arXiv subjects

Nagisa Hiroshima

Publications and source records attributed to Nagisa Hiroshima.

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

Evading Cosmological Strong Coupling in Non-minimally Coupled Vector Gravity

Recent analyses of Proca theories with non-minimal curvature couplings have uncovered an additional scalar degree of freedom with an identically vanishing propagation speed, $c_s^2=0$, signaling a scale-dependent strong-coupling problem. In this {\it Letter}, we show that an additional derivative interaction can lift this zero-speed degeneracy and restore a non-degenerate quadratic dynamics for the scalar perturbations. In an open region of parameter space, the scalar kinetic matrix is positive definite and the high-frequency propagation speeds are real and positive. At quadratic order, this removes the specific signature of strong coupling associated with the $c_s^2=0$ mode. We also uncover a non-uniform limit as the temporal vector condensate approaches zero: at fixed nonzero $A_0$, the formal extreme-ultraviolet regime develops a ghost, while the kinetic structure of the exactly vanishing-condensate branch is different. The momentum scale at which this ghost appears is pushed toward increasingly high values as $A_0\to0$. Whether the ghost scale ultimately lies above the EFT cutoff depends on an independent determination of the EFT cutoff. Finally, we identify a stable de Sitter fixed point with $A_0\neq0$, surrounded by a finite region in which the scalar no-ghost and high-frequency stability conditions remain satisfied.

hep-th↗

arXiv2026

Revisiting the limits on dark matter annihilation cross-section and decay lifetime in light of electron and positron fluxes

We revisit the upper bound on the annihilation cross-section, $\langleσv\rangle$ of a stable dark matter (DM) of mass $500-10^{14}$ GeV by considering five different channels: $W^+W^-$, $b\bar{b}$, $μ^+μ^-$, $τ^+τ^-$, and $e^+e^-$. We use the observed electron and positron fluxes from CALET, DAMPE, HESS, positron flux from AMS-02, and gamma-ray flux from HAWC, GRAPES-3, CASA-MIA to constrain the annihilation cross-section. We also consider unstable DM of mass $10^3-10^{16}$~GeV decaying to $W^+W^-$, $b\bar{b}$, $μ^+μ^-$, $τ^+τ^-$, and $e^+e^-$ and derive the corresponding lower bound on the DM lifetime, $τ_{\rm DM}$. We find that the latest AMS-02 data provide the most stringent constraints on $\langleσv\rangle$ for DM masses below 2 TeV, while HESS yields the strongest limits for $M_{\rm DM}\gtrsim2$ TeV. The HESS gives a much more stringent limit on the DM lifetime, excluding $τ_{\rm DM\rightarrowμ^+μ^-}\lesssim\mathcal{O}(10^{30})$ s for a 10 TeV mass of DM. The limits on $\langleσv\rangle$ derived from the $e^+e^-$ flux are competitive with those from $γ$-ray and neutrino observations for DM masses in the range $10^5$--$10^{11}$ GeV, and become the most stringent beyond this range. For decaying DM, the $e^+e^-$ flux provides the strongest constraints on the DM lifetime over the mass range $10^3$--$10^9$ GeV.

hep-ph↗

arXiv2025

An Analytic Prescription for $t$-channel Singularities

The $t$-channel singularity is a divergence in the scattering amplitude which occurs when a stable particle propagating in $t$-channel scattering process becomes an on-shell state. Such situations appear either in the system of collider experiments or in the context of the cosmological particle production. No scheme which is generally applicable is known. In this work, we propose a new formulation to identify and remove the source of the divergence. The scheme is fully analytical and various applications can be expected. This work provides a valuable tool in this research field.

hep-ph↗

arXiv2025

Classifying extended Higgs models through the trilinear Higgs boson coupling measurement at future colliders

We investigate the trilinear Higgs boson coupling derived from the functional forms of various extended Higgs potentials. In light of experimental constraints on Higgs boson couplings, we focus on extended Higgs models in which the trilinear Higgs boson coupling is predominantly determined by the Standard Model (SM) Higgs field. Such models include the nearly aligned Higgs effective field theory, classically scale-invariant models, pseudo-Nambu-Goldstone boson scenarios, tadpole-induced models, and others. We also consider higher-order corrections, including top quark and new particle contributions that are often neglected, and discuss their impact on the trilinear Higgs boson coupling. Finally, we show to what extent the functional forms of the Higgs potentials can be probed at future colliders.

