arXiv subjects

Hiroshi Okada

Publications and source records attributed to Hiroshi Okada.

At least 19 recordsLinked to original sources

arXiv2026

Can a minimal radiative seesaw explain the LZ 248 keV event?

We interpret the recently reported 248~keV nuclear recoil event in the LUX-ZEPLIN (LZ) experiment via inelastic dark matter scattering within the minimal Scotogenic model. A sub-MeV mass splitting between neutral inert scalars suppresses low-energy scattering while permitting signals from the high-velocity halo tail. Crucially, co-annihilation with nearly degenerate right-handed fermions accommodates the thermal relic density for dark matter masses up to $\sim {\cal O} (1)$~TeV, extending the viable range significantly beyond the pure inert doublet model limit while evading direct detection bounds. However, to resolve the severe tension with IceCube neutrino limits on solar capture, we extend this minimal framework by introducing a hidden $U(1)_X$ gauge symmetry that naturally leads us to tiny $λ_5$ coupling at the one-loop level. This realizes an isospin-violating scenario that suppresses dark matter capture in the Sun while preserving the coherent scattering signal in the Xenon-based LZ detector. We numerically verify that this extended framework naturally generates neutrino masses and satisfies constraints from Big Bang Nucleosynthesis and indirect detection, providing a robust and testable solution to the LZ anomaly.

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arXiv2026

Radiative double inverse seesaw and dark matter in an alternative gauged $U(1)_{B-L}$ model

We propose a concrete theoretical framework for the double inverse seesaw mechanism within an alternative gauged $U(1)_{B-L}$ model, where the tiny neutrino masses emerge radiatively at the loop level in accordance with the 't Hooft naturalness criterion. By assigning non-universal $B-L$ charges of $(-4, -4, 5)$ to the right-handed neutrinos, we introduce vector-like fermions and two inert singlet scalars that circulate in the loop to generate the required mass terms. The spontaneous breaking of the $U(1)_{B-L}$ symmetry leaves a remnant $Z_2$ symmetry, which naturally stabilizes these new particles as dark matter (DM) candidates. We systematically investigate both fermionic and bosonic DM scenarios, assuming their interactions are predominantly mediated by the $B-L$ gauge boson ($Z'$). Our comprehensive analysis of the relic density, direct detection, and collider bounds reveals that the fermionic DM scenario is strongly favored. In contrast, the bosonic DM via the $Z'$ portal is severely constrained and largely ruled out by the latest direct detection experiments such as LZ, PandaX-4T, and XENONnT.

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arXiv2026

Entanglement Signatures of Kinetic-Mixing Portals in Dark Monopole Scattering

We examine the interaction between the Standard Model (SM) fermions and the topological dark magnetic monopoles mediated via a kinetic-mixing portal to investigate the generation of quantum entanglement in particle-portal scattering. While the conventional phenomenology only takes into account total cross-sections, decay widths, and missing-energy signatures, we employ an information-theoretic approach to explain the scattering event. We quantify the quantum correlations transported over the portal boundary by calculating the Von Neumann entropy ($S_{\text{ent}}$) and the subsystem purity deterioration ($γ< 1$) analytically. Our results demonstrate that the non-perturbative core form factor of the dark monopole controls its high-energy momentum transfer and that the Von Neumann entropy increases quadratically with the topological magnetic charge ($g_m^2$) and the kinetic-mixing parameter ($ε^2$). This paradigm provides an information-theoretic "microscope" to restrict portal parameter spaces and explore underlying topological structures without using traditional energy signatures by establishing quantum decoherence and purity loss as new, complementary observables.

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arXiv2026

Dirac one-loop seesaw in a non-invertible fusion rule

We propose a radiative Dirac neutrino mass model stabilized by a non-invertible fusion rule originating from a $Z_3 \times Z_3'$ gauging. The imposed symmetry forbids tree-level Yukawa couplings and ensures that neutrino masses are generated only at the one-loop level through the exchange of exotic fermions and inert scalars. This minimal framework simultaneously accommodates neutrino masses and mixings consistent with current oscillation data, while providing a viable dark matter candidate. We analyze lepton flavor violating processes and lepton anomalous magnetic moments, finding that all contributions remain well below present experimental bounds. In the dark matter sector, the bosonic singlet emerges as a promising candidate with relic density compatible with cosmological observations, whereas the fermionic option is strongly disfavored due to suppressed annihilation cross sections. Our study demonstrates that non-invertible fusion rules can serve as a powerful organizing principle for constructing minimal and phenomenologically consistent extensions of the Standard Model, linking neutrino physics and dark matter within a unified radiative framework.

