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Takumi Kuwahara

Publications and source records attributed to Takumi Kuwahara.

At least 19 recordsLinked to original sources
arXiv2026

Composite Asymmetric Dark Matter Assisted by Primordial Black Holes Evaporation

We investigate a cogenesis scenario for composite asymmetric dark matter framework: a dark sector has a similar strong dynamics to quantum chromodynamics in the standard model, and the dark-sector counterpart of baryons is the dark matter candidate. The Hawking evaporation of primordial black holes plays the role of a source of heavy scalar particles whose $CP$-violating decay into quarks and dark quarks provides particle--anti-particle asymmetries in baryons and dark matter, respectively. Primordial black holes should evaporate after the electroweak phase transition and before the big-bang nucleosynthesis for explaining the baryon asymmetry of the Universe and for consistent cosmology. We find that this scenario explains the observed values for both baryon and dark matter energy densities when the heavy scalar particles have a mass of $10^6 \text{--} 10^9\, \mathrm{GeV}$ and the primordial black holes have masses of $10^7 \text{--} 10^9\,\mathrm{g}$.

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arXiv2026

Direct Detection and Cosmological Constraints of Dark Matter with Dark Dipoles

We study a fermionic dark matter candidate that couples to the standard model particles exclusively through electric and magnetic dipole operators mediated by a massive dark photon. Such dipole portals naturally arise in dark sectors where the dark matter is neutral under a hidden $U(1)_D$, and they lead to phenomenology distinct from conventional vector-current interactions. We consider the direct-detection signals arising from dark matter-nucleus scattering including the Migdal effect, dark matter-electron scattering, and semiconductor targets, which allow sensitivity to sub-GeV dark matter masses, together with the cosmological bounds from such as thermal relic abundance, cosmic microwave background, big-bang nucleosynthesis, and cosmic-rays. We find that the dark dipole coupling can be largely constrained by direct detection (in particular, electric dipole coupling). However, the cosmological observations have already constrained most of the parameter space, in particular for magnetic dipole interactions of $U(1)_D$ for sub-GeV dark matter. For the dark matter mass below 10 MeV, the semiconductor (in particular, using skipper-CCD) experiments can play a crucial role in probing the dark dipole interactions: future low-threshold experiments utilizing the semiconductor targets can further extend the constraints. Our results have demonstrated that the sub-GeV dark matter with dark dipole interactions can be still safe from the direct-detection constraints, and the future low-threshold semiconductor experiments may play a significant role in constraining the dark dipole interactions.

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arXiv2026

Revisiting Q-ball Interactions with Matters

Q-ball dark matter is one of the candidates for the macroscopic dark matter: Q-ball is a non-topological solitonic configuration, whose stability can be ensured by global charge and energy conservation. One of the crucial factors for discovering signatures from the Q-ball dark matter, is the interactions of the Q-ball dark matter with ordinary matter. In particular, the scattering of ordinary matter off the Q-ball dark matter is important for the direct detection searches, such as paleo-detectors. It was conjectured that quarks incident on the Q-ball were reflected as anti-quarks with a probability of order unity, but it costs the energy of the squark in the Q-ball, which cannot be paid in the scattering of ordinary matter off the Q-ball dark matter. In addition, once a proton is reflected as an anti-proton, the Q-ball obtains the electromagnetic charge. In this study, we revisit the scattering process of quarks with the Q-ball with taking into account the energy cost of the scattering and the electromagnetic charge-up of the Q-ball.

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arXiv2025

Composite asymmetric dark matter with a dark photon portal: Multimessenger tests

Composite asymmetric dark matter (ADM) is the framework that naturally explains the coincidence of the baryon density and the dark matter density of the Universe. Through a portal interaction sharing particle-antiparticle asymmetries in the Standard Model and dark sectors, dark matter particles, which are dark-sector counterparts of baryons, can decay into antineutrinos and dark-sector counterparts of mesons (dark mesons) or dark photon. Subsequent cascade decay of the dark mesons and the dark photon can also provide electromagnetic fluxes at late times of the Universe. The cosmic-ray constraints on the decaying dark matter with the mass of $1$--$10$~GeV has not been well studied. We perform comprehensive studies on the decay of the composite ADM by combining the astrophysical constraints from $e^\pm$ and $γ$-ray. The constraints from cosmic-ray positron measurements by AMS-02 are the most stringent at $\gtrsim2$~GeV: a lifetime should be larger than the order of $10^{26}$~s, corresponding to the cutoff scale of the portal interaction of about $10^8 \text{--} 10^9 \, \mathrm{GeV}$. We also perform the dedicated analysis for the neutrino monoenergetic signals at Super-Kamiokande and Hyper-Kamiokande due to the atmospheric neutrino background in the energy range of our interest.

