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H. Sagawa

Publications and source records attributed to H. Sagawa.

At least 73 records · Page 4Linked to original sources
arXiv2018

The Cosmic-Ray Energy Spectrum between 2 PeV and 2 EeV Observed with the TALE detector in monocular mode

We report on a measurement of the cosmic ray energy spectrum by the Telescope Array Low-Energy Extension (TALE) air fluorescence detector. The TALE air fluorescence detector is also sensitive to the Cherenkov light produced by shower particles. Low energy cosmic rays, in the PeV energy range, are detectable by TALE as "Cherenkov Events". Using these events, we measure the energy spectrum from a low energy of $\sim 2$ PeV to an energy greater than 100 PeV. Above 100 PeV TALE can detect cosmic rays using air fluorescence. This allows for the extension of the measurement to energies greater than a few EeV. In this paper, we will describe the detector, explain the technique, and present results from a measurement of the spectrum using $\sim 1000$ hours of observation. The observed spectrum shows a clear steepening near $10^{17.1}$ eV, along with an ankle-like structure at $10^{16.2}$ eV. These features present important constraints on galactic cosmic rays origin and propagation models. The feature at $10^{17.1}$ eV may also mark the end of the galactic cosmic rays flux and the start of the transition to extra-galactic sources.

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arXiv2018

Evidence of Intermediate-Scale Energy Spectrum Anisotropy of Cosmic Rays E$\geq$10$^{19.2}$ eV with the Telescope Array Surface Detector

An intermediate-scale energy spectrum anisotropy has been found in the arrival directions of ultra-high energy cosmic rays of energies above $10^{19.2}$ eV in the northern hemisphere, using 7 years of data from the Telescope Array surface detector. A relative energy distribution test is done comparing events inside oversampled spherical caps of equal exposure, to those outside, using the Poisson likelihood ratio. The center of maximum significance is at $9^h$$16^m$, $45^{\circ}$. and has a deficit of events with energies $10^{19.2}$$\leq$$E$$<$$10^{19.75}$ eV and an excess for $E$$\geq$$10^{19.75}$ eV. The post-trial probability of this energy anisotropy, appearing by chance anywhere on an isotropic sky, is found by Monte Carlo simulation to be $9$$\times$$10^{-5}$ ($3.74$$σ_{global}$).

astro-ph.HE↗
arXiv2018

Stationary waves and slowly moving features in the night upper clouds of Venus

At the cloud top level of Venus (65-70 km altitude) the atmosphere rotates 60 times faster than the underlying surface, a phenomenon known as superrotation. Whereas on Venus's dayside the cloud top motions are well determined and Venus general circulation models predict a mean zonal flow at the upper clouds similar on both day and nightside, the nightside circulation remains poorly studied except for the polar region. Here we report global measurements of the nightside circulation at the upper cloud level. We tracked individual features in thermal emission images at 3.8 and 5.0 $\mathrm{μm}$ obtained between 2006 and 2008 by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) onboard Venus Express and in 2015 by ground-based measurements with the Medium-Resolution 0.8-5.5 Micron Spectrograph and Imager (SpeX) at the National Aeronautics and Space Administration Infrared Telescope Facility (NASA/IRTF). The zonal motions range from -110 to -60 m s$^{-1}$, consistent with those found for the dayside but with larger dispersion. Slow motions (-50 to -20 m s$^{-1}$) were also found and remain unexplained. In addition, abundant stationary wave patterns with zonal speeds from -10 to +10 m s$^{-1}$ dominate the night upper clouds and concentrate over the regions of higher surface elevation.

astro-ph.EP↗
arXiv2018

Isoscalar and Isovector spin response in $sd-$ shell nuclei

The spin magnetic dipole transitions and the neutron-proton spin-spin correlations in $sd-$shell even-even nuclei with $N=Z$ are investigated using shell model wave functions taking into accout enhanced isoscalar (IS) spin-triplet pairing as well as the effective spin operators. It was shown that the IS pairing and the effective spin operators gives a large quenching effect on the IV spin transitions to be consistent with observed data by $(p,p')$ experiments. On the other hand, the observed IS spin strength show much smaller quenching effect than expected by the calculated results. The IS pairing gives a substantial quenching effect on the spin magnetic dipole transitions, especially on the isovector (IV) ones. Consequently, an enhanced isoscalar spin-triplet pairing interaction enlarges the proton-neutron spin-spin correlation deduced from the difference between the isoscalar (IS) and the IV sum rule strengths. The beta-decay rates and the IS magnetic moments of $sd-$shell are also examined in terms of the IS pairing as well as the effective spin operators.

