arXiv subjects

Koji Ishiwata

Publications and source records attributed to Koji Ishiwata.

At least 19 recordsLinked to original sources

arXiv2026

A lower bound on primordial power spectrum from halo substructure

We investigate how the primordial curvature perturbation of a certain wavelength scale affects the halo and subhalo structure. Primordial power spectrum considered in this paper features a nearly scale-invariant form with a cutoff at the wavenumber $ k = \mathcal{O}(1)~$Mpc$^{-1}$ and an additional log-normal bump at smaller scales. We compute the host halo evolution, as well as the subhalo mass function. To be consistent with the observations of stellar streams and gravitational lensing data, the amplitude of the bump that is typically the same or larger than $10^{-9}$ of the primordial curvature perturbation should be present at the wavenumbers of $10$ - $30$ Mpc$^{-1}$.

astro-ph.CO↗

arXiv2026

Collective excitations in magnetic topological insulators and axion dark matter search

We investigate collective excitations in magnetic topological insulators (TIs) and their impact on axion detection. In the three-dimensional TI model with the Hubbard term, the effective action of magnons and amplitude modes is formulated by dynamical susceptibility under the antiferromagnetic and ferromagnetic states. One of the amplitude modes is identified as ``axionic'' quasi-particle and its effective coupling to the electromagnetic fields turns out to be roughly unchanged, or suppressed by up to two orders of magnitude, compared to the previous estimate, which may drastically change the sensitivity of the axion search using``axion' in magnetic TIs.

hep-ph↗

arXiv2025

Perturbative limits on axion-SU(2) gauge dynamics during inflation from the energy density of spin-2 particles

We investigate the conditions under which the perturbative treatment of the backreaction of spin-2 particles on the dynamics of an axion-SU(2) gauge field system breaks down during cosmic inflation. This condition is based on the ratio of the energy density of spin-2 particles from the SU(2) gauge field to that of the background field. The perturbative treatment breaks down when this ratio exceeds unity. We show that this occurs within a parameter space nearly identical to the strong backreaction regime identified in previous studies. However, in some cases, the ratio exceeds unity even before the system enters the strong backreaction regime. Our results suggest that attempts to study the strong backreaction regime using perturbation theory are necessarily limited. Reliable calculations require non-perturbative treatments, such as three-dimensional lattice simulations.

hep-ph↗

arXiv2025

Supermassive black holes from inflation constrained by dark matter substructure

Recent James Webb Space Telescope observations of high-redshift massive galaxy candidates have initiated renewed interest in the important mystery around the formation and evolution of our Universe's largest supermassive black holes (SMBHs). We consider the possibility that some of them were seeded by the direct collapse of primordial density perturbations from inflation into primordial black holes and analyze the consequences of this on current dark matter substructures assuming non-Gaussian primordial curvature perturbation distributions. We derive bounds on the enhanced curvature perturbation amplitude from the number of dwarf spheroidal galaxies in our Galaxy, observations of stellar streams and gravitational lensing. We find this bound region significantly overlaps with that required for SMBH seed formation and enables us to probe Gaussian and non-Gaussian curvature perturbations corresponding to the SMBH seeds in the range ${\cal O}(10^5$\text{--}$10^{12}) M_\odot$.

astro-ph.CO↗

arXiv2023

Supersymmetric Baryogenesis in a Hybrid Inflation Model

We study baryogenesis in a hybrid inflation model which is embedded to the minimal supersymmetric model with right-handed neutrinos. Inflation is induced by a linear combination of the right-handed sneutrinos and its decay reheats the universe. The decay products are stored in conserved numbers, which are transported under the interactions in equilibrium as the temperature drops down. We find that at least a few percent of the initial lepton asymmetry is left under the strong wash-out due to the lighter right-handed (s)neutrinos. To account for the observed baryon number and the active neutrino masses after a successful inflation, the inflaton mass and the Majorana mass scale should be $10^{13}\,{\rm GeV}$ and ${\cal O}(10^{9}$-$10^{10})\,{\rm GeV}$, respectively.

hep-ph↗

arXiv2022

Topology-insensitive axion mass in magnetic topological insulators

We study the axion in three-dimensional topological insulators with magnetic impurities under finite temperature. We find a stable antiferromagnetic ground state and ferromagnetic metastable state. In both magnetic states, the mass of the axion is found to be up to eV scale and it approaches zero near the phase boundary of the magnetic state. This result applies to both normal and topological insulator phases, i.e., the axion mass is insensitive to the topological states, and it will have a direct impact on the targeted mass range of particle axion dark matter in future experiments.

