Kazunori Kohri

High Energy Accelerator Research Organization, Tsukuba, Ibaraki, Japan

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Publications (144)550.55 Total impact

  • Kazunori Kohri · Chia-Min Lin · Tomohiro Matsuda
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    ABSTRACT: If the beginning of inflation is defined at the moment when the vacuum energy of the inflaton starts to dominate, the energy density of the other fields at that moment is (by definition) comparable to the inflaton. Although the fraction will be small at the horizon exit due to the inflationary expansion, they can alter the scale dependence of the spectrum. At the same time, velocity of the inflaton field may not coincide with the slow-roll (attractor) velocity. Those dynamics could be ubiquitous but can easily alter the scale dependence of the spectrum. Since the scale dependence is currently used to constrain or even exclude inflation models, it is very important to measure its shift, which is due to the dynamics that does not appear in the original inflation model. Considering typical examples, we show that the spectral index of the tensor mode is a useful measure of such effect. Precise measurement of the higher runnings of the scalar mode will be helpful in discriminating the source.
    No preview · Article · Jan 2016 · Physics Letters B
  • Yoshihiko Oyama · Kazunori Kohri · Masashi Hazumi
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    ABSTRACT: Observations of the 21 cm line radiation coming from the epoch of reionization have a great capacity to study the cosmological growth of the Universe. Also, CMB polarization produced by gravitational lensing has a large amount of information about the growth of matter fluctuations at late time. In this paper, we investigate their sensitivities to the impact of neutrino property on the growth of density fluctuations, such as the total neutrino mass, the effective number of neutrino species (extra radiation), and the neutrino mass hierarchy. We will show that by combining a precise CMB polarization observations such as Simons Array with a 21 cm line observation such as Square kilometer Array (SKA) phase 1 and a baryon acoustic oscillation observation (Dark Energy Spectroscopic Instrument:DESI) we can measure effects of non-zero neutrino mass on the growth of density fluctuation if the total neutrino mass is larger than 0.1eV. Additionally, the combinations can strongly improve errors of the bounds on the effective number of neutrino species sigma(N_nu) ~ 0.06-0.09 at 95 % C.L.. Finally, by using SKA phase 2, we can determine the neutrino mass hierarchy at 95 % C.L. if the total neutrino mass is similar to or smaller than 0.1 eV.
    No preview · Article · Oct 2015 · Journal of Cosmology and Astroparticle Physics
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    ABSTRACT: We study the effects of dark-matter annihilation during the epoch of big-bang nucleosynthesis on the primordial abundances of light elements. We improve the calculation of the light-element abundances by taking into account the effects of anti-nucleons emitted by the annihilation of dark matter and the interconversion reactions of neutron and proton at inelastic scatterings of energetic nucleons. Comparing the theoretical prediction of the primordial light-element abundances with the latest observational constraints, we derive upper bounds on the dark-matter pair-annihilation cross section. Implication to some of particle-physics models are also discussed.
    Full-text · Article · Sep 2015 · Physics Letters B
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    ABSTRACT: The Cherenkov Telescope Array (CTA) will have a unique chance of discovery for a large range of masses in Weakly Interacting Massive Particles models of dark matter. The principal target for dark matter searches with CTA is the centre of the Galactic Halo. The best strategy is to perform CTA observations within a few degrees of the Galactic Centre, with the Galactic Centre itself and the most intense diffuse emission regions removed from the analysis. Assuming a cuspy dark matter density profile for the Milky Way, 500 hours of observations in this region provide sensitivities to and below the thermal cross-section of dark matter annihilations, for masses between a few hundred GeV and a few tens of TeV; therefore CTA will have a significant chance of discovery in some models. Since the dark matter density in the Milky Way is far from certain in the inner kpc region, other targets are also proposed for observation, like ultra-faint dwarf galaxies such as Segue 1 with 100 hours per year proposed. Beyond these two observational targets, further alternatives, such as Galactic dark clumps, will be considered closer to the actual date of CTA operations. Sensitivity predictions for dark matter searches are given on the various targets taking into account the latest instrument response functions expected for CTA together with a discussion on the systematic uncertainties from the backgrounds.
    Full-text · Conference Paper · Aug 2015
