arXiv subjects

Nobuchika Okada

Publications and source records attributed to Nobuchika Okada.

At least 19 recordsLinked to original sources

arXiv2026

Endothermic Z'-Portal Dark Matter: LZ-LHC Complementarity

Motivated by the $2.6σ$ high-energy recoil event recently reported by the LUX-ZEPLIN (LZ) Collaboration, we consider an endothermic $Z^\prime$-portal Majorana dark matter framework and discuss the complementarity between the LZ event and LHC searches for a $Z^\prime$ resonance. As a concrete realization, we consider a gauged U(1) ${B-L}$ extension of the Standard Model. The phenomenology of the framework is essentially controlled by two free parameters: the U(1) ${B-L}$ gauge coupling $g_{BL}$ and the $Z^\prime$ boson mass $m_{Z^\prime}$. For a fixed $m_{Z^\prime}$, the observed dark matter abundance requires the dark matter mass to be near the $Z^\prime$ resonance, $m_{\rm DM} \sim m_{Z^\prime}/2$, and sets a lower bound on $g_{BL}$. Complementarily, LHC searches for a $Z^\prime$ resonance set an upper bound on $g_{BL}$. The parameter space allowed by the dark matter abundance and LHC constraints can account for the recent LZ event. The synergy between future $Z^\prime$ resonance searches at the High-Luminosity LHC and the LZ experiment may provide a test of this framework.

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arXiv2026

Inelastic $B-L$ scalar dark matter and the LUX-ZEPLIN event

We show that our previously proposed inelastic scalar dark matter (DM) model in gauged $U(1)_{B-L}$ symmetry naturally accounts for the $2.6σ$ high-energy recoil event recently reported by the LUX-ZEPLIN (LZ) collaboration. To satisfy the thermal relic abundance, TeV-scale scalar DM $S$ with $m_S \simeq m_{Z'}/2$ for resonant annihilation and the gauge coupling constant $g_{B-L} \sim 0.5$ are required, where $m_{Z^\prime}$ is the mass of $U(1)_{B-L}$ gauge boson $Z^{\prime}$. The resulting predicted DM-nucleon inelastic cross section of $σ_\mathrm{SI} \sim 10^{-45}$ cm${}^2$ is in excellent agreement with the LZ event. This scenario can be decisively tested for future collider searches for $Z^\prime$ boson resonance.

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arXiv2026

Disentangling new physics with quantum entanglement in $t\bar{t}$ production at future lepton colliders

We investigate quantum entanglement and Bell-inequality violation in top-antitop pair production at future lepton colliders such as the International Linear Collider (ILC) and multi-TeV muon colliders. Within the Standard Model (SM), the process proceeds through $s$-channel $γ$ and $Z$ exchange and exhibits characteristic spin-correlation patterns that encode a non-trivial amount of entanglement. We then examine how these features are modified in several well-motivated extensions of the SM:(i) a neutral scalar mediator that couples to charged leptons and top quarks via Yukawa interactions and contributes as an additional $s$-channel exchange; (ii) the minimal gauged $U(1)_{B-L}$ model, which introduces a new neutral gauge boson $Z'$ coupling vectorially to SM fermions; and (iii) a Randall-Sundrum scenario, in which the exchange of massive Kaluza-Klein gravitons arising from a warped extra dimension induces additional spin-dependent interactions. For all cases, we evaluate quantum-information observables, including the entanglement marker, the concurrence, and the maximal Clauser-Horne-Shimony-Holt parameter, and study their dependence on the center-of-mass energy, scattering angle, and model parameters. We find that, relative to the SM expectation, the entanglement is typically reduced in the scalar-mediator scenario, while sizable deviations can arise in the $U(1)_{B-L}$ and Randall-Sundrum cases for phenomenologically relevant regions of parameter space. These results demonstrate the potential of quantum-information observables as sensitive probes of new particles and their interaction patterns in future lepton colliders.

