arXiv subjects

Motoko Fujiwara

Publications and source records attributed to Motoko Fujiwara.

At least 19 recordsLinked to original sources

arXiv2026

Sommerfeld Enhancement in Spin-1 Electroweak Dark Matter

We study a renormalizable spin-1 electroweakly interacting dark matter (DM) model in which the DM particle is the neutral component of a $Z_2$-odd $\mathrm{SU(2)}_L$ triplet vector boson. The model predicts an additional $Z_2$-even heavy vector triplet, $W'^{\pm}$ and $Z'$, which is generically heavier than the DM particle and whose mass is closely related to the DM mass. Taking into account the Sommerfeld enhancement due to long-range electroweak interactions, we evaluate the thermal relic abundance of the spin-1 DM. We find that the observed relic abundance is reproduced through the freeze-out mechanism for DM masses ($m_V$) in the range $3.6~\mathrm{TeV} \lesssim m_V \lesssim 9.2~\mathrm{TeV}$ within a perturbative regime. A heavier DM mass is favored when the heavy vector boson mass approaches the DM mass, since annihilation processes into a heavy vector boson and a Standard Model particle significantly enhance the effective annihilation cross section. This behavior is distinctive from spin-0 and spin-$1/2$ electroweak DM scenarios, which typically predict a DM mass around $3~\mathrm{TeV}$. We further investigate indirect detection prospects and find that the Cherenkov Telescope Array Observatory (CTAO) will probe the entire viable parameter region. In particular, for $m_V \gtrsim 7.5~\mathrm{TeV}$, the model predicts a characteristic double-peak gamma-ray signature: one peak arising from the unresolved $\gamma\gamma$ and $Z\gamma$ channels, and the other from the $Z'\gamma$ annihilation channel.

hep-ph↗

arXiv2026

A radiative neutrino mass model with semi-annihilating boosted dark matter

Boosted dark matter can provide a distinctive signature of a non-minimal dark sector. In this work, we construct a radiative neutrino mass model that allows for the production of boosted dark matter in the present universe through semi-annihilation. A Dirac fermion serves as the dark matter candidate, which semi-annihilates into an anti-dark matter particle and a neutrino. Taking into account the relevant experimental and theoretical constraints, we find that the boosted dark matter can be probed by DARWIN if its mass lies in the range of 20--50 MeV and the semi-annihilation proceeds through a resonance with a tuning of $\mathcal{O}(0.1)\%$. Furthermore, the monochromatic neutrino produced simultaneously with the boosted dark matter can also be detected at Hyper-Kamiokande and DUNE in part of the allowed parameter space.

hep-ph↗

arXiv2025

Dark matter spikes with strongly self-interacting particles

An unavoidable prediction of scenarios with Dark Matter (DM) self-interactions is the existence of number changing processes that convert $n$ initial DM particles into $m$ final ones ($n\to m$ processes), possibly accompanied by Standard Model particles. We argue that the $n\rightarrow m$ processes could be probed in DM spikes at the center of galaxies, where the high density may allow sizable rates. We systematically study the implications of the $n \to m$ processes in DM spikes, including other possible interactions involving DM, such as annihilation and self-scattering. We find that for $n\geq3$, the spike is significantly depleted for $n\to m$ cross-sections favored by DM production via thermal freeze-out. On the other hand, the semi-annihilation of two DM particles into one DM particle and one Standard Model particle preserves in general the structure of the spike. Such density modifications significantly affect phenomenological studies of both astrophysics and particle DM processes around DM spikes.

hep-ph↗

arXiv2025

Boosted dark matter from semi-annihilations in the galactic center

In some scenarios, the dark matter relic abundance is set by the semi-annihilation of two dark matter particles into one dark matter particle and one Standard Model particle. These semi-annihilations might still be occurring today in the Galactic Center at a significant rate, generating a flux of boosted dark matter particles. We investigate the possible signals of this flux component in direct detection and neutrino experiments for sub-GeV dark matter masses. We show that for typical values of the semi-annihilation cross-section, the sensitivity of current experiments to the spin-independent dark matter-proton scattering cross-section can be several orders of magnitude larger than current constraints from cosmic-ray boosted dark matter. We also argue that the upcoming DARWIN and DUNE experiments may probe scattering cross-sections as low as $10^{-37}\,{\rm cm}^2$ for masses between 30 MeV and 1 GeV.

hep-ph↗

arXiv2025

Resummed multi-line gamma-ray spectra for Cherenkov Telescopes from heavy spin-1 dark matter