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    ABSTRACT: We perform simulations of Cherenkov Telescope Array (CTA) observations of a young supernova remnant RX J1713.7-3946. This target is not only one of the brightest sources ever discovered in very high-energy gamma rays but also well observed in other wavebands. In X-rays, the emission is dominated by synchrotron radiation, which links directly to the existence of high-energy electrons. Radio observations of CO and HI gas have revealed a highly inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds. Therefore gamma rays from hadronic interactions are naturally expected. However, the spectrum in GeV energy range measured by Fermi/LAT indicates more typical of leptonic emission from accelerated electrons. Despite lots of multi-wavelength information, the competing interpretations have led to much uncertainty in the quest of unraveling the true origin of the gamma-ray emission from RX~J1713.7--3946. CTA will achieve highest performance ever in sensitivity, angular resolution, and energy resolution. We estimate CTA capability to examine the emission mechanisms of the gamma rays through simulated spatial distribution, spectra, and their time variation.
    No preview · Article · Aug 2015
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    ABSTRACT: List of contributions from the CTA Consortium presented at the 34th International Cosmic Ray Conference, 30 July - 6 August 2015, The Hague, The Netherlands.
    Full-text · Article · Aug 2015
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    Kazunori Kohri · Kunihito Ioka · Yutaka Fujita · Ryo Yamazaki
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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).
    Full-text · Article · May 2015
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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.
    Full-text · Article · Mar 2015
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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.
    Preview · Article · Aug 2014 · Physical Review D
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    ABSTRACT: The Large Size Telescopes, LSTs, located at the center of the Cherenkov Telescope Array, CTA, will be sensitive for low energy gamma-rays. The camera on the LST focal plane is optimized to detect low energy events based on a high photon detection efficiency and high speed electronics. Also the trigger system is designed to detect low energy showers as much as possible. In addition, the camera is required to work stably without maintenance in a few tens of years. In this contribution we present the design of the camera for the first LST and the status of its development and production.
    No preview · Conference Paper · Aug 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.
    Full-text · Article · Aug 2014 · Physical Review D
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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.
    Preview · Article · May 2014 · Journal of Cosmology and Astroparticle Physics
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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.
    Preview · Article · May 2014 · Physical Review D
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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.
    Preview · Article · May 2014
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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.
    Full-text · Article · May 2014 · Journal of Cosmology and Astroparticle Physics
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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.
    Full-text · Article · Apr 2014 · Journal of Cosmology and Astroparticle Physics
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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].
    Preview · Article · Mar 2014 · Journal of Physics Conference Series
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    Toru Shibata · Yutaka Ohira · Kazunori Kohri · Ryo Yamazaki
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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.
    Preview · Article · Mar 2014 · Astroparticle Physics
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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) W 28 with 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 ∼ 0.3 keV and a density of ∼ 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 γ-ray peaks with an upper-limit flux of 2.1 × 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 γ-ray emission site indicate that the TeV γ-ray of W 28 is π0-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 ≲ 1500 μG.
    Full-text · Article · Feb 2014 · Publications- Astronomical Society of Japan
  • Tomohiro Harada · Chul-Moon Yoo · Kazunori Kohri

    No preview · Article · Dec 2013 · Physical Review D

Publication Stats

5k Citations
550.55 Total Impact Points


  • 2011-2014
    • High Energy Accelerator Research Organization
      • • Theory Center
      • • Institute of Particle and Nuclear Studies
      Tsukuba, Ibaraki, Japan
    • The Graduate University for Advanced Studies
      • Department of Particle and Nuclear Physics
      Миура, Kanagawa, Japan
    • Ibaraki University
      Mito-shi, Ibaraki, Japan
  • 2013
    • Universidad de Jaén
      • High Technical College (Jaen)
      Jaén, Andalusia, Spain
  • 2009-2011
    • Tohoku University
      • Department of Physics
      Sendai-shi, Miyagi, Japan
    • University of Tuebingen
      • Institute for Astronomy and Astrophysics
      Tübingen, Baden-Württemberg, Germany
  • 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
  • 2000-2003
    • Kyoto University
      • • Yukawa Institute for Theoretical Physics
      • • Department of Physics II
      Kioto, Kyōto, Japan