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arXiv2026

Discovery prospects of a singly-charged scalar at $μ$TRISTAN

In this article, we study the associated production of a singly-charged ($Δ^+$) scalar along with a $W^+$ boson in the newly proposed $μ^+μ^+$ collider (also known as $μ$TRISTAN) at $\sqrt{s} = 2~$ TeV. Such a singly-charged scalar is naturally accommodated in an extremely well-motivated neutrino mass model, namely, the Type-II seesaw model. This model, beside providing a viable explanation of neutrino mass generation, also allows for lepton flavor violating (LFV) processes. Since LFV processes are not allowed in the Standard Model (SM), we focus on the discovery prospect of the singly-charged scalar in the Type-II seesaw model at $μ$TRISTAN through a LFV process, owing to the advantage of this process being free of any SM background. Additionally, this article also proposes a method to indicate if the underlying theory follows a Normal or an Inverted hierarchy depending on the distribution of lepton flavors in the final state.

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arXiv2026

Stochastic gravitational wave spectrum from cosmic string emitting gauge bosons and Majorana fermions

The effect of particle radiation on the spectrum of the stochastic gravitational wave background (SGWB) from cosmic strings is studied. We consider cosmic strings in an Abelian-Higgs model coupling with Majorana fermion whose mass is generated by the Higgs field, motivated by a gauged $U(1)_{B-L}$ model, in which the Majorana fermions are identified with right-handed neutrinos. Taking the energy loss by particle radiation into account, we evaluate the resultant SGWB spectrum and demonstrate the emergence of very high frequency cutoff due to the particle radiation.

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arXiv2026

A Model-Independent Approach to First-Order Phase Transitions, Gravitational Waves, and Primordial Magnetic Fields

We employ a model-independent Effective Field Theory (EFT) to analyze the possibility of a strong First-Order Phase Transition (FOPT) in extensions Beyond the Standard Model (BSM). We find that sizable deviations in the Higgs cubic and quartic interactions that are still allowed experimentally could lead to a strong FOPT, whereas the Higgs interactions to the top quark yield a weak FOPT. We also study the Gravitational Wave (GW) power spectra corresponding to the strong FOPT and find that they could be detectable in future experiments. In particular, we find that deformations of the Higgs quartic coupling have the dominant impact on the FOPT, with a GW signal that could be probed by a number of future experiments, such as LISA, BBO, and DECIGO. We also study the magnetic field produced by the corresponding FOPT and find that it could explain the primordial magnetic field puzzle. We find that for the size of deformations that could induce a strong FOPT, a scale of NP can be as low as $\sim 4\text{--}5~\text{TeV}$ for deformations in the Higgs cubic coupling, and $\sim 9\text{--}11~\text{TeV}$ for deformations in the Higgs quartic coupling. This highlights the synergy between collider searches and GW experiments in probing the Higgs couplings, specifically the Higgs quartic coupling.

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arXiv2026

Good flavor search in SU(5): a machine learning approach

We revisit the fermion mass problem of the $SU(5)$ grand unified theory using machine learning techniques. The original $SU(5)$ model proposed by Georgi and Glashow is incompatible with the observed fermion mass spectrum. Two remedies are known to resolve this discrepancy, one is through introducing a new interaction via a 45-dimensional field, and the other via a 24-dimensional field. We investigate which modification is more beautiful, defining the beauty as proximity to the original Georgi-Glashow $SU(5)$ model. Our analysis shows that, in both supersymmetric and non-supersymmetric scenarios, the model incorporating the interaction with the 24-dimensional field is more beautiful under this criterion. We then generalise these models by introducing a continuous parameter $y$, which takes the value 3 for the 45-dimensional field and 1.5 for the 24-dimensional field. Numerical optimisation reveals that $y \approx 0.8$ yields the closest match to the original $SU(5)$ model, indicating that this value corresponds to the most beautiful model according to our definition.