Electroweakly interacting stable spin-1 particle in the $(1-10)$ TeV mass range can be a dark matter candidate with rich testability. In particular, one or even two gamma-ray line-like features are expected to be a smoking-gun signature for indirect detection in this scenario. The presence of large Sudakov logarithmic corrections, though, significantly complicates the theoretical prediction of the gamma-ray spectrum. We resum these corrections at the next-to-leading-log (NLL) accuracy using Soft-Collinear Effective field Theory (SCET). Rather interestingly, we find that the LL- and NLL-resummed endpoint spectra for this model are, up to an overall factor, identical to already existing calculations in the contexts of spin-$0$ and spin-$1/2$ (i.e. wino-like) scenarios. We discuss how this non-trivial "exact universality" irrespective of DM spin at these accuracies comes about despite the completely different SCET operator bases. Our resummations allow us to reduce the uncertainty, demonstrated in the energy spectrum with distinctive two peaks from annihilations into $γγ, Z γ$ channel and a photon with $Z_2$-even extra heavy neutral boson $Z'$. We discuss the prospect of improving accuracy further, which is crucial for the heavier DM mass region and realistic resolution in future gamma-ray observations.

hep-ph↗

arXiv2024

Neutrino Diffusion within Dark Matter Spikes

Multi-messenger observations of astrophysical transients provide powerful probes of the underlying physics of the source as well as beyond the Standard Model effects. We explore transients that can occur in the vicinity of supermassive black holes at the center of galaxies, including tidal disruption events (TDEs), certain types of blazars, or even supernovae. In such environments, the dark matter (DM) density can be extremely high, resembling a dense spike or core. We study a novel effect of neutrino diffusion sustained via frequent scatterings off DM particles in these regions. We show that for transients occurring within DM spikes or cores, the DM-neutrino scattering can delay the arrival of neutrinos with respect to photons, but this also comes with a suppression of the neutrino flux and energy loss. We apply these effects to the specific example of TDEs, and demonstrate that currently unconstrained parameter space of DM-neutrino interactions can account for the sizable $O$(days) delay of the tentative high-energy neutrinos observed from some TDEs.

hep-ph↗

arXiv2024

New comprehensive description of the scaling evolution of the cosmological magneto-hydrodynamic system

We study the evolution of primordial magnetic fields until the recombination epoch, which is constrained by the conservation of magnetic helicity density if they are maximally helical and by the Hosking integral if they are non-helical. We combine these constraints with conditions obtained by estimating time scales of energy dissipation processes to describe the evolution of magnetic field strength and magnetic coherence length analytically. The dissipation processes depend on whether magnetic or kinetic energy is dominant, whether the decay dynamics is linear or not, and whether the dominant dissipation term is shear viscosity or drag force. We apply the description to compare constraints on primordial magnetic fields at different epochs in the early universe and argue that magnetogenesis before the electroweak symmetry breaking is not feasible.

astro-ph.CO↗

arXiv2024

Tidal Disruption Events and Dark Matter Scatterings with Neutrinos and Photons

Stars can be tidally disrupted when passing near a black hole, and the debris can induce a flux of high-energy neutrinos. It has been discussed that there are hints in IceCube data of high-energy neutrinos produced in Tidal Disruption Events. The emitting region of neutrinos and photons in these astrophysical events is likely to be located in the vicinity of the central black hole, where the dark matter density might be significantly larger than in the outer regions of the galaxy. We explore the potential attenuation of the emitted neutrino and photon fluxes due to interactions with dark matter particles around the supermassive black hole of the host galaxies of AT2019dsg, AT2019fdr and AT2019aalc, and study the implications for some well-motivated models of dark matter-neutrino and dark matter-photon interactions. Furthermore, we discuss the complementarity of our constraints with values of the dark matter-neutrino scattering cross section proven to alleviate some cosmological tensions.

hep-ph↗

arXiv2023

Vortex Creep Heating vs. Dark Matter Heating in Neutron Stars

Dark matter particles captured in neutron stars deposit their energy as heat. This DM heating effect can be observed only if it dominates over other internal heating effects in NSs. In this work, as an example of such an internal heating source, we consider the frictional heating caused by the creep motion of neutron superfluid vortex lines in the NS crust. The luminosity of this heating effect is controlled by the strength of the interaction between the vortex lines and nuclei in the crust, which can be estimated from the many-body calculation of a high-density nuclear system as well as through the temperature observation of old NSs. We show that both the temperature observation and theoretical calculation suggest that the vortex creep heating dominates over the DM heating. The vortex-nuclei interaction must be smaller than the estimated values by several orders of magnitude to overturn this.

