0)$, where the orbital magnetic moment fluctuates as a function of the field, the currents are irregularly distributed paramagnetically or diamagnetically inside the bulk region. As the temperature is raised under such a weak field, however, the currents in the bulk region are immediately reduced and finally there only remains the diamagnetic current flowing along the edge. At the same time, the usual Landau diamagnetism results for the total magnetic moment. The origin of this dramatic temperature dependence is seen to be in the multiple reflection of electron waves by the boundary confining potential, which becomes important once the coherence length of electrons gets longer than the system length. Under a stronger field $(ω_c\gsimω_0)$, on the other hand, the currents in the bulk region cause de Haas-van Alphen effect at low temperature as $T\lsim\hbarω_c$. As the temperature gets higher $(T\gsim\hbarω_c)$ under such a strong field, the bulk currents are reduced and the Landau diamagnetism by the edge current is recovered.

cond-mat.mes-hall↗

arXiv1998

Effects of Magnetic Field on Josephson Current in SNS System

The effect of a magnetic field on Josephson current has been studied for a superconductor/normal-metal/superconductor (SNS) system, where N is a two-dimensional electron gas in a confining potential. It is found that the dependence of Josephson currents on the magnetic field are sensitive to the width of the normal metal. If the normal metal is wide and contains many channels (subbands), the current on a weak magnetic field shows a dependence similar to a Fraunhofer-pattern in SIS system and, as the field gets strong, it shows another type of oscillatory dependence on the field resulting from the Aharonov-Bohm interference between the edge states. As the number of channels decreases (i.e. normal metal gets narrower), however, the dependence in the region of the weak field deviates from a clear Fraunhofer pattern and the amplitude of the oscillatory dependence in the region of the strong field is reduced.

cond-mat.mes-hall↗