hep-ph↗

arXiv2022

Constraining the primordial curvature perturbation using dark matter substructure

We investigate the primordial curvature perturbation by the observation of dark matter substructure. Assuming a bump in the spectrum of the curvature perturbation in the wavenumber of k>1 Mpc^{-1}, we track the evolution of the host halo and subhalos in a semi-analytic way. Taking into account possible uncertainties in the evaluation of the tidal stripping effect on the subhalo growth, we find a new robust bound on the curvature perturbation with a bump from the number of observed dwarf spheroidal galaxies in our Galaxy and the observations of the stellar stream. The upper limit on the amplitude of the bump is O(10^{-7}) for k~10^3 Mpc^{-1}. Furthermore we find the boost factor, which is crucial for the indirect detection of dark matter signals, is up to O(10^4) due to the bump that is allowed in the current observational bounds.

astro-ph.CO↗

arXiv2022

Subcritical regime of hybrid inflation with modular $A_4$ symmetry

We consider a supergravity model that has the modular $A_4$ symmetry and discuss the interplay between the neutrino mixing and inflation. The model contains right-handed neutrinos that have the Majorana masses and additional Yukawa couplings to the waterfall field. In the model an active neutrino is massless and we find that only the inverted hierarchy is allowed and the Majorana phase is predicted to be around $\pm (120\text{--}180)^\circ$ from the observed neutrino mixing data. In the early universe, one of right-handed sneutrinos plays the role of the inflaton field. Focusing on the subcritical regime of the hybrid inflation that is consistent with the cosmic microwave background data, we analyze the dynamics of the scalar sector and derive an upper bound $\mathcal{O}(10^{10})~{\rm GeV}$ on the scale of the Majorana mass.

hep-ph↗

arXiv2022

Axion-Gauge Field Dynamics with Backreaction

Phenomenological success of inflation models with axion and SU(2) gauge fields relies crucially on control of backreaction from particle production. Most of the previous study only demanded the backreaction terms in equations of motion for axion and gauge fields be small on the basis of order-of-magnitude estimation. In this paper, we solve the equations of motion with backreaction for a wide range of parameters of the spectator axion-SU(2) model. First, we find a new slow-roll solution of the axion-SU(2) system in the absence of backreaction. Next, we obtain accurate conditions for stable slow-roll solutions in the presence of backreaction. Finally, we show that the amplitude of primordial gravitational waves sourced by the gauge fields can exceed that of quantum vacuum fluctuations in spacetime by a large factor, without backreaction spoiling slow-roll dynamics. Imposing additional constraints on the power spectra of scalar and tensor modes measured at CMB scales, we find that the sourced contribution can be more than ten times the vacuum one. Imposing further a constraint of scalar modes non-linearly sourced by tensor modes, the two contributions can still be comparable.

hep-ph↗

arXiv2022

Subcritical hybrid inflation in a generalized superconformal model

We study a generalized superconformal model that gives rise to a subcritical regime of $D$-term hybrid inflation. Exhibiting the model both in a Jordan frame and in the Einstein frame, the effective potential of the subcritical regime is derived in the Einstein frame. It turns out the inflaton-waterfall field dynamics leads to various types of inflaton potential. Consequently the tensor-to-scalar ratio is found to range from $10^{-4}$ ($10^{-3}$) to $0.1$ for getting 60 (50) $e$-folds before the end of inflation.

hep-ph↗

arXiv2021

Sommerfeld-enhanced dark matter searches with dwarf spheroidal galaxies

We study observable signals from dark matter that self-annihilates via the Sommerfeld effect in dwarf spheroidal galaxies (dSphs). Since the effect of the Sommerfeld enhancement depends on the velocity of dark matter, it is crucial to determine the profile of dSphs to compute the J-factor, i.e., the line-of-sight integral of density squared. In our study we use the prior distributions of the parameters for satellite density profiles in order to determine the J-factor, making most out of the recent developments in the N-body simulations and semi-analytical modeling for the structure formation. As concrete models, we analyze fermionic dark matter that annihilates via a light scalar and Wino dark matter in supersymmetric models. We find that, with the more realistic prior distributions that we adopt in this study, the J-factor of the most promising dSphs is decreased by a factor of a few, compared with earlier estimates based on non-informative priors. Nevertheless, the Cherenkov Telescope Array should be able to detect the thermal Wino dark matter by pointing it toward best classical or ultrafaint dSphs for 500 hours.