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arXiv2025

Mediator Decay through mixing with Degenerate Spectrum

The decay of the mediator particle into standard model (SM) particles plays a significant role in exploring the dark sector scenario. We consider such a decay, taking the dark photon mediator as an example that mixes with the SM photon. We find that it requires a careful analysis of the decay rate in the presence of an SM vector boson (e.g., $Z$ boson, $ρ$ meson, and true muonium, etc.) nearly degenerate with the mediator particle in mass. The decay rate of the mediator particle calculated in the mass eigenstate basis {\bf does not} agree with the correct result, given by the imaginary parts of the poles for the vector boson propagators, when the mixing parameter is smaller than a specific value. In such a case, the decay rate calculated by treating the mixing as a perturbative parameter is in agreement with the correct result. We clarify specific values for the mixing parameter quantitatively using several concrete examples of the SM vector bosons degenerate with the dark photon. When the mass mixing between the vector boson and dark photon is smaller (larger) than the decay width of the vector boson, the latter (former) method to calculate the decay rate of the mediator particle gives the correct result.

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arXiv2024

Quantum Theory of Dark Matter Scattering

Dark matter self-scattering is one of key ingredients for small-scale structure of the Universe, while dark matter annihilation is important for the indirect measurements. There is a strong correlation between the velocity-dependent self-scattering cross section and the Sommerfeld enhancement factor for the dark matter annihilation cross section. In this study, we formulate a direct relation between them by the use of Watson's (initial state/final state) theorem and Omnès solution, and our formulation reproduces the Sommerfeld enhancement factor, which directly computed by solving the Schrödinger equation, from the scattering phase shift.

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arXiv2023

Dark Vector Mesons at LHC Forward Detector Searches

Confining gauge dynamics in dark sector is promising to provide dark matter with a mass in the range of sub-GeV to GeV. These dark sectors consist of composite particles, such as dark baryons and dark mesons, that are neutral under the standard-model charge. Dark photon is introduced as a portal matter between the dark sector and the standard-model sector in order to alleviate cosmological problems, and dark hadrons are produced through the same dark photon at accelerator-based experiments. As dark vector mesons and dark pions have the similar masses, dark vector mesons can be long-lived particles, which will be explored by far-detector experiments. In this study, we study the future prospect of the LHC forward-detector experiments, FASER2 and FACET, for exploring the dark vector mesons. When dark photon is heavier than dark pions, the LHC forward-detector searches can be comparable to DarkQuest, and the invisible decay searches of dark photons can also explore the same parameter space. Meanwhile, when dark photon is lightest in the dark sector, their future prospect will be comparable to the visible decay searches for dark photons at LHCb, Belle-II, and HPS.

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arXiv2023

Perturbative Unitarity of Strongly Interacting Massive Particle Models

Dark pion is a promising candidate for the strongly interacting massive particle dark matter. A large pion self-coupling $m_π/f_π$ tends to be required for correct relic abundance, and hence the partial-wave amplitudes can violate the perturbative unitarity even for the coupling within naïve perturbative regime. We improve the partial-wave amplitudes in order to satisfy the optical theorem. We demonstrate that the improvement is relevant only for semi-relativistic pions, and thus this does not affect the self-scattering cross section at the cosmic structures. We also discuss the impact of the improvement of the $πππ\to ππ$ scattering process, and we find that there is an upper bound on $m_π$ at which the correct relic abundance is never achieved even at large $m_π/f_π$ due to the optical theorem.