hep-ph↗

arXiv2023

Vortex Creep Heating in Neutron Stars

Recent observations of old warm neutron stars suggest the presence of a heating source in these stars, requiring a paradigm beyond the standard neutron-star cooling theory. In this work, we study the scenario where this heating is caused by the friction associated with the creep motion of neutron superfluid vortex lines in the crust. As it turns out, the heating luminosity in this scenario is proportional to the time derivative of the angular velocity of the pulsar rotation, and the proportional constant $J$ has an approximately universal value for all neutron stars. This $J$ parameter can be determined from the temperature observation of old neutron stars because the heating luminosity is balanced with the photon emission at late times. We study the latest data of neutron star temperature observation and find that these data indeed give similar values of $J$, in favor of the assumption that the frictional motion of vortex lines heats these neutron stars. These values turn out to be consistent with the theoretical calculations of the vortex-nuclear interaction.

astro-ph.HE↗

arXiv2022

New description of the scaling evolution of the cosmological magneto-hydrodynamic system

We present a new description of cosmological evolution of the primordial magnetic field under the condition that it is non-helical and its energy density is larger than the kinetic energy density. We argue that the evolution can be described by four different regimes, according to whether the decay dynamics is linear or not, and whether the dominant dissipation term is the shear viscosity or the drag force. Using this classification and conservation of the Hosking integral, we present analytic models to adequately interpret the results of various numerical simulations of field evolution with variety of initial conditions. It is found that, contrary to the conventional wisdom, the decay of the field is generally slow, exhibiting the inverse transfer, because of the conservation of the Hosking integral. Using the description proposed here, we may trace the intermediate evolution history of the magnetic field and clarify whether each process governing its evolution is frozen or not, which is essential to follow the evolution of relatively weak magnetic fields.

astro-ph.CO↗

arXiv2022

Capture of Electroweak Multiplet Dark Matter in Neutron Stars

If dark matter has a sizable scattering cross section with nucleons, it can efficiently be captured by a neutron star. Its energy is then transferred to the neutron star as heat through the scattering and annihilation inside the star. This heating effect may be detectable via dedicated temperature observations of nearby old pulsars, providing an alternative method for dark matter searches. In this paper, we show that for electroweak multiplet dark matter this search strategy can probe the parameter region which is out of reach of future dark matter direct detection experiments. To see this systematically, we classify such dark matter candidates in terms of their electroweak charges and investigate the effect of ultraviolet physics by means of higher-dimensional effective operators. We then show that if the effect of ultraviolet physics is sizable, the dark matter-nucleon elastic scattering cross section becomes sufficiently large, whilst if it is suppressed, then the mass splittings among the components of the DM multiplet get small enough so that the inelastic scattering processes are operative. In any case, the electroweak multiplet dark matter particles are efficiently captured in neutron stars, making the search strategy with the temperature observation of old neutron stars promising.

hep-ph↗

arXiv2021

Vanishing or non-vanishing rainbow? Reduction formulas of electric dipole moment

In this paper, we derive a simplified formula of electric dipole moments (EDMs) of a fermion. In the Standard Model, it is well-known that non-trivial cancellations between some rainbow-type diagrams induced by $W$ boson exchanges occur in the calculation of the neutron EDM at the two-loop level due to the gauge symmetry. The fermion self-energy and the vertex correction are related through the Ward-Takahashi identity, and this relation causes the exact cancellation of the EDM. We derive EDM formulas for a more general setup by introducing the form factors for the fermion self-energy and the vertex correction so that the derived formulas can be applicable to a larger class of models. We conclude that the non-zero EDM contributions are induced from rainbow-type diagrams with the chirality flipping effects for internal fermions. We also discuss the other possible generalization of the EDM calculation which is applicable to the other classes of models.

hep-ph↗

arXiv2021

Gamma-ray line from electroweakly interacting non-abelian spin-1 dark matter

We study gamma-ray line signatures from electroweakly interacting non-abelian spin-1 dark matter (DM). In this model, $Z_2$-odd spin-1 particles including a DM candidate have the SU(2)$_L$ triplet-like features, and the Sommerfeld enhancement is relevant in the annihilation processes. We derive the annihilation cross sections contributing to the photon emission and compare with the SU(2)$_L$ triplet fermions, such as Wino DM in the supersymmetric Standard Model. The Sommerfeld enhancement factor is approximately the same in both systems, while our spin-1 DM predicts the larger annihilation cross sections into $γγ/ Z γ$ modes than those of the Wino by $\frac{38}{9}$. This is because a spin-1 DM pair forms not only $J=0$ but also $J=2$ partial wave states where $J$ denotes the total spin angular momentum. Our spin-1 DM also has a new annihilation mode into $Z_2$-even extra heavy vector and photon, $Z' γ$. For this mode, the photon energy depends on the masses of DM and the heavy vector, and thus we have a chance to probe the mass spectrum. The latest gamma-ray line search in the Galactic Center region gives a strong constraint on our spin-$1$ DM. We can probe the DM mass for $\lesssim 25.3~$TeV by the Cherenkov Telescope Array experiment even if we assume a conservative DM density profile.

