arXiv subjects

Kazunori Kohri

Publications and source records attributed to Kazunori Kohri.

At least 91 records · Page 5Linked to original sources

arXiv2019

MeV-scale reheating temperature and thermalization of oscillating neutrinos by radiative and hadronic decays of massive particles

From a theoretical point of view, there is a strong motivation to consider an MeV-scale reheating temperature induced by long-lived massive particles with masses around the weak scale, decaying only through gravitational interaction. In this study, we investigate lower limits on the reheating temperature imposed by big-bang nucleosynthesis assuming both radiative and hadronic decays of such massive particles. For the first time, effects of neutrino self-interactions and oscillations are taken into account in the neutrino thermalization calculations. By requiring consistency between theoretical and observational values of light element abundances, we find that the reheating temperature should conservatively be $T_{\rm RH} \gtrsim 1.8$ MeV in the case of the 100% radiative decay, and $T_{\rm RH} \gtrsim$ 4-5 MeV in the case of the 100% hadronic decays for particle masses in the range of 10 GeV to 100 TeV.

hep-ph↗

arXiv2019

Microwave Spectro-Polarimetry of Matter and Radiation across Space and Time

This paper discusses the science case for a sensitive spectro-polarimetric survey of the microwave sky. Such a survey would provide a tomographic and dynamic census of the three-dimensional distribution of hot gas, velocity flows, early metals, dust, and mass distribution in the entire Hubble volume, exploit CMB temperature and polarisation anisotropies down to fundamental limits, and track energy injection and absorption into the radiation background across cosmic times by measuring spectral distortions of the CMB blackbody emission. In addition to its exceptional capability for cosmology and fundamental physics, such a survey would provide an unprecedented view of microwave emissions at sub-arcminute to few-arcminute angular resolution in hundreds of frequency channels, a data set that would be of immense legacy value for many branches of astrophysics. We propose that this survey be carried-out with a large space mission featuring a broad-band polarised imager and a moderate resolution spectro-imager at the focus of a 3.5m aperture telescope actively cooled to about 8K, complemented with absolutely-calibrated Fourier Transform Spectrometer modules observing at degree-scale angular resolution in the 10-2000 GHz frequency range. We propose two observing modes: a survey mode to map the entire sky as well as a few selected wide fields, and an observatory mode for deeper observations of regions of specific interest.

astro-ph.CO↗

arXiv2019

Prospective constraints on the primordial black hole abundance from the stochastic gravitational-wave backgrounds produced by coalescing events and curvature perturbations

For a variety of on-going and planned gravitational-wave (GW) experiments, we study expected constraints on the fraction ($f_{\rm PBH}$) of primordial black holes (PBHs) in dark matter by evaluating the energy-density spectra of two kinds of stochastic GW backgrounds. The first one is produced from an incoherent superposition of GWs emitted from coalescences of all the binary PBHs. The second one is induced through non-linear mode couplings of large primordial curvature perturbations inevitably associated with the generation of PBHs in the early Universe. In this paper, we focus on the PBHs with their masses of $10^{-8}M_{\odot}\leq M_{\mathrm{PBH}} < 1M_{\odot}$, since they are not expected to be of a stellar origin. In almost all ranges of the masses, we show that the experiments are sensitive to constrain the fraction for $10^{-5} \lesssim f_{\rm PBH} \lesssim 1$ by considering the GWs from coalescing events and $10^{-13} \lesssim f_{\rm PBH} \lesssim 1$ by considering the GWs from curvature perturbations. Exceptionally, only in a narrow range of masses for $M_{\mathrm{PBH}} \simeq 10^{-7} M_{\odot}$, the fraction cannot be constrained for $f_{\rm PBH} \lesssim 10^{-13}$ by those two GW backgrounds.

astro-ph.CO↗

arXiv2019

Ultracompact minihalos associated with stellar-mass primordial black holes

The possibility that primordial black hole binary mergers of stellar mass can explain the signals detected by the gravitational-wave interferometers has attracted much attention. In this scenario, primordial black holes can comprise only part of the entire dark matter, say, of order 0.1 %. This implies that most of the dark matter is accounted for by a different component, such as Weakly Interacting Massive Particles. We point out that in this situation, very compact dark matter minihalos, composed of the dominant component of the dark matter, are likely to be formed abundantly in the early Universe, with their formation redshift and abundance depending on primordial non-Gaussianity. They may be detected in future experiments via pulsar observations.