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↗

arXiv2025

Explosive production of Higgs particles and implications for heavy dark matter

It is widely believed that the parameter space for Higgs-portal dark matter that achieves the relic abundance through thermal freeze-out has already been tightly constrained, typically at masses on the order of ${\cal O}(10-100)$ GeV. We point out the possibility that the multiple Higgs production due to its self-interaction dramatically changes this picture. We show that the multiplicity can be as large as ${\cal O}(200)$ for the parameters of the Standard Model Higgs, independently of the kinematics of the particle production process. Consequently, heavy Higgs-portal dark matter of $m_χ\gtrsim{\cal O}(1)$ TeV can achieve the required relic abundance in the same mechanism with that for canonical weakly interacting massive particle models.

hep-ph↗

arXiv2024

Search for heavy dark matter from dwarf spheroidal galaxies: leveraging cascades and subhalo models

The Fermi Large Area Telescope (Fermi-LAT) has been widely used to search for Weakly Interacting Massive Particle (WIMP) dark matter signals due to its unparalleled sensitivity in the GeV energy band. The leading constraints for WIMP by Fermi-LAT are obtained from the analyses of dwarf spheroidal galaxies within the Local Group, which are compelling targets for dark matter searches due to their relatively low astrophysical backgrounds and high dark matter content. In the meantime, the search for heavy dark matter with masses above TeV remains a compelling and relatively unexplored frontier. In this study, we utilize 14-year Fermi-LAT data to search for dark matter annihilation and decay signals in 8 classical dwarf spheroidal galaxies within the Local Group. We consider secondary emission caused by electromagnetic cascades of prompt gamma rays and electrons/positrons from dark matter, which enables us to extend the search with Fermi-LAT to heavier dark matter cases. We also update the dark matter subhalo model with informative priors respecting the fact that they reside in subhalos of our Milky Way halo aiming to enhance the robustness of our results. We place constraints on dark matter annihilation cross section and decay lifetime for dark matter masses ranging from $10^3$ GeV to $10^{11}$ GeV, where our limits are more stringent than those obtained by many other high-energy gamma-ray instruments.

astro-ph.HE↗

arXiv2023

Dark Matter Search with Cherenkov Telescope Array

Many models of dark matter (DM) are now widely considered and probed intensively with accelerators, underground detectors, and astrophysical experiments. Among the various approaches, high-energy astrophysical observations are extremely useful to complement laboratory searches for some DM candidates. In the near future, the Cherenkov Telescope Array (CTA) should enable us to access much heavier weakly interacting massive particles, as well as a broad range of other DM candidates. In this talk, we describe DM searches with CTA.

astro-ph.HE↗

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

Semi-analytical frameworks for subhalos from the smallest to the largest scale

Substructures of dark matter halo, called subhalos, provide important clues to understand the nature of dark matter. We construct a useful model to describe the properties of subhalo mass functions based on the well-known analytical prescriptions, the extended Press-Schechter theory. The unevolved subhalo mass functions at arbitrary mass scales become describable without introducing free parameters. The different host halo evolution histories are directly recast to their subhalo mass functions. As applications, we quantify the effects from (i) the Poisson fluctuation, (ii) the host mass scatter, and the (iii) different tidal evolution models on observables in the current Universe with this scheme. The Poisson fluctuation dominates in the number count of the mass ratio to the host of $\sim {\cal O}(10^{-2})$, where the intrinsic scatter is smaller by a factor of a few. The host-mass scatter around its mean does not affect the subhalo mass function. Different models of the tidal evolution predict a factor of $\sim2$ difference in numbers of subhalos with $\lesssim {\cal O}(10^{-5})$, while the dependence of the Poisson fluctuation on the tidal evolution models is subtle. The scheme provides a new tool for investigating the smallest-scale structures of our Universe which are to be observed in near future experiments.

astro-ph.CO↗

arXiv2021

Impacts of new small-scale N-body simulations on dark matter annihilations constrained from cosmological 21cm line observations

We revisit constraints on annihilating dark matter based on the global 21cm signature observed by EDGES. For this purpose, we used the numerical data of the latest N-body simulation performed by state-of-the-art standard in order to estimate the boost factor at high redshifts ($z$ = 10 - 100), which enhances the annihilation of dark matter in course of structure formation. By taking into account to what fraction injected energy from dark matter annihilation contributes to ionization, excitation and heating of intergalactic medium during dark ages, we estimated how large the global 21cm absorption can be. By assuming the thermal freezeout scenario, we find that $m_{\rm DM} < 15$ GeV and $m_{\rm DM} < 3$ GeV have been excluded at 95$\%$ C.L. for the annihilation modes into $b\bar{b}$ and $e^+ e^-$, respectively.

