Kazunori Kohri

The Graduate University for Advanced Studies, Миура, Kanagawa, Japan

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Publications (133)524.45 Total impact

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    ABSTRACT: We explain the excess of the antiproton fraction recently reported by the AMS-02 experiment by considering collisions between cosmic-ray protons accelerated by a local supernova remnant (SNR) and the surrounding dense cloud. The same "pp collisions" provide the right branching ratio to fit the observed positron excess simultaneously without a fine tuning. The supernova happened in relatively lower metalicity than the major cosmic-ray sources. The cutoff energy of electrons marks the supernova age of ~10^{5} years, while the antiproton excess may extend to higher energy. Both antiproton and positron fluxes are completely consistent with our predictions in Fujita, Kohri, Yamazaki and Ioka (2009).
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    Kazunori Kohri, Chia-Min Lin, Tomohiro Matsuda
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    ABSTRACT: At the beginning of inflation, when the vacuum energy starts to dominate, there could be many dynamical fields in the Universe. At the same time, velocity of the inflaton may not coincide with the slow-roll (attractor) velocity. Although these additional degrees of freedom may neither enhance nor suppress the curvature perturbation, they can easily alter the scale-dependence of the spectrum. Therefore, if the perturbations exit horizon during the early stage of inflation where these effects are still not negligible, one might observe peculiar scale dependence in the spectrum. We show that the effect can be measured using the running of the tensor mode.
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    Carsten Rott, Kazunori Kohri, Seong Chan Park
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    ABSTRACT: Superheavy dark matter may show its presence in high energy neutrino signals detected on earth. From the latest results of IceCube, we could set the strongest lower bound on the lifetime of dark matter beyond 100 TeV around $10^{28} {\rm sec}$. The excess around a PeV is noticed and may be interpreted as the first signal of DM even though further confirmation and dedicated searches are invited.
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    ABSTRACT: We show that the Li problems can be solved in the next-to-minimal supersymmetric standard model where the slepton as the next-to-lightest SUSY particle is very long-lived. Such a long-lived slepton induces exotic nuclear reactions in big-bang nucleosynthesis, and destroys and produces the 7Li and 6Li nuclei via bound state formation. We study cases where the lightest SUSY particle is singlino-like neutralino and bino-like neutralino to present allowed regions in the parameter space which is consistent with the observations on the dark matter and the Higgs mass.
    Physical Review D 08/2014; 90(3):035003. DOI:10.1103/PhysRevD.90.035003 · 4.86 Impact Factor
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    Kazunori Kohri, Tomohiro Matsuda
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    ABSTRACT: At the beginning of inflation there could be extra dynamical scalar fields that will soon disappear (become static) before the end of inflation. In the light of multi-field inflation, those extra degrees of freedom may not change the prediction of the original spectrum of the curvature perturbation. One can remove such fields introducing extra number of e-foldings prior to $N_e\sim 60$, however such extra e-foldings may make the trans-Planckian problem worse due to the Lyth bound. We show that such extra scalar fields can change the running of the spectral index to give correction of $\pm 0.01$ without modifying other cosmological parameters. The ambiguity in the running of the spectral index, which could be generated by such additional dynamical fields, can be used to nullify the tension between BICEP2 and other experiments.
    Journal of Cosmology and Astroparticle Physics 05/2014; 2015(02). DOI:10.1088/1475-7516/2015/02/019 · 5.88 Impact Factor
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    Kazunori Kohri, Tomohiro Nakama, Teruaki Suyama
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    ABSTRACT: Supermassive black holes and intermediate mass black holes are believed to exist in the Universe. There is no established astrophysical explanation for their origin and considerations have been made in the literature that those massive black holes (MBHs) may be primordial black holes (PBHs), black holes which are formed in the early universe (well before the matter-radiation equality) due to the direct collapse of primordial overdensities. This paper aims at discussing the possibility of excluding the PBH scenario as the origin of the MBHs. We first revisit the constraints on PBHs obtained from the CMB distortion that the seed density perturbation causes. By adopting a recent computation of the CMB distortion sourced by the seed density perturbation and the stronger constraint on the CMB distortion set by the COBE/FIRAS experiment used in the literature, we find that PBHs in the mass range $6\times 10^4~M_\odot \sim 5 \times 10^{13}~M_\odot$ are excluded. Since PBHs lighter than $6 \times 10^4~M_\odot$ are not excluded from the non-observation of the CMB distortion, we propose a new method which can potentially exclude smaller PBHs as well. Based on the observation that large density perturbations required to create PBHs also result in the copious production of ultracompact minihalos (UCMHs), compact dark matter halos formed at around the recombination, we show that weakly interacting massive particles (WIMPs) as dark matter annihilate efficiently inside UCMHs to yield cosmic rays far exceeding the observed flux. Our bound gives severe restriction on the compatibility between the particle physics models for WIMPs and the PBH scenario as the explanation of MBHs.
    Physical Review D 05/2014; 90(8). DOI:10.1103/PhysRevD.90.083514 · 4.86 Impact Factor