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arXiv2026

Radiative lepton model in a non-invertible fusion rule

We propose a radiatively induced lepton mass model introducing a $Z_2$ gauging $Z_5$ fusion rule. In our framework, the charged-lepton mass matrix is generated at one-loop level via dynamical breaking of the fusion rule. On the other hand, the neutrino mass matrix is induced at the one-loop level without breaking the fusion rule. As a direct consequence of the loop induced charged lepton masses, we can also consider lepton flavor violations, electron and muon $g-2$, and charged-lepton electric dipole moments that come into our valid phenomenological discussion. Then, we perform numerical analysis and show some interesting tendency on the Dirac $CP$ phase, two Majorana phases, charged-lepton electric dipole moments and the neutrinoless double beta decay, all of which depends on their arguments where we fix the absolute values of our free parameters in order to satisfy experimental data of the lepton masses and mixing angles.

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arXiv2026

Zee models with a non-invertible $Z_M$ symmetry

We investigate Zee models by incorporating a non-invertible $Z_M$ symmetry. The models are systematically classified based on their symmetry assignments, which dictate structures for the Yukawa couplings and the neutrino mass matrix. By evaluating the consistency of these mass structures with current experimental data, we identify the viable model candidates. Focusing on a representative benchmark model based on the non-invertible $Z_7$ symmetry, we perform a detailed numerical analysis. Our results yield characteristic predictions for neutrino observables and charged lepton flavor violating processes within the allowed parameter space.

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arXiv2026

A theoretical account of tiny multi-Higgs vacuum expectation values from non-invertible symmetry

We propose a novel mechanism to explain the naturally small vacuum expectation values (VEVs) of exotic multi-Higgs fields by employing non-invertible symmetries. Specifically, we introduce an $SU(2)_L$ quartet $H_4$ and a quintet $H_5$ within the framework of the minimal Fibonacci fusion rule (FFR). This non-invertible symmetry strictly forbids the generation of tree-level VEVs for these exotic fields. However, once the symmetry is broken, these VEVs are generated radiatively at the one-loop level.This mechanism is highly minimal, as it requires no additional loop-inducing particles. We demonstrate that for various benchmark energy scales, the resulting VEVs are naturally suppressed to the order of $10^{-3}-10^{-2}$ GeV, satisfying experimental constraints from the $ρ$ parameter. Finally, we illustrate the phenomenological viability of this setup by applying it to three representative neutrino mass models: the type-III seesaw, the Dirac neutrino seesaw, and the inverse seesaw mechanisms.

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arXiv2026

Dynamical CP Violation from Non-Invertible Selection Rules

We propose a novel mechanism in which leptonic CP-violating phases are generated dynamically through the radiative breaking of non-invertible selection rules. In this framework, tree-level mass matrices, initially constrained by a CP-like symmetry within a non-invertible structure, acquire flavor-dependent phases once loop corrections are incorporated. Furthermore, these corrections can also generate mass terms, thereby addressing the mass hierarchy problem. As an illustrative example, we employ the Inverse Seesaw (ISS) model to demonstrate how the Majorana mass of the light sterile neutrino $N_L$ arises via this mechanism while simultaneously realizing CP violation. Although our analysis is carried out within the ISS framework, the mechanism has broader implications, potentially offering new perspectives on CP-related problems such as the strong CP problem, leptogenesis, and baryogenesis. This work thus establishes a foundation for exploring the dynamical breaking of non-invertible selection rules as a novel origin of CP violation in particle physics.

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arXiv2026

Lepton seesaw model in a modular $A_4$ symmetry

We propose a lepton seesaw model to get the same mass-origin of charged-leptons and neutrinos introducing a modular $A_4$ symmetry. In this scenario, the mass matrix of charged-leptons is induced via seesaw-like mechanism while the mass matrix of neutrino is realized via an inverse seesaw mechanism with help of vector-like fermions and an isospin singlet and triplet scalar field with nonzero vacuum expectation values. In analyzing the model we concentrate on fixed points of modulus $τ$ that is favored by a flux compactification of Type IIB string theory. We search for allowed region at nearby these fixed points and find good solutions at nearby $τ=ω$ with several good predictions.