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arXiv2026

Leptogenesis and neutrino mass with one right-handed neutrino and Higgs inflaton

We propose a novel and minimal setup where the observed baryon asymmetry of the Universe and neutrino oscillation data can be satisfied with only one right-handed neutrino (RHN) and a second Higgs doublet with the latter being also responsible for driving cosmic inflation. While inflation is realised via non-minimal coupling of the Higgs to gravity, baryon asymmetry is generated via Affleck-Dine leptogenesis. Due to the presence of only two new fields beyond the standard model (BSM), the proposed setup remains very predictive with only a small allowed parameter space consistent with the PLANCK 2018 and ACT 2025 data simultaneously. The preferred mass spectrum of the BSM particles also keeps the detection prospects alive at terrestrial experiments.

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arXiv2026

Signatures of Long-Lived Heavy Neutral Leptons from Neutrinophilic Charged Higgs Pair Production at the LHC

In the neutrinophilic Higgs doublet framework, the neutrino Dirac Yukawa couplings can be sizable because of the small vacuum expection value of the extra Higgs doublet, even for a low seesaw scale. Due to this structure, the neutrinophilic charged Higgs bosons, once created, decay dominantly into heavy neutral leptons (HNLs) and charged leptons. This is a new mechanism to produce a gauge singlet HNL without suppressed cross sections. In the standard seesaw, one HNL can be long-lived, when the lightest neutrino is sufficiently light. We investigate displaced vertex signatures of the long-lived HNLs produced from the decays of the charged Higgs pair at the high luminosity LHC. We consider one displaced vertex as well as two displaced vertices signatures and perform a dedicated simulation to identify the displaced leptons. We find that high statistical significance can be achieved for the observation of one displaced vertex for charged Higgs pair production cross section $>\mathcal{O}(1)$ fb. On the other hand, the observation of two displaced vertices is challenging even for charged Higgs pair production cross section of $\mathcal{O}(10)$ fb.

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arXiv2026

Novel and Updated Bounds on Flavor-violating Z Interactions in the Lepton Sector

We investigate the experimental bounds on the Flavor-Violating (FV) couplings of the $Z$ boson to the charged leptons. In addition to the direct LHC searches for FV $Z$ decays to leptons, we investigate indirect bounds from flavor-conserving $Z$ decays to leptons at 1-loop, bounds from LEP searches, Electroweak Precision Observables (EWPO), $\ell_{i}\to\ell_{j}γ$ decays, $\ell_{i}\to3\ell_{j}$ decays, $\ell_{i}\to\ell_{j}+\text{inv.}$ decays, FV meson decays to leptons, FV $τ$ decays to $μ(e)$ + mesons, muon conversion in nuclei, and from muonium-antimuonium oscillations. For FV $Z$ couplings to $τμ$, we find that $τ\toμγ$ yields the strongest bounds, with a level reaching $\mathcal{O}(10^{-5})$, followed by bounds from $τ\to3μ(μee)$. For FV $Z$ couplings to $τe$, we find that the strongest bounds arise from the decay $τ\toμμe$, reaching $\mathcal{O}(10^{-7})$ as well, with bounds from $τ\to3e$ also yielding strong bounds. For FV $Z$ couplings to $μe$, we find that the strongest bounds are obtained from the decay $μ\to3e$, reaching $\mathcal{O}(10^{-11})$, with bounds from $μ\to eγ$, muon conversion, $K_{L}^{0}\rightarrowμe$ and $μ\to e+\text{inv.}$ also providing strong bounds. We also study projections from future experiments, such as the FCC-ee, Belle II and the Mu2e experiment. For the $Z$ couplings to $τμ$, we find that future experiments could improve the bound to $\mathcal{O}(10^{-6})$, whereas for the $Z$ couplings to $τe$, we find that future experiments could improve the bound to $\mathcal{O}(10^{-8})$, and for the $Z$ couplings to $μe$, they could improve the bound to $\mathcal{O}(10^{-13})$