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    Yohei Kikuta, Kazunori Kohri, Eunseong So
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    ABSTRACT: We discuss possibilities to observe stochastic gravitational wave backgrounds produced by the electroweak phase transition in the early universe. Once the first-order phase transition occurs, which is still predicted in a lot of theories beyond the standard model, collisions of nucleated vacuum bubbles and induced turbulent motions can become significant sources of the gravitational waves. Detections of such gravitational wave backgrounds are expected to reveal the Higgs sector physics. In particular, through pulsar timing experiments planned in Square Kilometre Array (SKA) under construction, we will be able to detect the gravitational wave in near future and distinguish particle physics models by comparing the theoretical predictions to the observations.
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    Kazunori Kohri, C. S. Lim, Chia-Min Lin
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    ABSTRACT: In this paper, we carefully calculated the tensor-to-scalar ratio, the running spectral index, and the running of running spectrum for (extra) natural inflation in order to compare with recent BICEP2 data, PLANCK satellite data and future 21 cm data. We discovered that the prediction for running spectral index and the running of running spectrum in natural inflation is different from that in the case of extra natural inflation. Near future observation for the running spectral index may achieve enough accuracy to allow us distinguishing between extra natural inflation from natural inflation. Distinguishing the models by using the running of running spectrum is not impossible but would be more challenging for future experiments.
    Journal of Cosmology and Astroparticle Physics 05/2014; 2014(08). DOI:10.1088/1475-7516/2014/08/001 · 5.88 Impact Factor
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    ABSTRACT: We investigate the issue of how accurately we can constrain the lepton number asymmetry xi_nu = mu_nu/T_nu in the Universe by using future observations of 21 cm line fluctuations and cosmic microwave background (CMB). We find that combinations of the 21 cm line and the CMB observations can constrain the lepton asymmetry better than big-bang nucleosynthesis (BBN). Additionally, we also discuss constraints on xi_nu in the presence of some extra radiation, and show that the 21 cm line observations can substantially improve the constraints obtained by CMB alone, and allow us to distinguish the effects of the lepton asymmetry from the ones of extra radiation.
    Journal of Cosmology and Astroparticle Physics 04/2014; 2014(09). DOI:10.1088/1475-7516/2014/09/014 · 5.88 Impact Factor
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    ABSTRACT: We propose helium-4 spallation processes induced by long-lived stau in supersymmetric standard models, and investigate an impact of the processes on light elements abundances. We show that, as long as the phase space of helium-4 spallation processes is open, they are more important than stau-catalyzed fusion and hence constrain the stau property. This talk is based on the work of ref. [1].
    Journal of Physics Conference Series 03/2014; 485(1). DOI:10.1088/1742-6596/485/1/012020
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    ABSTRACT: Because the production cross sections of γγ rays, electrons, and positrons (e±) made in p-p collisions, σpp→γσpp→γ and σpp→e±σpp→e±, respectively, are kinematically equivalent with respect to the parent pion-production cross section σpp→πσpp→π, we obtain σpp→e±σpp→e± directly from the machine data on σpp→γσpp→γ. In Paper I (Sato et al. [1]), we give explicitly σpp→γσpp→γ, reproducing quite well the accelerator data with LHC, namely σpp→e±σpp→e± is applicable enough over the wide energy range from GeV to 20 PeV for projectile proton energy. We dicuss in detail the relation between the cross sections, and present explicitly σpp→e±σpp→e± that are valid into the PeV electron energy.
    Astroparticle Physics 03/2014; DOI:10.1016/j.astropartphys.2014.01.001 · 4.45 Impact Factor
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    ABSTRACT: We report on spatially resolved X-ray spectroscopy of the north-eastern part of the mixed morphology supernova remnant (SNR) W28 with {\it XMM-Newton}. The observed field of view includes a prominent and twisted shell emission forming the edge of this SNR as well as part of the center-filled X-ray emission brightening toward the south-west edge of the field of view. The shell region spectra are in general represented by an optically thin thermal plasma emission in collisional ionization equilibrium with a temperature of $\sim$0.3~keV and a density of $\sim$10~cm$^{-3}$, which is much higher than the density obtained for inner parts. In contrast, we detected no significant X-ray flux from one of the TeV $\gamma$-ray peaks with an upper-limit flux of 2.1$\times$10$^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the 2--10~keV band. The large flux ratio of TeV to X-ray, larger than 16, and the spatial coincidence of the molecular cloud and the TeV $\gamma$-ray emission site indicate that the TeV $\gamma$-ray of W28 is $\pi^{0}$-decay emission originating from collisions between accelerated protons and molecular cloud protons. Comparing the spectrum in the TeV band and the X-ray upper limit, we obtained a weak upper limit on the magnetic field strength $B\lesssim$ 1500~$\mu$G.
    Publications- Astronomical Society of Japan 02/2014; 66(3). DOI:10.1093/pasj/psu031 · 2.01 Impact Factor
  • Tomohiro Harada, Chul-Moon Yoo, Kazunori Kohri
    Physical Review D 12/2013; 89(2). DOI:10.1103/PhysRevD.89.029903 · 4.86 Impact Factor