astro-ph.CO↗

arXiv2019

Dependence of accessible dark matter annihilation cross-sections on the density profiles of dwarf spheroidal galaxies with the Cherenkov Telescope Array

Dwarf spheroidal galaxies are excellent targets in gamma-ray searches for dark matter. We consider dark matter searches in dwarf spheroidal galaxies (dSphs) with the Cherenkov Telescope Array (CTA). The aim of this work is to reveal a quantitative and precise dependence of the accessible dark matter annihilation cross-sections on the dark matter density profiles of dSphs and on the distance to them. In most data analyses, researchers have assumed point-like signals from dSphs because it is difficult to resolve the expected emission profiles with current gamma-ray observatories. In future however, CTA will be able to resolve the peak emission profiles in dSphs. We take several variations of the dark matter density profile of Draco dSph as examples and analyze the simulated observations of with CTA. We derive the accessible region of the dark matter annihilation cross-section with each dark matter density profile. The accessible region of the annihilation cross-section can differ by a factor of 10 among plausible profiles. We also examine the dependence on the distance to the target dSphs by assuming the same profiles of dSphs at different distances. Closer targets are better due to the higher J-factor, while their spatial extension significantly degrades our reach to the annihilation cross-section compared to the value expected from a simple distance-scaling of the J-factor. Spatial extension of the source affects the probable parameter region in energy-dependent ways. In some gamma-ray energy ranges, this behaviour becomes moderately dependent on the properties of the observation facility.

astro-ph.HE↗

arXiv2019

Spins of primordial black holes formed in the matter-dominated phase of the Universe

Angular momentum plays very important roles in the formation of PBHs in the matter-dominated phase if it lasts sufficiently long. In fact, most collapsing masses are bounced back due to centrifugal force, since angular momentum significantly grows before collapse. As a consequence, most of the formed PBHs are rapidly rotating near the extreme value $a_{*}=1$, where $a_{*}$ is the nondimensional Kerr parameter at their formation. The smaller the density fluctuation $σ_{H}$ at horizon entry is, the stronger the tendency towards the extreme rotation. Combining the effect of angular momentum with that of anisotropy, we estimate the black hole production rate. We find that the production rate suffers from suppression dominantly due to angular momentum for a smaller value of $σ_{H}$, while due to anisotrpopy for a larger value of $σ_{H}$. We argue that matter domination significantly enhances the production of PBHs despite the suppression. If the matter-dominated phase does not last so long, the effect of the finite duration significantly suppresses PBH formation and weakens the tendency towards large spins. (abridged)

gr-qc↗

arXiv2018

Electroweak Vacuum Collapse induced by Vacuum Fluctuations of the Higgs Field around Evaporating Black Holes

In this paper, we discuss the Higgs vacuum stability around evaporating black holes. We provide a new approach to investigate the false vacuum decay around the black hole and clearly show how vacuum fluctuations of the Higgs induce a gravitational collapse of the vacuum. Furthermore, we point out that the backreaction of the Hawking radiation can not be ignored and the gravitational vacuum decay is exponentially suppressed. However, a large number of the evaporating (or evaporated) primordial black holes threaten the existence of the Universe and we obtain a new upper bound on the evaporating PBH abundance from the vacuum stability. Finally, we show that the high-order corrections of the BSM or QG would not destabilize the Higgs potential, otherwise even a single evaporating black hole triggers a collapse of the electroweak vacuum.