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arXiv2026

Novel and Updated Bounds on Flavor-Violating Z Interactions in the Quark Sector

We derive bounds on the flavor-violating (FV) couplings of the $Z$ boson to quarks and present future sensitivity projections. Our analysis shows that the current bounds on the FV couplings are $\mathcal{O}(10^{-9})$ for the $Z$ couplings to $cu$ and $sd$, $\mathcal{O}(10^{-7})$ for $bd$, $\mathcal{O}(10^{-6})$ for $bs$, and $\mathcal{O}(10^{-3})$ for $tu$ and $tc$. Overall, low-energy flavor experiments provide significantly stronger constraints on these FV couplings than current collider searches.

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arXiv2026

Novel Bounds From The Weak Gravity and Festina Lente Conjectures

We demonstrate that the Weak Gravity Conjecture (WGC) and the Festina-Lente Conjecture (FLC) yield novel bounds on fifth force searches and milli-Charged Particles (mCPs), as well as on the scale of inflation and on the effective Higgs quartic interaction. In particular, we find that combining the FLC with inflation leads to stronger bounds on mCPs than what the simple application of the FLC provides. Furthermore, we have explored the implications of naturalness on both the FLC and WGC, and have found that these conjectures place a lower limit on the charge of a $U(1)$ gauge group.

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arXiv2026

Radiative Seesaw Model with Baryon Number Violation and Upper Limit on Neutron-anti-Neutron Transition Time

The minimal scotogenic model where small neutrino masses arise via radiative seesaw, is known to provide a unified framework for neutrino mass and origin of matter via leptogenesis. However if the inflation reheat temperature of the universe is below the sphaleron reheating temperature, then leptogenesis fails and one way to understand the origin of matter would be to add an effective interaction involving the right handed neutrino (RHN) $N$ of the form $\frac{1}{Λ^2}N u_Rd_Rd_R$. This model can lead to observable neutron-anti-neutron ($n-\bar{n}$) oscillation. We show that if RHNs are produced non-thermally, we can get a cosmological upper limit on the transition time $τ_{n-\bar{n}}$, which is within the reach of the planned ESS HIBEAM/NNBAR experiment. The proton stability is guaranteed by the scotogenic $Z_2$ invariance, which prevents the appearance of the Dirac mass term for the neutrino

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arXiv2026

Unified Origin of Curvature Perturbation and Baryon Asymmetry of the Universe

We propose a unified framework that describes both the curvaton mechanism for generating primordial density fluctuations and the Affleck-Dine (AD) mechanism for baryogenesis. By introducing a complex scalar field (AD field) carrying a baryon/lepton number and its potential consisting of quadratic and quartic terms with a small baryon/lepton-number-violating mass term, we investigate the evolution of the scalar field during the radiation-dominated era following inflation. We set the initial conditions such that the quartic term dominates the scalar potential, and the angular component of the AD field is non-zero. We focus on a scenario where the AD field sufficiently dominates the energy density of the universe before its decay. We show that the radial component of the AD field can be identified with the curvaton to solely produce the Planck normalized scalar power spectrum while the evolution of the angular component is crucial for generating the observed baryon asymmetry of the universe. Additionally, we find that the amplitude of scalar bispectrum $f_{NL}$ is negative, which is consistent with the current Planck data and testable in future observations such as CMB-S4, LiteBIRD, LSS, and 21-cm experiments. In our estimation of the scalar power spectrum and bispectrum, we develop a novel analytical scheme for computing scalar fluctuations based on the $δN$ formalism, which allows us to deal with the evolution of curvaton with polynomial potential more accurately in comparison to the existing analytical methods.

