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Search Results: 1 - 10 of 627 matches for " Kazunori Kohri "
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Cosmological scenario of stop NLSP with gravitino LSP and the cosmic lithium problem
Kohri, Kazunori;Santoso, Yudi
High Energy Physics - Phenomenology , 2008,
Abstract: The discrepancy on Li^7 and Li^6 abundances between the observational data and the standard Big Bang Nucleosynthesis theory prediction has been a nagging problem in astrophysics and cosmology, given the highly attractive and succesful Big Bang paradigm. One possible solution of this lithium problem is through hadronic decays of a massive metastable particle which alter the primordial element abundances. We explore this possibility using gravitino dark matter framework in which the next lightest supersymmetric particle (NLSP) is typically long-lived. We found that stop NLSP can provide an attractive solution to the lithium problem.
Generating Primordial Black Holes Via Hilltop-Type Inflation Models
Laila Alabidi,Kazunori Kohri
Physics , 2009, DOI: 10.1103/PhysRevD.80.063511
Abstract: It has been shown that black holes would have formed in the early Universe if, on any given scale, the spectral amplitude of the Cosmic Microwave Background (CMB) exceeds 10^(-4). This value is within the bounds allowed by astrophysical phenomena for the small scale spectrum of the CMB, corresponding to scales which exit the horizon at the end of slow-roll inflation. Previous work by Kohri et. al. (2007) showed that for black holes to form from a single field model of inflation, the slope of the potential at the end of inflation must be flatter than it was at horizon exit. In this work we show that a phenomenological Hilltop model of inflation, satisfying the Kohri et. al. criteria, could lead to the production of black holes, if the power of the inflaton self-interaction is less than or equal to 3, with a reasonable number or e-folds. We extend our analysis to the running mass model, and confirm that this model results in the production of black holes, and by using the latest WMAP year 5 bounds on the running of the spectral index, and the black hole constraint we update the results of Leach et. al. (2000) excluding more of parameter space.
Can neutrino-cooled accretion disk be an origin of gamma-ray bursts?
Kazunori Kohri,Shin Mineshige
Physics , 2002, DOI: 10.1086/342166
Abstract: It is often considered that a massive torus with solar mass or so surrounding a stellar-mass black hole may be a central engine of a gamma-ray burst. We study the properties of such massive accretion tori (or disks) based on the $\alpha$ viscosity model. For surface density exceeding about $10^{20}$ g cm$^{-2}$, which realizes when about a solar-mass material is contained within a disk with a size of $\sim 5 \times 10^6$ cm, we find that (1) luminosity of photons is practically zero due to significant photon trapping, (2) neutrino cooling dominates over advective cooling, (3) pressure of degenerate electrons dominates over pressure of gas and photons, and (4) magnetic field strength exceeds the critical value of about $4 \times 10^{13}$ G, even if we take 0.1 % of the equi-partition value. The possible observable quantum electrodynamical (QED) effects arising from super-critical fields are discussed. Most interestingly, photon splitting may occur, producing significant number of photons of energy below $\sim 511$ keV, thereby possibly suppressing e$^\pm$ pair creation.
Ambiguity in running spectral index with an extra light field during inflation
Kazunori Kohri,Tomohiro Matsuda
Physics , 2014, DOI: 10.1088/1475-7516/2015/02/019
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 alter the time-dependence of the original spectrum of the curvature perturbation. It is possible to 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 adding significant contribution to the spectral index. The corrections to the spectral index (and the amplitude) could be important in considering global behavior of the corrected spectrum, although they can be neglected in the estimation of the spectrum and its spectral index at the pivot scale. The ambiguity in the running of the spectral index, which could be due to such fields, can be used to nullify tension between BICEP2 and Planck experiments.
Cosmology with Long-Lived Charged Massive Particles
Kazunori Kohri,Tomo Takahashi
Physics , 2009, DOI: 10.1016/j.physletb.2009.11.051
Abstract: We investigate the evolution of the bound state of negatively charged massive particles (CHAMPs) with light elements and discuss its cosmological consequences and the constraint. By numerically solving the Boltzmann equation, we study the time evolutions of such bound states. Since most of negative CHAMPs are captured by He4, its bound state is positively charged and couples with the electromagnetic plasma. When charged particles constitute a dominant non-relativistic component, density fluctuations of matter cannot grow due to the acoustic damping. This results in the suppression of matter power spectrum from which a severe constraint can be obtained. By arguing constraints from other aspects of cosmology, we show that the constraint from large scale structure gives most stringent one in some representative cases.
Possible Origin of Rapid Variability of Gamma-Ray Bursts due to Convective Energy Transfer in Hyperaccretion Disks
Norita Kawanaka,Kazunori Kohri
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19733.x
Abstract: We investigate the effects of the convection in the hyperaccretion disk around a stellar-mass black hole, which is considered to be the central engine of gamma-ray bursts (GRBs), with simple analytical calculations. If the convective energy transfer in the vertical direction becomes efficient compared with the inward advective energy transport, the hyperaccretion disk is expected to be hotter and the neutrino emission due to the electron-positron annihilation becomes the most efficient cooling process. We find that the sequence of the thermal equilibrium solutions for the convective hyperaccretion disk would have a viscously unstable branch, especially when the viscosity parameter is relatively small (alpha <~ 0.01). This means that the sporadic mass accretion onto a black hole would occur in this disk. We propose that this process can be the origin of the highly variable lightcurves observed in the prompt emissions of GRBs.
Constraining the co-genesis of Visible and Dark Matter with AMS-02 and Xenon-100
Kazunori Kohri,Narendra Sahu
Physics , 2013, DOI: 10.1103/PhysRevD.88.103001
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.
Consistency Relations for Large Field Inflation
Takeshi Chiba,Kazunori Kohri
Physics , 2014, DOI: 10.1093/ptep/ptu123
Abstract: Consistency relations for chaotic inflation with a monomial potential and natural inflation and hilltop inflation are given which involve the scalar spectral index $n_s$, the tensor-to-scalar ratio $r$ and the running of the spectral index $\alpha$. The measurement of $\alpha$ with $O(10^{-3})$ and the improvement in the measurement of $n_s$ could discriminate monomial model from natural/hilltop inflation models. A consistency region for general large field models is also presented.
Quintessence Cosmology and Varying $α$
Takeshi Chiba,Kazunori Kohri
Physics , 2001, DOI: 10.1143/PTP.107.631
Abstract: If the reported measurements of the time variation of the fine structure constant from observations of distant QSOs are correct, combined with the Oklo limit they would strongly constrain the class of the quintessence potential. If these results prove valid, future satellite experiment (STEP) should measure the induced violation of the weak equivalence principle. Future cosmological observations of nearby $(z \siml 0.5)$ absorption systems would make it clear whether the variation is significant or not.
Supernova Cosmology and the Fine Structure Constant
Takeshi Chiba,Kazunori Kohri
Physics , 2003, DOI: 10.1143/PTP.110.195
Abstract: We study the dependence of the peak luminosity of Type Ia supernovae on the fine structure constant $\alpha$. We find that decreasing (increasing) $\alpha$ enhances (reduces) the luminosity. Future experiments like SNAP could determine the variation of $\alpha$ to a precision of $10^{-2}$.
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