hep-ph↗

arXiv2018

Revisiting Big-Bang Nucleosynthesis Constraints on Long-Lived Decaying Particles

We study effects of long-lived massive particles, which decay during the big-bang nucleosynthesis (BBN) epoch, on the primordial abundances of light elements. Compared to the previous studies, (i) the reaction rates of the standard BBN reactions are updated, (ii) the most recent observational data of light element abundances and cosmological parameters are used, (iii) the effects of the interconversion of energetic nucleons at the time of inelastic scatterings with background nuclei are considered, and (iv) the effects of the hadronic shower induced by energetic high energy anti-nucleons are included. We compare the theoretical predictions on the primordial abundances of light elements with latest observational constraints, and derive upper bounds on relic abundance of the decaying particle as a function of its lifetime. We also apply our analysis to unstable gravitino, the superpartner of the graviton in supersymmetric theories, and obtain constraints on the reheating temperature after inflation.

hep-ph↗

arXiv2018

Electroweak Vacuum Instability and Renormalized Vacuum Field Fluctuations in Friedmann-Lemaitre-Robertson-Walker Background

The cosmological Higgs vacuum stability has been an attractive research subject and it is crucial to accurately follow the development of the Higgs fluctuations. In this work, we thoroughly investigate how the vacuum fluctuations of the Higgs field affect the stability of the electroweak vacuum in Friedmann-Lemaitre-Robertson-Walker (FLRW) background. Adopting adiabatic (WKB) approximation or adiabatic regularization methods, we clearly show that vacuum fluctuations of the Higgs field in the FLRW background depend on the curvature and also masses of the Higgs or other scalar fields. The Higgs fluctuations can generate true vacuum bubbles and trigger off a collapse of the electroweak vacuum. Furthermore we clearly show that the effective Higgs potential in the FLRW background is modified by the Higgs vacuum fluctuations. The vacuum fluctuations of the standard model fields can stabilize or destabilize the effective Higgs potential through backreaction effects. Considering the improved effective Higgs potential with the Higgs vacuum fluctuations $\left< { δϕ}^{ 2 } \right>$ in various backgrounds, we provide new cosmological constraints on the mass of the Higgs-coupled scalar fields and a quantitative description of the Higgs stability in the FLRW background.

hep-ph↗

arXiv2018

Primordial Black Hole Dark Matter and LIGO/Virgo Merger Rate from Inflation with Running Spectral Indices: Formation in the Matter- and/or Radiation-Dominated Universe

We study possibilities to explain the whole dark matter abundance by primordial black holes (PBHs) or to explain the merger rate of binary black holes estimated from the gravitational wave detections by LIGO/Virgo. We assume that the PBHs are originated in a radiation- or matter-dominated era from large primordial curvature perturbation generated by inflation. We take a simple model-independent approach considering inflation with large running spectral indices which are parametrized by $n_\text{s}, α_\text{s}$, and $β_\text{s}$ consistent with the observational bounds. The merger rate is fitted by PBHs with masses of $\mathcal{O}(10)$ $M_{\odot}$ produced in the radiation-dominated era. Then the running of running should be $β_\text{s} \sim 0.025$, which can be tested by future observation. On the other hand, the whole abundance of dark matter is consistent with PBHs with masses of asteroids ($\mathcal{O}(10^{-17})~M_{\odot}$) produced in an early matter-dominated era if a set of running parameters are properly realized.

astro-ph.CO↗

arXiv2018

Effect of Inhomogeneity on Primordial Black Hole Formation in the Matter Dominated Era

We investigate the effect of inhomogeneity on primordial black hole formation in the matter dominated era. In the gravitational collapse of an inhomogeneous density distribution, a black hole forms if apparent horizon prevents information of the central region of the configuration from leaking. Since information cannot propagate faster than the speed of light, we identify the threshold of the black hole formation by considering the finite speed for propagation of information. We show that the production probability $β_{inhom}(σ)$ of primordial black holes, where $σ$ is density fluctuation at horizon entry, is significantly enhanced from that derived in previous work in which the speed of propagation was effectively regarded as infinite. For $σ\ll 1$, we obtain $β_{inhom}\simeq 3.70 σ^{3/2}$, which is larger by about an order of magnitude than the probability derived in earlier work by assuming instantaneous propagation of information.

astro-ph.CO↗

arXiv2018

Semianalytic Calculation of Gravitational Wave Spectrum Nonlinearly Induced from Primordial Curvature Perturbations