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arXiv2026

Dynamical Determination of the Cut-off Scale in Loop-Induced Neutrino Mass Models with Non-Invertible Symmetry

We propose our framework as an effective field theory valid below the cut-off scale $Λ$, in which we explain the tiny scale of neutrino masses by integrating a non-invertible symmetry with the dynamical determination of the cut-off scale. In our model, we introduce three families of SU(2)$_L$ quintet fermions ($Σ_R$) and a quartet scalar ($ϕ_4$), both of which are charged under the Fibonacci fusion rule (FFR). A central feature of this construction is that the vacuum expectation value (VEV) $v_4$ of $ϕ_4$ is induced at the one-loop level via dynamical symmetry breaking. To resolve the inherent arbitrariness of the cut-off scale $Λ$ in loop-induced VEV models, we identify $Λ$ with the scale at which the SU(2)$_L$ gauge coupling $g_2$ encounters the renormalization group evolution (RGE) of $g_2$, naturally fixing the physical cut-off within the range of approximately $10^5$ to $10^7$ GeV. Quantitatively, this framework yields $v_4\sim 0.07-0.1$ GeV, which in turn leads to neutrino Yukawa couplings on the order of $10^{-3}$. This result provides a significantly more natural explanation for the neutrino mass hierarchy compared to standard seesaw models, which typically require couplings as small as $10^{-6}$ or less. Notably, our approach maintains a relatively simple particle content and does not necessitate additional (gauge) bosons for symmetry breaking.

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arXiv2026

A Minimal Realization of Radiative Dirac Neutrino Masses via a Non-Invertible Fusion Rule

We propose a minimal one-loop radiative framework for Dirac neutrino mass matrix. As a consequence, the Yukawa hierarchies among the SM fermions are alleviated, and radiative type-I seesaw framework is realized. To regulate divergent loop contributions, we introduce an effective cutoff scale $Λ\sim 100 \, {\rm TeV}$. By introducing a scalar leptoquark and imposing appropriate assignments of ising fusion rule to the particle content, we successfully realize a minimal construction. Furthermore, the presence of the leptoquark leads to rich phenomenology, including semi-leptonic decays, neutral meson mixing, lepton flavor violations and lepton $g-2$, thereby rendering the model experimentally testable. After formulating each sector of our model, we perform a comprehensive numerical analysis, taking into account all relevant experimental constraints for both normal and inverted hierarchies of neutrino masses. Our analysis reveals characteristic tendencies within the viable parameter space.

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arXiv2026

Zee-Babu model in a non-holomorphic modular $A_4$ symmetry and modular stabilization

We study a Zee-Babu neutrino model in a non-holomorphic modular $A_4$ symmetry, and we construct a model so that there are minimum free parameters (two complex parameters). We find only the normal hierarchy is allowed. Moreover, the allowed region to satisfy the neutrino oscillation data is localized at nearby $τ=ω$. The small absolute deviation plays a crucial role in fitting two mixings of $s^2_{23}$ and $s^2_{12}$. In addition, we obtain several predictions on Majorana and Dirac CP phases, and neutrinoless double beta decay as shown in our chi square numerical analysis. We also study modulus stabilization within the framework of non-supersymmetric models. In the end, we compute the expansion of modular forms at nearby $τ=ω$ in the Appendix so that one can apply them for a model and understand its analytical structure.

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arXiv2026

Radiative Dirac Neutrino Masses from Modular $S_3$ Symmetry in an Axion Model

We present a unified axion model framework that simultaneously addresses the origin of neutrino masses, leptonic flavor structure, the strong CP problem, and dark matter. The model is based on a global $U(1)_{\rm PQ}$ symmetry combined with a modular $S_3$ symmetry and is realized within a novel class of KSVZ-type axion model. Exotic colored fermions and scalars mediate radiative neutrino mass generation at the one loop-level. The PQ charge assignment forbids tree-level neutrino masses and leaves a residual $Z_3$ symmetry that ensures the Dirac nature of neutrinos. In the minimal realization, the neutrino mass matrix is of rank two, predicting one massless neutrino. Consequently, the sum of neutrino masses is constrained for both the normal and inverted hierarchies. We analyze the implications for charged lepton flavor violation and the lepton $g-2$. The axion emerging from this framework dynamically resolves the strong CP problem and accounts for the observed dark matter abundance. Notably, the predicted axion-photon coupling is within reach of upcoming experiments and consistent with existing astrophysical and cosmological bounds.