hep-ph↗

arXiv2021

Electric dipole moments in the extended scotogenic models

Electric dipole moments (EDMs) of charged leptons arise from a new source of CP violation in the lepton sector. In this paper, we calculate the EDMs of the charged leptons in the minimal scotogenic model with two singlet fermions, and the models extended with one or two triplet fermions instead of the singlet fermions, taking into account the constraints of the neutrino oscillation data, the charged lepton flavor violation and perturbative unitarity bound for the Yukawa couplings. We show that the hybrid model with one singlet and one triplet fermions predicts an electron EDM larger than the other models in both normal and inverted neutrino mass hierarchy. We find some parameter space has already been ruled out by the current upper bound of the electron EDM and further parameter space can be explored by future experiments.

hep-ph↗

arXiv2020

A model of electroweakly interacting non-abelian vector dark matter

We propose an electroweakly interacting spin-1 dark matter (DM) model. The electroweak gauge symmetry, SU(2)$_L\times$U(1)$_Y$, is extended into SU(2)$_0\times$SU(2)$_1 \times $SU(2)$_2 \times$U(1)$_Y$. A discrete symmetry exchanging SU(2)$_0$ and SU(2)$_2$ is imposed. This discrete symmetry stabilizes the DM candidate. The spin-1 DM particle ($V^0)$ and its SU(2)$_L$ partners ($V^\pm$) interact with the Standard Model (SM) electroweak gauge bosons without any suppression factors. Consequently, pairs of DM particles efficiently annihilate into the SM particles in the early universe, and the measured value of the DM energy density is easily realized by the thermal freeze-out mechanism. The model also predicts a heavy vector triplet ($W'^\pm$ and $Z'$) in the visible sector. They contribute to the DM annihilation processes. The mass ratio of $Z'$ and $V^0$ determines values of various couplings, and constraints on $W'$ and $Z'$ restrict regions of the parameter space that are viable for DM physics. We investigate the constraints from perturbative unitarity of scalar and gauge couplings, the Higgs signal strength, $W'$ search at the LHC, and DM direct detection experiments. It is found that the relic abundance of $V^0$ explains the right amount of the DM energy density for 3 TeV $\lesssim m_{V^0} \lesssim$ 19 TeV.

hep-ph↗

arXiv2020

Maximum value of the spin-independent cross section in the THDM+a

We investigate the maximum value of the spin-independent cross section ($σ_\text{SI}$) in a dark matter (DM) model called the two-Higgs doublet model + a (THDM+a). This model can explain the measured value of the DM energy density by the freeze-out mechanism. Also, $σ_\text{SI}$ is suppressed by the momentum transfer at the tree level, and loop diagrams give the leading contribution to it. The model prediction of $σ_\text{SI}$ highly depends on values of $c_1$ and $c_2$ that are the quartic couplings between the gauge singlet CP-odd state ($a_0$) and Higgs doublet fields ($H_1$ and $H_2$), $c_1 a_0^2 H_1^\dagger H_1$ and $c_2 a_0^2 H_2^\dagger H_2$. We discuss the upper and lower bounds on $c_1$ and $c_2$ by studying the stability of the electroweak vacuum, the condition for the potential bounded from the below, and the perturbative unitarity. We find that the condition for the stability of the electroweak vacuum gives upper bounds on $c_1$ and $c_2$. The condition for the potential to be bounded from below gives lower bounds on $c_1$ and $c_2$. It also constrains the mixing angle between the two CP-odd states. The perturbative unitarity bound gives the upper bound on the Yukawa coupling between the dark matter and $a_0$ and the quartic coupling of $a_0$. Under these theoretical constraints, we find that the maximum value of the $σ_\text{SI}$ is $\sim 5\times 10^{-47}$ cm$^2$ for $m_A = $ 600 GeV, and the LZ and XENONnT experiments can see the DM signal predicted in this model near future.

hep-ph↗

arXiv2019

Loop corrections to dark matter direct detection in a pseudoscalar mediator dark matter model

If dark matter (DM) is a fermion and its interactions with the standard model particles are mediated by pseudoscalar particles, the tree-level amplitude for the DM-nucleon elastic scattering is suppressed by the momentum transfer in the non-relativistic limit. At the loop level, on the other hand, the spin-independent contribution to the cross section appears without such suppression. Thus, the loop corrections are essential to discuss the sensitivities of the direct detection experiments for the model prediction. The one-loop corrections were investigated in the previous works. However, the two-loop diagrams give the leading order contribution to the DM-gluon effective operator ($\barχ χG^{a}_{μν} G^{a μν} $) and have not been correctly evaluated yet. Moreover, some interaction terms which affect the scattering cross section were overlooked. In this paper, we show the cross section obtained by the improved analysis and discuss the region where the cross section becomes large.

hep-ph↗