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arXiv2022

LHC Lifetime Frontier and Visible Decay Searches in Composite Asymmetric Dark Matter Models

The LHC lifetime frontier will probe dark sector in near future, and the visible decay searches at fixed-target experiments have been exploring dark sector. Composite asymmetric dark matter with dark photon portal is a promising framework explaining the coincidence problem between dark matter and visible matter. Dark strong dynamics provides rich structure in the dark sector: the lightest dark nucleon is the dark matter, while strong annihilation into dark pions depletes the symmetric components of the dark matter. Dark photons alleviate cosmological problems. Meanwhile, dark photons make dark hadrons long-lived in terrestrial experiments. Moreover, the dark hadrons are produced through the very same dark photon. In this study, we discuss the visible decay searches for composite asymmetric dark matter models. For a few GeV dark nucleons, the LHC lifetime frontier, MATHUSLA and FASER, has a potential to discover their decay when kinetic mixing angle of dark photon is $ε\gtrsim 10^{-4}$. On the other hand, fixed-target experiments, in particular SeaQuest, will have a great sensitivity to dark pions with a mass below GeV and with kinetic mixing $ε\gtrsim 10^{-4}$ in addition to the LHC lifetime frontier. These projected sensitivities to dark hadrons in dark photon parameter space are comparable with the future sensitivities of dark photon searches, such as Belle-II and LHCb.

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arXiv2021

Maximally self-interacting dark matter: models and predictions

We study self-interacting dark matter (SIDM) scenarios, where the $s$-wave self-scattering cross section almost saturates the Unitarity bound. Such self-scattering cross sections are singly parameterized by the dark matter mass, and are featured by strong velocity dependence in a wide range of velocities. They may be indicated by observations of dark matter halos in a wide range of masses, from Milky Way's dwarf spheroidal galaxies to galaxy clusters. We pin down the model parameters that saturates the Unitarity bound in well-motivated SIDM models: the gauged $L_μ - L_τ$ model and composite asymmetric dark matter model. We discuss implications and predictions of such model parameters for cosmology like the $H_{0}$ tension and dark-matter direct-detection experiments, and particle phenomenology like the beam-dump experiments.

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arXiv2020

Lessons from $T^μ_{~ μ}$ on inflation models: two-loop renormalization of $η$ in the scalar QED

A non-minimal coupling $η$ has been attracting growing interest particularly in the context of inflation models, though its quantum nature is not clear yet. We study the renormalization of a non-minimal coupling in the scalar quantum electrodynamics (QED). We find ${\it no}$ inhomogeneous term of the renormalization group equation (RGE) at the two-loop level. This is similar to other theories, where an inhomogeneous term of the RGE appears only at a higher-loop order: e.g., four-loop order in $λϕ^{4}$ theory.

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arXiv2020

Lessons from $T^μ_{~ μ}$ on inflation models: two-scalar theory and Yukawa theory

We demonstrate two properties of the trace of the energy-momentum tensor $T^μ_{~ μ}$ in the flat spacetime. One is the decoupling of heavy degrees of freedom; i.e., heavy degrees of freedom leave no effect for low-energy $T^μ_{~ μ}$-inserted amplitudes. This is intuitively apparent from the effective field theory point of view, but one has to take into account the so-called trace anomaly to explicitly demonstrate the decoupling. As a result, for example, in the $R^{2}$ inflation model, scalaron decay is insensitive to heavy degrees of freedom when a matter sector ${\it minimally}$ couples to gravity (up to a non-minimal coupling of a matter scalar field other than the scalaron). The other property is a quantum contribution to a non-minimal coupling of a scalar field. The non-minimal coupling disappears from the action in the flat spacetime, but leaves the so-called improvement term in $T^μ_{~ μ}$. We study the renormalization group equation of the non-minimal coupling to discuss its quantum-induced value and implications for inflation dynamics. We work it out in the two-scalar theory and Yukawa theory.