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arXiv2026

Quasi two-zero texture in Type-II seesaw at fixed points from modular $A_4$ symmetry

We study a quasi two-zero neutrino texture based on a type-II seesaw model with modular $A_4$ symmetry to evade the cosmological bound on the sum of neutrino masses while keeping some predictability in the neutrino sector. Working on three fixed points for modulus, we discuss predictions of the model and show the allowed points satisfying the cosmological bound on neutrino mass from both CMB and CMB+BAO data.

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arXiv2026

Two-loop rainbow neutrino masses in a non-invertible symmetry

We propose two-loop rainbow type of the neutrino mass model via ${Z}_2$ gauging of ${Z}_6$ non-invertible symmetry in which we introduce three families of isospin doublet vector-like fermions, heavy right-handed neutrinos and isospin doublet and singlet bosons. All new fields, which have nonzero charges under the non-invertible symmetry, can be dark matter candidates, since the non-invertible symmetry possesses a remnant ${Z}_2$ symmetry that plays a role in assuring the stability of our dark matter candidate. Even though the non-invertible symmetry is dynamically broken at one-loop level, its violation does not affect our scenario. In this paper, we especially consider the lightest mode of the neutral components in the doublet vector-like fermions as our main dark matter candidate. The dark matter is potentially degenerated to the other two families of neutral fermions, since the mass difference is induced at one-loop level. Thus, we consider our dark matter candidate in rather simpler co-annihilation system among their particles. Considering all the constraints of neutrino oscillation data, lepton flavor violations, muon $g-2$ and the relic density of dark matter, we perform the numerical analysis and show some allowed regions for these phenomenology. Due to our dark matter nature, the sum of neutrino masses in case of normal hierarchy is larger than that in case of inverted hierarchy, which is opposite situation compared with typical active neutrino models.

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arXiv2025

A novel realization of linear seesaw model in a non-invertible selection rule with the assistance of $\mathbb Z_3$ symmetry

We propose a novel realization of linear seesaw model in a non-invertible selection rule with the assistance of $\mathbb Z_3$ symmetry. In our framework, Dirac mass matrices are generated at one-loop level, dynamically breaking the non-invertible symmetry while the symmetry is invariant under the tree-level. In addition to the active neutrino masses, the model exhibits rich and testable phenomenology such as non-unitarity bound, lepton flavor violations, lepton anomalous magnetic moment, and dark matter candidate. After describing our model, we carry out numerical analysis and show some results for our physical parameters.

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arXiv2025

Radiative neutrino mass models from non-invertible selection rules

We apply non-invertible selection rules coming from a fusion algebra to radiative neutrino mass models where fields are labeled by the elements in the algebra. Since non-invertible selection rules only hold at tree level, radiative corrections naturally explain the origin of tiny neutrino masses. Furthermore, a remnant symmetry of the fusion algebra protects the stability of dark matter, which is conventionally imposed in radiative neutrino models. We also find that interesting neutrino mass textures are realized by assigning fields to family-dependent elements in the algebra.

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arXiv2025

Radiative inverse seesaw model with hidden $U(1)$ gauge symmetry enhancing lepton $g-2$

We propose a new inverse seesaw model based on hidden local $U(1)$ symmetry framework where inverse seesaw mechanism is induced at one loop level. A Majorana mass term of singlet fermion is forbidden by the $U(1)$ symmetry and it is generated at one-loop level by introducing relevant particle contents to get loop diagram, inducing inverse seesaw mechanism. The same particle contents also contribute to lepton magnetic(electric) dipole moment and lepton flavor violating decays without chiral suppression. We can then obtain sizable muon anomalous magnetic dipole moment that accommodate with deviation from the standard model prediction. The constraints from lepton flavor violating decays and electron magnetic(electric) dipole moment are also discussed to explore testability of the model.

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