Whether or not the primordial gravitational wave (GW) produced during inflation is sufficiently strong to be observable, GWs are necessarily produced from the primordial curvature perturbations in the second order of perturbation. The induced GWs can be enhanced by curvature perturbations enhanced at small scales or by the presence of matter-dominated stages of the cosmological history. We analytically calculate the integral in the expression of the power spectrum of the induced GWs which is a universal part independent of the spectrum of the primordial curvature perturbations. This makes the subsequent numerical integrals significantly easy. In simple cases, we derive fully analytic formulas for the induced GW spectrum.

gr-qc↗

arXiv2018

Slow-roll corrections in multi-field inflation: a separate universes approach

In view of cosmological parameters being measured to ever higher precision, theoretical predictions must also be computed to an equally high level of precision. In this work we investigate the impact on such predictions of relaxing some of the simplifying assumptions often used in these computations. In particular, we investigate the importance of slow-roll corrections in the computation of multi-field inflation observables, such as the amplitude of the scalar spectrum $P_ζ$, its spectral tilt $n_s$, the tensor-to-scalar ratio $r$ and the non-Gaussianity parameter $f_{NL}$. To this end we use the separate universes approach and $δN$ formalism, which allows us to consider slow-roll corrections to the non-Gaussianity of the primordial curvature perturbation as well as corrections to its two-point statistics. In the context of the $δN$ expansion, we divide slow-roll corrections into two categories: those associated with calculating the correlation functions of the field perturbations on the initial flat hypersurface and those associated with determining the derivatives of the e-folding number with respect to the field values on the initial flat hypersurface. Using the results of Nakamura & Stewart '96, corrections of the first kind can be written in a compact form. Corrections of the second kind arise from using different levels of slow-roll approximation in solving for the super-horizon evolution, which in turn corresponds to using different levels of slow-roll approximation in the background equations of motion. We consider four different levels of approximation and apply the results to a few example models. The various approximations are also compared to exact numerical solutions.

astro-ph.CO↗

arXiv2017

Constraints on small-scale primordial power by annihilation signals from extragalactic dark matter minihalos

We revisit constraints on small-scale primordial power from annihilation signals from dark matter minihalos. Using gamma rays and neutrinos from extragalactic minihalos and assuming the delta-function primordial spectrum, we show the dependence of the constraints on annihilation modes, the mass of dark matter, and the annihilation cross section. We report both conservative constraints by assuming minihalos are fully destructed when becoming part of halos originating from the standard almost-scale invariant primordial spectrum, and optimistic constraints by neglecting destruction.

astro-ph.CO↗

arXiv2017

High-energy Neutrinos from Multi-body Decaying Dark Matter

Since the report of the PeV-TeV neutrinos by the IceCube Collaboration, various particle physics models have been proposed to explain the neutrino spectrum by dark matter particles decaying into neutrinos and other Standard Model particles.In such scenarios, simultaneous $γ$-ray emission is commonly expected. Therefore, multi-messenger connections are generally important for the indirect searches of dark matters. The recent development of $γ$-ray astronomy puts stringent constraints on the properties of dark matter, especially by observations with the Fermi $γ$-ray satellite in the last several years. Motivated by the lack of $γ$-ray as well as the shape of the neutrino spectrum observed by IceCube, we discuss a scenario in which the DM is a PeV scale particle which couples strongly to other invisible particles and its decay products do not contain a charged particle. As an example to realize such possibilities, we consider a model of fermionic dark matter that decays into a neutrino and many invisible fermions. The dark matter decay is secluded in the sense that the emitted products are mostly neutrinos and dark fermions. One remarkable feature of this model is the resulting broadband neutrino spectra around the energy scale of the dark matter. We apply this model to multi-PeV dark matter, and discuss possible observable consequences in light of the IceCube data. In particular, this model could account for the large flux at medium energies of $\sim10-100$ TeV, possibly as well as the second peak at PeV, without violating the stringent $γ$-ray constraints from Fermi and air-shower experiments such as CASA-MIA.

