Kazunori Kohri

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

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Publications (127)313.77 Total impact

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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.
    08/2014;
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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. · 4.69 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.
    05/2014;
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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.
    05/2014;
  • 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). · 6.04 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). · 6.04 Impact Factor
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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; · 4.78 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). · 2.44 Impact Factor
  • Tomohiro Harada, Chul-Moon Yoo, Kazunori Kohri
    12/2013; 89(2).
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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.
    10/2013;
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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). · 6.04 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). · 4.69 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. Since the configuration that exists prior to inflation could be expanded to give the super horizon-scale perturbation, the configuration may source modulation of the CMB. Using simple analysis of the $\delta N$ formalism, we show a catalog of models in which the asymmetry is suppressed on the smaller scale. Problems of these models are also discussed.
    Journal of Cosmology and Astroparticle Physics 08/2013; 2014(08). · 6.04 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.
    07/2013;
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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). · 4.69 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. · 6.04 Impact Factor
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    ABSTRACT: We show that the production cross-sections of $\gamma$ and e$^\pm$ in p-p collision, $\sigma_{pp\rightarrow \gamma}$ and $\sigma_{pp\rightarrow {\rm e}^\pm}$ respectively, may be kinematically equivalent without back to that of the parent pion $\sigma_{pp\rightarrow \pi}$. So we can obtain straightforwardly $\sigma_{pp\rightarrow {\rm e}^\pm}$ once we have $\sigma_{pp\rightarrow \gamma}$ with help of the machine data. We discuss in detail the relation between them, and present how reliable for $\sigma_{pp\rightarrow {\rm e}^\pm}$ even in the TeV region. Based on these studies, we give also the emissivities of $\gamma$ and e$^\pm$ in the Galaxy, and present the positron fraction observed at the solar system, the ratio of e$^+$ to [e$^+$ + e$^-$]. It is now in a vital open question that it appears to rise as the energy gets higher, $\gtrsim$\,10\,GeV, far beyond the expectation with the standard model today. The aim of the present paper is to give the production cross-section of e$^\pm$ firmly even in the TeV region, in order to see quantitatively how much deviated from the standard model in the yield of e$^\pm$ and its propagation to the Earth.
    04/2013;
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    Kazunori Kohri, Chia-Min Lin, Tomohiro Matsuda
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    ABSTRACT: In this paper, the curvature perturbation generated by the modulated curvaton decay is studied by a direct application of $\delta N$-formalism. Our method has a sharp contrast with the {\it non-linear formalism} which may be regarded as an indirect usage of $\delta N$-formalism. We first show that our method can readily reproduce results in previous works of modulation of curvaton. Then we move on to calculate the case where the curvaton mass (and hence also the decay rate) is modulated. The method can be applied to the calculation of the modulation in the freezeout model, in which the heavy species are considered instead of the curvaton. Our method explains curvaton and various modulation on an equal footing.
    Journal of Cosmology and Astroparticle Physics 03/2013; 2013(06). · 6.04 Impact Factor
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    ABSTRACT: We discuss the issue of how precisely we can measure the primordial power spectrum by using future observations of 21 cm fluctuations and cosmic microwave background (CMB). For this purpose, we investigate projected constraints on the quantities characterizing primordial power spectrum: the spectral index n_s, its running alpha_s and even its higher order running beta_s. We show that future 21 cm observations in combinations with CMB would accurately measure above mentioned observables of primordial power spectrum. We also discuss its implications to some explicit inflationary models.
    Journal of Cosmology and Astroparticle Physics 03/2013; · 6.04 Impact Factor
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    ABSTRACT: The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project.
    Astroparticle Physics 03/2013; 43:3-18. · 4.78 Impact Factor

Publication Stats

3k Citations
313.77 Total Impact Points

Institutions

  • 2011–2014
    • The Graduate University for Advanced Studies
      • Department of Particle and Nuclear Physics
      Миура, Kanagawa, Japan
  • 2011–2013
    • High Energy Accelerator Research Organization
      • Theory Center
      Tsukuba, Ibaraki, Japan
  • 1997–2013
    • The University of Tokyo
      • • Institute for the Physics and Mathematics of the Universe (IPMU)
      • • Department of Physics
      • • Institute for Cosmic Ray Research
      Edo, Tōkyō, Japan
  • 2010
    • The University of Edinburgh
      • School of Physics and Astronomy
      Edinburgh, Scotland, United Kingdom
    • Saga University
      • Department of Physics
      Сага Япония, Saga, Japan
  • 2005–2010
    • Tohoku University
      • Department of Physics
      Sendai, Kagoshima, Japan
  • 2–2010
    • Lancaster University
      • Department of Physics
      Lancaster, ENG, United Kingdom
  • 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
    • Waseda University
      • Department of Computer Science and Engineering
      Edo, Tōkyō, Japan
  • 2000–2003
    • Kyoto University
      • • Yukawa Institute for Theoretical Physics
      • • Department of Physics II
      Kyoto, Kyoto-fu, Japan