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arXiv2025

Impact of Non-Thermal Leptogenesis with Early Matter Domination on Gravitational Waves from First-order Phase Transition

We study the impact of non-thermal leptogenesis on the spectrum of gravitational waves (GWs) produced by a strong first-order phase transition in the early Universe. We consider a scenario in which a heavy scalar field, $ϕ$, dominates the energy density of the early Universe and decays into heavy right-handed neutrinos (RHNs). The subsequent decay of RHNs generates a lepton asymmetry, which is partially converted into the observed baryon asymmetry via the sphaleron process. The $ϕ$-dominated era and the entropy injection from the decays of $ϕ$ and RHNs leave characteristic imprints on the GW spectrum, such as damping and modified frequency dependence, that distinguish it from the standard cosmological evolution. We identify the parameter space in which non-thermal leptogenesis is successful, leading to distinctive GW spectral features. We show that these GW signals can fall within the sensitivity ranges of future detectors such as ET, DECIGO and BBO. If observed, they would provide valuable insights into the thermal history and dynamics of the early Universe.

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arXiv2025

Nambu-Goldstone boson phenomenology in Domain-Wall Standard Model

We investigate the Domain-Wall Standard Model (DWSM), a five-dimensional framework in which all Standard Model (SM) particles are localized on a domain wall embedded in a non-compact extra spatial dimension. A distinctive feature of this setup is the emergence of a Nambu-Goldstone (NG) boson, arising from the spontaneous breaking of translational invariance in the extra dimension due to the localization of SM chiral fermions. This NG boson couples via Yukawa interactions to SM fermions and their Kaluza-Klein (KK) excitations. We study the phenomenology of this NG boson and derive constraints from astrophysical processes (supernova cooling), Big Bang Nucleosynthesis (BBN), and collider searches for KK-mode fermions at the Large Hadron Collider (LHC). The strongest limits arise from LHC data: we reinterpret existing mass bounds on squarks and sleptons in simplified supersymmetric models (assuming a massless lightest neutralino), as well as limits on exotic hadrons containing long-lived squarks or long-lived charged sleptons in the regime of extremely small Yukawa couplings. From this analysis, we obtain a conservative lower bound of 1 TeV on the masses of KK-mode quarks and charged leptons. Finally, we discuss the prospects for producing KK-mode fermions at future high-energy lepton colliders and outline strategies to distinguish their signatures from those of sfermions.

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arXiv2025

Non-perturbative Origin of Electroweak Scale via Higgs-portal: Dyson-Schwinger in Conformally Invariant Scalar Sector

We investigate conformally extended Standard Model with a hidden scalar $ϕ$. It is shown that due to non-perturbative dynamics in the hidden sector, $ϕ$ develops a vacuum expectation value (vev) in the form of a mass gap which triggers the electroweak symmetry breaking (EWSB) and dynamically generates the SM Higgs boson mass. For estimating the non-perturbatively generated mass scale, we solve the hierarchy of Dyson-Schwinger Equations in form of partial differential equations using the exact solution known via a novel technique developed by Bender, Milton and Savage. We employ Jacobi Elliptic function as exact background solution and show that the mass gap that arises in the hidden sector can be transmuted to the EW sector, expressed in terms of Higgs-portal mixed quartic coupling $β$ and self interaction quartic coupling $λ_ϕ$ of $ϕ$. We identify the suitable parameter space where the observed SM Higgs boson can be successfully generated . Finally, we discuss how this idea of non-perturbative EW scale generation can serve as a new starting point for better realistic model building in the context of resolving the hierarchy problem in the Standard Model.

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arXiv2025

Smooth hybrid inflation in light of ACT DR6 data

Smooth hybrid inflation is a hybrid inflation model which is free from topological defects and predicts the density perturbation with the spectral index of about $0.97$. We show that the prediction on the spectral index is robust regardless the power of nonrenormalizable terms and meets with the latest results reported by Atacama Cosmology Telescope. Viable scenarios for baryogenesis and dark matter are also discussed.

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