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    ABSTRACT: We apply the axion-photon conversion mechanism to the 130GeV gamma-ray line observed by Fermi satellite. Mono-energetic axions or axion-like particles (ALPs) can be produced through annihilation or decay processes of dark matter. Then the axion converts to the gamma-ray in Galactic magnetic fields along its flight to the Earth, which can explain the observations. This mechanism suppresses productions of any continuum components of gamma-rays and cosmic-ray anti-protons, which is severe constraints to the models trying to explain the observation from unknown new particles. In our mechanism, the gamma-ray spatial distribution depends on both the dark matter profile and the magnetic field configuration, which will be tested by future gamma-ray observations, e.g., through HESS II, CTA, GAMMA-400. As an example, we discuss possible scenarios for the 130GeV axion emissions in supersymmetric axion models.
    Physical Review D 10/2013; 91(6). DOI:10.1103/PhysRevD.91.063532 · 4.86 Impact Factor
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    ABSTRACT: In this paper, we consider high scale (100 TeV) supersymmetry (SUSY) breaking motivated by recent experimental results of LHC and realize the idea of hilltop supernatural inflation in concrete particle physics models based on flipped-SU(5) and Pati-Salam models in the framework of supersymmetric grand unified theories (SUSY GUTs). The inflaton can be a flat direction including right-handed sneutrino and the waterfall field is a GUT Higgs. The spectral index is $n_s=0.96$ which fits very well with recent data by PLANCK satellite. There is no both thermal and non-thermal gravitino problems. Non-thermal leptogenesis can be resulted from the decay of right-handed sneutrino which plays (part of) the role of inflaton.
    Journal of Cosmology and Astroparticle Physics 09/2013; 2014(01). DOI:10.1088/1475-7516/2014/01/029 · 5.88 Impact Factor
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    Tomohiro Harada, Chul-Moon Yoo, Kazunori Kohri
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    ABSTRACT: Based on a physical argument, we derive a new analytic formula for the amplitude of density perturbation at the threshold of primordial black hole formation in the universe dominated by a perfect fluid with the equation of state $p=w\rho c^{2}$ for $w\ge 0$. The formula gives $\delta^{\rm UH}_{H c}=\sin^{2}[\pi \sqrt{w}/(1+3w)]$ and $\tilde{\delta}_{c}=[3(1+w)/(5+3w)]\sin^{2}[\pi\sqrt{w}/(1+3w)]$, where $\delta^{\rm UH}_{H c}$ and $\tilde{\delta}_{c}$ are the amplitude of the density perturbation at the horizon crossing time in the uniform Hubble slice and the amplitude measure used in numerical simulations, respectively, while the conventional one gives $\delta^{\rm UH}_{H c}=w$ and $\tilde{\delta}_{c}=3w(1+w)/(5+3w)$. Our formula shows a much better agreement with the result of recent numerical simulations both qualitatively and quantitatively than the conventional formula. For a radiation fluid, our formula gives $\delta^{\rm UH}_{H c}=\sin^{2}(\sqrt{3}\pi/6)\simeq 0.6203$ and $\tilde{\delta}_{c}=(2/3)\sin^{2}(\sqrt{3}\pi/6)\simeq 0.4135$. We also discuss the maximum amplitude and the cosmological implications of the present result.
    Physical Review D 09/2013; 88(8). DOI:10.1103/PhysRevD.88.084051 · 4.86 Impact Factor
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    Kazunori Kohri, Chia-Min Lin, Tomohiro Matsuda
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    ABSTRACT: We demonstrate that a topological defect can explain the hemispherical power asymmetry of the CMB. The first point is that a defect configuration, which already exists prior to inflation, can source asymmetry of the CMB. The second point is that modulation mechanisms, such as the curvaton and other modulation mechanisms, can explain scale-dependence of the asymmetry. Using a simple analysis of the $\delta N$ formalism, we show models in which scale-dependent hemispherical power asymmetry is explained by primordial configuration of a defect.
    Journal of Cosmology and Astroparticle Physics 08/2013; 2014(08). DOI:10.1088/1475-7516/2014/08/026 · 5.88 Impact Factor
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    ABSTRACT: The two arrays of the Very High Energy gamma-ray observatory Cherenkov Telescope Array (CTA) will include four Large Size Telescopes (LSTs) each with a 23 m diameter dish and 28 m focal distance. These telescopes will enable CTA to achieve a low-energy threshold of 20 GeV, which is critical for important studies in astrophysics, astroparticle physics and cosmology. This work presents the key specifications and performance of the current LST design in the light of the CTA scientific objectives.
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    Kazunori Kohri, Narendra Sahu
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    ABSTRACT: We study a non-thermal scenario in a two-Higgs doublet extension of the standard model (SM), augmented by an U(1)_{\rm B-L} gauge symmetry. In this set up, it is shown that the decay product of a weakly coupled scalar field just above the electroweak scale can generate visible and dark matter (DM) simultaneously. The DM is unstable because of the broken B-L symmetry. The lifetime of DM (\approx 5\times 10^{25} sec) is found to be much longer than the age of the Universe, and its decay to the SM leptons at present epoch can explain the positron excess observed at the AMS-02. The relic abundance and the direct detection constraint from Xenon-100 can rule out a large parameter space just leaving the $B-L$ breaking scale around \approx 2 - 4 TeV.
    Physical Review D 06/2013; 88(10). DOI:10.1103/PhysRevD.88.103001 · 4.86 Impact Factor
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    ABSTRACT: Assuming that inflation is succeeded by a phase of matter domination, which corresponds to a low temperature of reheating Tr < 109GeV, we evaluate the spectra of gravitational waves induced in the post-inflationary universe. We work with models of hilltop-inflation with an enhanced primordial scalar spectrum on small scales, which can potentially lead to the formation of primordial black holes. We find that a lower reheat temperature leads to the production of gravitational waves with energy densities within the ranges of both space and earth based gravitational wave detectors.
    Journal of Cosmology and Astroparticle Physics 05/2013; 2013(05):033. DOI:10.1088/1475-7516/2013/05/033 · 5.88 Impact Factor