astro-ph.CO↗

arXiv2020

Structure Formation Models Weaken Limits on WIMP Dark Matter from Dwarf Spheroidal Galaxies

Dwarf spheroidal galaxies that form in halo substructures provide stringent constraints on dark matter annihilation. Many ultrafaint dwarfs discovered with modern surveys contribute significantly to these constraints. At present, because of the lack of abundant stellar kinematic data for the ultrafaints, non-informative prior assumptions are usually made for the parameters of the density profiles. Based on semi-analytic models of dark matter subhalos and their connection to satellite galaxies, we present more informative and realistic satellite priors. We show that our satellite priors lead to constraints on the annihilation rate that are between a factor of 2 and a factor of 7 weaker than under non-informative priors. As a result, the thermal relic cross section can at best only be excluded (with 95% probability) for dark matter masses of $\lesssim 40$ GeV from dwarf spheroidal data, assuming annihilation into $b\bar{b}$.

astro-ph.CO↗

arXiv2020

MeV-scale reheating temperature and cosmological production of light sterile neutrinos

We investigate how sterile neutrinos with a range of masses influence cosmology in MeV-scale reheating temperature scenarios. By computing the production of sterile neutrinos through the combination of mixing and scattering in the early Universe, we find that light sterile neutrinos, with masses and mixings as inferred from short-baseline neutrino oscillation experiments, are consistent with big-bang nucleosynthesis (BBN) and cosmic microwave background (CMB) radiation for the reheating temperature of ${\cal O}(1)$ MeV if the parent particle responsible for reheating decays into electromagnetic components (radiative decay). In contrast, if the parent particle mainly decays into hadrons (hadronic decay), the bound from BBN becomes more stringent. In this case, the existence of the light sterile neutrinos can be cosmologically excluded, depending on the mass and the hadronic branching ratio of the parent particle.

hep-ph↗

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

Halo Substructure Boosts to the Signatures of Dark Matter Annihilation

The presence of dark matter substructure will boost the signatures of dark matter annihilation. We review recent progress on estimates of this subhalo boost factor---a ratio of the luminosity from annihilation in the subhalos to that originating the smooth component---based on both numerical $N$-body simulations and semi-analytic modelings. Since subhalos of all the scales, ranging from the Earth mass (as expected, e.g., the supersymmetric neutralino, a prime candidate for cold dark matter) to galaxies or larger, give substantial contribution to the annihilation rate, it is essential to understand subhalo properties over a large dynamic range of more than twenty orders of magnitude in masses. Even though numerical simulations give the most accurate assessment in resolved regimes, extrapolating the subhalo properties down in sub-grid scales comes with great uncertainties---a straightforward extrapolation yields a very large amount of the subhalo boost factor of $\gtrsim$100 for galaxy-size halos. Physically motivated theoretical models based on analytic prescriptions such as the extended Press-Schechter formalism and tidal stripping modeling, which are well tested against the simulation results, predict a more modest boost of order unity for the galaxy-size halos. Giving an accurate assessment of the boost factor is essential for indirect dark matter searches and thus, having models calibrated at large ranges of host masses and redshifts, is strongly urged upon.

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↗

arXiv2018

Modeling evolution of dark matter substructure and annihilation boost

We study evolution of dark matter substructures, especially how they lose the mass and change density profile after they fall in gravitational potential of larger host halos. We develop an analytical prescription that models the subhalo mass evolution and calibrate it to results of N-body numerical simulations of various scales from very small (Earth size) to large (galaxies to clusters) halos. We then combine the results with halo accretion histories, and calculate the subhalo mass function that is physically motivated down to Earth-mass scales. Our results --- valid for arbitrary host masses and redshifts --- show reasonable agreement with those of numerical simulations at resolved scales. Our analytical model also enables self-consistent calculations of the boost factor of dark matter annhilation, which we find to increase from tens of percent at the smallest (Earth) and intermediate (dwarfs) masses to a factor of several at galaxy size, and to become as large as a factor of $\sim$10 for the largest halos (clusters) at small redshifts. Our analytical approach can accommodate substructures in the subhalos (sub-subhalos) in a consistent framework, which we find to give up to a factor of a few enhancement to the annihilation boost. Presence of the subhalos enhances the intensity of the isotropic gamma-ray background by a factor of a few, and as the result, the measurement by Fermi Large Area Telescope excludes the annihilation cross section greater than $\sim$$4\times 10^{-26}$ cm$^3$ s$^{-1}$ for dark matter masses up to $\sim$200 GeV.

astro-ph.CO↗