hep-ph↗

arXiv2017

CMB bounds on disk-accreting massive Primordial Black Holes

Stellar-mass Primordial Black Holes (PBH) have been recently reconsidered as a Dark Matter (DM) candidate, after the aLIGO discovery of several binary BH mergers with masses of tens of $M_\odot$. Matter accretion on such massive objects leads to the emission of high-energy photons, capable of altering the ionization and thermal history of the universe. This in turn affects the statistical properties of the cosmic microwave background (CMB) anisotropies. Previous analyses have assumed spherical accretion. We argue that this approximation likely breaks down and that an accretion disk should form in the dark ages. Using the most up-to-date tools to compute the energy deposition in the medium, we derive constraints on the fraction of DM in PBH. Provided that disks form early on, even under conservative assumptions for accretion, these constraints exclude a monochromatic distribution of PBH with masses above $\sim 2\, M_\odot$ as the dominant form of DM. The bound on the median PBH mass gets more stringent if a broad, log-normal mass function is considered. A deepened understanding of non-linear clustering properties and BH accretion disk physics would permit an improved treatment and possibly lead to more stringent constraints.

astro-ph.CO↗

arXiv2017

Axion-Like Particles and Recent Observations of the Cosmic Infrared Background Radiation

The CIBER collaboration released their first observational data of the Cosmic IR background (CIB) radiation, which has significant excesses at around the wavelength $\sim$ 1 $μ$m compared to theoretically-inferred values. The amount of the CIB radiation has a significant influence on the opaqueness of the Universe for TeV gamma-rays emitted from distant sources such as AGNs. With the value of CIB radiation reported by the CIBER experiment, through the reaction of such TeV gamma-rays with the CIB photons, the TeV gamma-rays should be significantly attenuated during propagation, which would lead to energy spectra in disagreement with current observations of TeV gamma ray sources. In this article, we discuss a possible resolution of this tension between the TeV gamma-ray observations and the CIB data in terms of axion [or Axion-Like Particles (ALPs)] that may increase the transparency of the Universe by the anomaly-induced photon-axion mixing. We find a region in the parameter space of the axion mass, $m_a \sim 7 \times 10^{-10} - 5 \times 10^{-8}$eV, and the axion-photon coupling constant, $1.2 \times 10^{-11} {\rm GeV}^{-1} \lesssim g_{aγ} \lesssim 8.8 \times 10^{-10} {\rm GeV}^{-1}$ that solves this problem.

hep-ph↗

arXiv2017

Electroweak Vacuum Instability and Renormalized Higgs Field Vacuum Fluctuations in the Inflationary Universe

In this work, we investigated the electroweak vacuum instability during or after inflation. In the inflationary Universe, i.e., de Sitter space, the vacuum field fluctuations $\left< {δϕ}^{ 2 } \right>$ enlarge in proportion to the Hubble scale $H^{2}$. Therefore, the large inflationary vacuum fluctuations of the Higgs field $\left< {δϕ}^{ 2 } \right>$ are potentially catastrophic to trigger the vacuum transition to the negative-energy Planck-scale vacuum state and cause an immediate collapse of the Universe. However, the vacuum field fluctuations $\left< {δϕ}^{ 2 } \right>$, i.e., the vacuum expectation values have an ultraviolet divergence, and therefore a renormalization is necessary to estimate the physical effects of the vacuum transition. Thus, in this paper, we revisit the electroweak vacuum instability from the perspective of quantum field theory (QFT) in curved space-time, and discuss the dynamical behavior of the homogeneous Higgs field $ϕ$ determined by the effective potential ${ V }_{\rm eff}\left( ϕ\right)$ in curved space-time and the renormalized vacuum fluctuations $\left< {δϕ}^{ 2 } \right>_{\rm ren}$ via adiabatic regularization and point-splitting regularization. We simply suppose that the Higgs field only couples the gravity via the non-minimal Higgs-gravity coupling $ξ(μ)$. In this scenario, the electroweak vacuum stability is inevitably threatened by the dynamical behavior of the homogeneous Higgs field $ϕ$, or the formations of AdS domains or bubbles unless the Hubble scale is small enough $H< Λ_{I} $.

hep-ph↗