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arXiv2019

Baryon-Dark Matter Coincidence in Mirrored Unification

About 80\% of the mass of the present Universe is made up of the unknown (dark matter), while the rest is made up of ordinary matter. It is a very intriguing question why the {\it mass} densities of dark matter and ordinary matter (mainly baryons) are close to each other. It may be hinting the identity of dark matter and furthermore structure of a dark sector. A mirrored world provides a natural explanation to this puzzle. On the other hand, if mirror-symmetry breaking scale is low, it tends to cause cosmological problems. In this letter, we propose a mirrored unification framework, which breaks mirror-symmetry at the grand unified scale, but still addresses the puzzle. The dark matter mass is strongly related with the dynamical scale of QCD, which explains the closeness of the dark matter and baryon masses. Intermediate-energy portal interactions share the generated asymmetry between the visible and dark sectors. Furthermore, our framework is safe from cosmological issues by providing low-energy portal interactions to release the superfluous entropy of the dark sector into the visible sector.

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arXiv2019

Renormalization Effects on Electric Dipole Moments in Electroweakly Interacting Massive Particle Models

The extended models of the standard model with a single Majorana fermion could be realized as the low-energy effective theory of the well-motivated ultraviolet models. We study the electric dipole moments generated by the effective operators which are composed of the Majorana fermion and the standard model Higgs bosons, especially focusing on the renormalization effects of the effective operators. We give the one-loop anomalous dimension of the effective operators from the scale where the operators are generated to the electroweak scale. In this proceedings, we focused on the electric dipole moments from electroweak Majorana fermions, triplet and 5-plet. We found the renormalization effects could give an enhancement factor being of the order of O(10)% for a triplet model and being more than two in a 5-plet fermion model.

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arXiv2019

Renormalization Effects on Electric Dipole Moments in Electroweakly Interacting Massive Particle Models

We study the renormalization effects on electric dipole moments in the models with new electroweakly interacting massive fermions. The electric dipole moments are generated by the effective operators which arise from integrating out heavy particles at some scale in the models. We give the renormalization group equation for the Wilson coefficients of the effective operators from the scale where the operators are generated to the electroweak scale. Our numerical studies focus on the electric dipole moments in the mini-split supersymmetric scenario and the electroweakly interacting massive particle dark matter scenario. It turns out that the renormalization effects can give an enhancement factor being of the order of O(10)% in the mini-split scenario and being more than two in the minimal dark matter model.

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arXiv2018

Ultraviolet Completion of a Composite Asymmetric Dark Matter Model with a Dark Photon Portal

Composite asymmetric dark matter scenarios naturally explain why the dark matter mass density is comparable with the visible matter mass density. Such scenarios generically require some entropy transfer mechanism below the composite scale; otherwise, their late-time cosmology is incompatible with observations. A tiny kinetic mixing between a dark photon and the visible photon is a promising example of the low-energy portal. In this paper, we demonstrate that grand unifications in the dark and the visible sectors explain the origin of the tiny kinetic mixing. We particularly consider an ultraviolet completion of a simple composite asymmetric dark matter model, where asymmetric dark matter carries a $B-L$ charge. In this setup, the longevity of asymmetric dark matter is explained by the $B-L$ symmetry, while the dark matter asymmetry originates from the $B-L$ asymmetry generated by thermal leptogenesis. In our minimal setup, the Standard Model sector and the dark sector are unified into $SU(5)_\mathrm{GUT} \times SU(4)_\mathrm{DGUT}$ gauge theories, respectively. This model generates required $B-L$ portal operators while suppressing unwanted higher-dimensional operators that could wash out the generated $B-L$ asymmetry.

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arXiv2018

Revisiting Flavor and CP Violation in Supersymmetric SU(5) with Right-Handed Neutrinos

We revisit the minimal supersymmetric SU(5) grand unified theory with three right-handed neutrinos in which universality conditions for soft-supersymmetry breaking parameters are imposed at an input scale above the unification scale. If the Majorana masses for the neutrinos are around $10^{15}$ GeV, large mixing angles and phases in the neutrino sector lead to flavor-violation and CP-violation in the right-handed down squark and left-handed slepton sectors. Since the observed Higgs boson mass and the proton decay constraints indicate sfermions have masses larger than a few TeV, flavor and CP constraints are less restrictive. We explore the constraints on models with a universal soft-supersymmetry breaking input parameters coming from proton stability, electric dipole moments, $μ\to eγ$ decays, and the Higgs mass observed at the LHC. Regions compatible with all constraints can be found if non-zero A-terms are taken.

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