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arXiv2018

Beyond the neutron-drip line: two-neutron decay of unbound nuclei

We discuss a decay of unbound nuclei beyond the neutron drip-line using a three-body model with a core nucleus and two valence neutrons. We particularly discuss the role of dineutron correlation between the valence neutrons in the two-neutron emission from the ground state of $^{26}$O and $^{10}$He nuclei. Our calculations clearly indicate that the emission of the two neutrons in the back-to-back direction is enhanced due to the dineutron correlation.

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arXiv2017

Interplay between isoscalar and isovector correlations in neutron-rich nuclei

The interplay between isoscalar and isovector correlations in the 1$^{-}$ states in neutron-rich (N$\neq$Z) even-even nuclei is studied, taking examples of the nuclei, $^{22}_{8}$O$_{14}$ and $^{24}_{8}$O$_{16}$. The excitation modes explored are isovector dipole and isoscalar compression dipole modes. The self-consistent Hartree-Fock plus the random-phase approximation with the Skyrme interaction, SLy4, is solved in coordinate space so as to take properly into account the continuum effect. The isovector peak induced by isoscalar correlation, the isoscalar peak induced by isovector correlation, and the possible collective states made by both isoscalar and isovector correlations, ("iS-iV pigmy resonance"), are shown. The strong neutron-proton interaction in nuclei can be responsible for controlling the isospin structure of normal modes. It is explicitly shown that in the scattering by isoscalar (isovector) particles on N$\neq$Z even-even nuclei isovector (isoscalar) strength in addition to isoscalar (isovector) strength may be populated.

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arXiv2017

Role of deformation in odd-even staggering in reaction cross sections for $^{30,31,32}$Ne and $^{36,37,38}$Mg isotopes

We discuss the role of pairing anti-halo effect in the observed odd-even staggering in reaction cross sections for $^{30,31,32}$Ne and $^{36,37,38}$Mg isotopes by taking into account the ground state deformation of these nuclei. To this end, we construct the ground state density for the $^{30,31}$Ne and $^{36,37}$Mg nuclei based on a deformed Woods-Saxon potential, while for the $^{32}$Ne and $^{38}$Mg nuclei we also take into account the pairing correlation using the Hartree-Fock-Bogoliubov method. We demonstrate that, when the one-neutron separation energy is small for the odd-mass nuclei, a significant odd-even staggering still appears even with finite deformation, although the degree of staggering is somewhat reduced compared to the spherical case. This implies that the pairing anti-halo effect in general plays an important role in generating the odd-even staggering in reaction cross sections for weakly bound nuclei.

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arXiv2017

Search for Anisotropy in the Ultra High Energy Cosmic Ray Spectrum using the Telescope Array Surface Detector

The Telescope Array (TA) experiment is located in the western desert of Utah, USA, and observes ultra high energy cosmic rays (UHECRs) in the Northern hemisphere. At the highest energies, $E>10$~EeV, the shape of cosmic ray energy spectrum may carry an imprint of the source density distribution along the line of sight different in different directions of the sky. In this study, we search for such directional variations in the shape of the energy spectrum using events observed with the Telescope Array's surface detector. We divide the TA field of view into two nearly equal-exposure regions: the "on-source" region which we define as $\pm 30^\circ$ of the supergalactic plane containing mostly nearby structures, and the complementary "off-source" region where the sources are further away on average. We compare the UHECR spectra in these regions by fitting them to the broken power law and comparing the resulting parameters. We find that the off-source spectrum has an earlier break at highest energies. The chance probability to obtain such or larger difference in statistically equivalent distributions is estimated as $6.2\pm1.1\times10^{-4}$ ($3.2σ$) by a Monte-Carlo simulation. The observed difference in spectra is in a reasonable quantitative agreement with a simplified model that assumes that the UHECR sources trace the galaxy distribution from the 2MRS catalogue, primary particles are protons and the magnetic deflections can be neglected.

astro-ph.HE↗
arXiv2016

Sum rule study for Double Gamow-Teller states

We study the sum rules of double Gamow-Teller (DGT) excitations through double spin-isospin operator $(σt_-)^2$. In general, $2^+$ states in the granddaughter nuclei have dominant transition strength in DGT excitations and $0^+$ states are weak, except in $T=1$ mother nuclei in which $0^+$ strength is competitive with 2$^+$ strength. The isospin selection of DGT is also discussed among 5 possible isospin states in granddaughter nuclei. A possibility to extract the unit cross section for the DGT transition strength is pointed out in the $(σt_-)^2$ excitation of double isobaric analog state (DIAS) in $T=1$ nuclei.