astro-ph.CO↗

arXiv2021

Axion mass in antiferromagnetic insulators

We calculate axion potential in antiferromagnetic insulators from path integral. It is derived from the effective potential for the order parameter of the antiferromagnetic phase in insulators. Static and dynamical axions are defined consistently from the potential. Consequently antiferromagnetic/paramagnetic and topological/normal orders are classified. The dynamical axion is predicted in all phases and its mass turns out to be various values up to eV scale.

hep-ph↗

arXiv2020

Probing heavy dark matter decays with multi-messenger astrophysical data

We set conservative constraints on decaying dark matter particles with masses spanning a very wide range ($10^4-10^{16}$ GeV). For this we use multimessenger observations of cosmic-ray (CR) protons/antiprotons, electrons/positrons, neutrinos/antineutrinos and gamma rays. Focusing on decays into the $\bar{b}b$ channel, we simulate the spectra of dark matter yields by using the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equations and the $\texttt{Pythia}$ package. We then propagate the CRs of dark matter origin till Earth by using the state-of-the-art numerical frameworks $\texttt{CRPropa}$, $\texttt{GALPROP}$ and $\texttt{HelMod}$ for the solution of the CR transport equation in the extragalactic, Galactic region and the heliosphere, respectively. Conservative limits are obtained by requiring that the predicted dark matter spectra at Earth be less than the observed CR spectra. Overall, we exclude dark matter lifetimes of $10^{28}$ s or shorter for all the masses investigated in this work. The most stringent constraints reach $10^{30}$ s for very heavy dark matter particles with masses in the range $10^{11}-10^{14}$ GeV.

astro-ph.HE↗

arXiv2019

Leptogenesis after superconformal subcritical hybrid inflation

We consider an extended version of superconformal subcritical hybrid inflation model by introducing three right-handed neutrinos that have the Majorana mass terms. In the model one of the right-handed sneutrinos plays a role of the inflaton field, and it decays to reheat the universe after inflation. The vacuum expectation value for the waterfall field gives an unconventional pattern of the light neutrino mass matrix, and the neutrino Yukawa couplings that determine the reheating temperature are not constrained by the neutrino oscillation data. Consequently thermal leptogenesis or sneutrino leptogenesis is realized.

hep-ph↗

arXiv2018

Probing pseudo Nambu-Goldstone boson dark matter at loop level

In the standard model extended by a single complex scalar boson with a softly broken global U(1) symmetry, a pseudo Nambu-Goldstone boson becomes a candidate for dark matter. In this paper, we discuss the direct detection of the pseudo Nambu-Goldstone boson dark matter. Since the tree-level amplitude for dark matter-nucleon scattering vanishes, higher order quantum corrections for the amplitude should be taken into account. We perform the calculation at the next-to-leading order in QCD in a systematic manner.

hep-ph↗

arXiv2018

Superconformal Subcritical Hybrid Inflation

We consider D-term hybrid inflation in the framework of superconformal supergravity. In part of the parameter space, inflation continues for subcritical inflaton field value. Consequently, a new type of inflation emerges, which gives predictions for the scalar spectral index and the tensor-to-scalar ratio that are consistent with the Planck 2015 results. The potential in the subcritical regime is found to have a similar structure to one in the simplest class of superconformal alpha attractors.

astro-ph.CO↗

arXiv2017

Longitudinal W boson scattering in a light scalar top scenario

Scalar tops in the supersymmetric model affect the potential of the standard model-like Higgs at the quatum level. In light of the equivalence theorem, the deviation of the potential from the standard model can be traced by longitudinal gauge bosons. In this work, high energy longitudinal W boson scattering is studied in a TeV-scale scalar top scenario. O(1-10%) deviation from the standard model prediction in the differential cross section is found depending on whether the observed Higgs mass is explained only by scalar tops or by additional contributions at a higher scale.

hep-ph↗

arXiv2017

$WW$ scattering in a radiative electroweak symmetry breaking scenario

A classically scale invariant (CSI) extension of the standard model (SM) induces radiative electroweak symmetry breaking and predicts anomalously large Higgs self-interactions. Hence, $W_LW_L$ scattering processes can be a good probe of the symmetry breaking mechanism. We develop a theoretical framework for perturbative computation and calculate $WW$ scattering amplitudes in a CSI model. It is shown that $W_LW_L$ scattering amplitudes satisfy the equivalence theorem, and that a large deviation of $W_LW_L$ differential cross sections from the SM predictions is predicted depending on the c.m. energy and scattering angle. The results are more accurate than those based on the effective-potential approach. A prescription to implement predictions of the CSI model to Monte Carlo event generators is also presented.

hep-ph↗