Publication Stats

4k Citations
524.45 Total Impact Points

Institutions

  • 2012–2014
    • The Graduate University for Advanced Studies
      Миура, Kanagawa, Japan
    • Ibaraki University
      Mito-shi, Ibaraki, Japan
  • 2011–2014
    • High Energy Accelerator Research Organization
      • • Theory Center
      • • Institute of Particle and Nuclear Studies
      Tsukuba, Ibaraki, Japan
  • 2009–2011
    • Tohoku University
      • Department of Physics
      Sendai-shi, Miyagi, Japan
  • 2010
    • The University of Edinburgh
      • School of Physics and Astronomy
      Edinburgh, Scotland, United Kingdom
  • 2007–2010
    • Lancaster University
      • Department of Physics
      Lancaster, England, United Kingdom
  • 1997–2010
    • The University of Tokyo
      • • Department of Physics
      • • Institute for Cosmic Ray Research
      Edo, Tōkyō, Japan
  • 2005–2008
    • Harvard-Smithsonian Center for Astrophysics
      • Institute for Theory and Computation
      Cambridge, Massachusetts, United States
  • 2003–2005
    • Osaka University
      • Department of Earth and Space Science
      Suika, Ōsaka, Japan
  • 2004
    • University of Ferrara
      Ferrare, Emilia-Romagna, Italy
  • 2000–2003
    • Kyoto University
      • Yukawa Institute for Theoretical Physics
      Kioto, Kyōto, Japan
  • 2002
    • RIKEN
      Вако, Saitama, Japan