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arXiv2016

Mean field approaches for $Ξ^-$ hypernuclei and current experimental data

Motivated by the recently observed hypernucleus (Kiso event) $^{15}_Ξ$C ($^{14}$N$+Ξ^-$), we identify the state of this system theoretically within the framework of the relativistic-mean-field and Skyrme-Hartree-Fock models. The $ΞN$ interactions are constructed to reproduce the two possibly observed $Ξ^-$ removal energies, $4.38\pm 0.25$ MeV or $1.11\pm 0.25$ MeV. The present result is preferable to be $^{14}{\rm N}({\rm g.s.})+Ξ^-(1p)$, corresponding to the latter value.

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arXiv2016

Search for EeV Protons of Galactic Origin

Cosmic rays in the energy range $10^{18.0}$ - $10^{18.5}$ eV are thought to have a light, probably protonic, composition. To study their origin one can search for anisotropy in their arrival directions. Extragalactic cosmic rays should be isotropic, but galactic cosmic rays of this type should be seen mostly along the galactic plane, and there should be a shortage of events coming from directions near the galactic anticenter. This is due to the fact that, under the influence of the galactic magnetic field, the transition from ballistic to diffusive behavior is well advanced, and this qualitative picture persists over the whole energy range. Guided by models of the galactic magnetic field that indicate that the enhancement along the galactic plane should have a standard deviation of about 20$^\circ$ in galactic latitude, and the deficit in the galactic anticenter direction should have a standard deviation of about 50$^\circ$ in galactic longitude, we use the data of the Telescope Array surface detector in $10^{18.0}$ to $10^{18.5}$ eV energy range to search for these effects. The data are isotropic. Neither an enhancement along the galactic plane nor a deficit in the galactic anticenter direction is found. Using these data we place an upper limit on the fraction of EeV cosmic rays of galactic origin at 1.3% at 95% confidence level.

astro-ph.HE↗
arXiv2016

Effect of isoscalar spin-triplet pairings on spin-isospin responses in $sd-$ shell nuclei

The spin magnetic dipole transitions and the neutron-proton spin-spin correlations in $sd-$shell even-even nuclei with $N=Z$ are investigated using shell model wave functions. The isoscalar spin-triplet pairing correlation provides a substantial quenching effect on the spin magnetic dipole transitions, especially the isovector (IV) ones. Consequently, an enhanced isoscalar spin-triplet pairing interaction influences the proton-neutron spin-spin correlation deduced from the difference between the isoscalar (IS) and the IV sum rule strengths. The effect of the $Δ$ ($Δ_{33}$ resonance)-hole coupling is examined in the IV spin transition and the spin-spin correlations of the ground states.

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arXiv2016

Quasi-particle random phase approximation with quasi-particle-vibration coupling: application to the Gamow-Teller response of the superfluid nucleus $^{120}$Sn

We propose a self-consistent quasi-particle random phase approximation (QRPA) plus quasi-particle-vibration coupling (QPVC) model with Skyrme interactions to describe the width and the line shape of giant resonances in open-shell nuclei, in which the effect of superfluidity should be taken into account in both the ground state and the excited states. We apply the new model to the Gamow-Teller resonance in the superfluid nucleus $^{120}$Sn, including both the isoscalar spin-triplet and the isovector spin-singlet pairing interactions. The strength distribution in $^{120}$Sn is well reproduced and the underlying microscopic mechanisms, related to QPVC and also to isoscalar pairing, are analyzed in detail.

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arXiv2016

New concept for pairing anti-halo effect as a localized wave packet of quasi-particle

The pairing anti-halo effect is a phenomenon that a pairing correlation suppresses a divergence of nuclear radius, which happens for single-particle states with orbital angular momenta of $l$ = 0 and 1 in the limit of vanishing binding energy. While this effect has mainly been discussed in terms of Hartree-Fock-Bogoliubov (HFB) theory, we here use a three-body model and provide its new intuitive concept as a localized wave packet for a quasi-particle, that is, a coherent superposition of a weakly bound and continuum wave functions due to a pairing interaction. We show that the one-particle density in the three-body model can be directly expressed with such quasi-particle wave functions, which have a close analog to wave functions in the HFB approximation.

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arXiv2016

First Upper Limits on the Radar Cross Section of Cosmic-Ray Induced Extensive Air Showers

TARA (Telescope Array Radar) is a cosmic ray radar detection experiment colocated with Telescope Array, the conventional surface scintillation detector (SD) and fluorescence telescope detector (FD) near Delta, Utah, U.S.A. The TARA detector combines a 40 kW, 54.1 MHz VHF transmitter and high-gain transmitting antenna which broadcasts the radar carrier over the SD array and within the FD field of view, towards a 250 MS/s DAQ receiver. TARA has been collecting data since 2013 with the primary goal of observing the radar signatures of extensive air showers (EAS). Simulations indicate that echoes are expected to be short in duration (~10 microseconds) and exhibit rapidly changing frequency, with rates on the order of 1 MHz/microsecond. The EAS radar cross-section (RCS) is currently unknown although it is the subject of over 70 years of speculation. A novel signal search technique is described in which the expected radar echo of a particular air shower is used as a matched filter template and compared to waveforms obtained by triggering the radar DAQ using the Telescope Array fluorescence detector. No evidence for the scattering of radio frequency radiation by EAS is obtained to date. We report the first quantitative RCS upper limits using EAS that triggered the Telescope Array Fluorescence Detector.

astro-ph.IM↗
arXiv2016

Decay dynamics of the unbound $^{25}$O and $^{26}$O nuclei

We study the ground and excited resonance states of $^{26}$O with a three-body model of $^{24}$O+n+n taking into account the coupling to the continuum. To this end, we use the new experimental data for the invariant mass spectroscopy of the unbound $^{25}$O and $^{26}$O nuclei, and present an update of three-body model calculations for the two-neutron decay of the $^{26}$O nucleus. With the new model inputs determined with the ground state decay of $^{26}$O, we discuss the di-neurtron correlations and a halo nature of this nucleus, as well as the structure of the excited states. For the energy of the 2$^+$ state, we achieve an excellent agreement with the experimental data with this calculation. We show that the 2$^+$ state consists predominantly of the $(d_{3/2})^2$ configuration, for which the pairing interaction between the valence neutrons slightly decreases its energy from the unperturbed one. We also discuss the structure of excited 0$^+$ states of the $^{26}$O nucleus. In particular, we show the existence of an excited 0$^+$ state at 3.38 MeV, which is mainly composed of the $(f_{7/2})^2$ configuration.

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arXiv2015

Isovector spin-singlet (T=1, S=0) and isoscalar spin-triplet (T=0, S=1) pairing interactions and spin-isospin response

We review several experimental and theoretical advances that emphasise common aspects of the study of T=1 and T=0 pairing correlations in nuclei. We first discuss several empirical evidences of the special role played by the T=1 pairing interaction. In particular, we show the peculiar features of the nuclear pairing interaction in the low density regime, and possible outcomes such as the BCS-BEC crossover in nuclear matter and, in an analogous way, in loosely bound nuclei. We then move to the competition between T=1 and T=0 pairing correlations. The effect of such competition on the low-lying spectra is studied in N=Z odd-odd nuclei by using a three-body model; it is shown that the inversion of the 0+ and 1+ states near the ground state, and the strong magnetic dipole transitions between them, can be considered as a clear manifestation of strong T=0 pairing correlations in these nuclei. The effect of T=0 pairing correlations is also quite evident if one studies charge-changing transitions. The Gamow-Teller (GT) states in N=Z+2 nuclei are studied here by using self-consistent HFB+QRPA calculations in which the T=0 pairing interaction is taken into account. Strong GT states are found, near the ground state of daughter nuclei; these are compared with available experimental data from charge-exchange reactions, and such comparison can pinpoint the value of the strength of the T=0 interaction. Pair transfer reactions are eventually discussed: while two-neutron transfer has been long proposed as a tool to measure the T=1 superfluidity in the nuclear ground states, the study of deuteron transfer is still in its infancy, despite its potential interest in revealing effects coming from both T=1 and T=0 interactions.

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