arXiv subjects

Nobuchika Okada

Publications and source records attributed to Nobuchika Okada.

At least 37 records · Page 2Linked to original sources

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv2025

Anomalies in String-inspired Non-local Extensions of QED

We investigate anomalies in the class of non-local field theories that have been proposed as an ultraviolet completion of 4-D Quantum Field Theory (QFT) with generalizing the kinetic energy operators to an infinite series of higher derivatives inspired by string field theory and ghost-free non-local approaches to quantum gravity. We explicitly calculate the vector and chiral anomalies in a string-inspired non-local extension of QED. We show that the vector anomaly vanishes as required by gauge-invariance and the Ward identity. On the other hand, although the chiral anomaly vanishes to the leading order with massless fermions, it nonetheless does not vanish with the massive fermions and we calculate it to the leading order in scale of non-locality. We also calculate the non-local vector and axial currents explicitly, and present an illustrative example by applying our results to the decay of π_0 \rightarrow γγ.

hep-th↗

arXiv2025

Non-perturbative Origin of the Electroweak Scale: RGE in Strongly-coupled Dark Gauge Theories via Dyson-Schwinger

We propose a novel pathway to generate the electroweak scale (EW) via non-perturbative dynamics of a dark gauge sector based on the SU(N) gauge group. Imposing the scale invariance of the theory, we investigate the electroweak symmetry breaking (EWSB) which is triggered dynamically via the condensation of gauge fields. Instead of the usual dimension-4 triggered breaking, a dimension 6 term (with Wilson coefficient compatible with SMEFT bounds) coupling the Higgs boson and the Yang-Mills field quadratic term in the Lagrangian provides feedback from the gauge to the Higgs sector. We provide a novel method to estimate a non-perturbative EW scale generation using the exact solution of a truncated set of the background equations of motion in Yang-Mills theory in terms of Jacobi elliptic functions and the exact beta-function valid in the strongly coupled regimes via the Dyson-Schwinger approach. Particularly, we find an analytical result for the Renormalization Group Equation (RGE) of the gauge coupling in the $SU(N)$ sector in the strongly-coupled regime. The dynamics studied in this paper pave the way to a more realistic model building with possible resolution to the hierarchy problem and, in general, dynamical generation of scales.

hep-ph↗

arXiv2025

Kination-like Era Driven by the Effective Inflaton/Higgs Potential

Based on the minimal $U(1)_X$ extended Standard Model, we explore cosmic inflation where the $U(1)_X$ Higgs field serves as the inflaton. We demonstrate that a stiff era with an equation of state $w > 1/3$ can emerge during the inflaton's oscillatory phase after inflation, driven by the Coleman-Weinberg potential of the inflaton, arising due to radiative corrections. This leads to significant modulation and enhancement of the irreducible stochastic gravitational wave (GW) background from inflation, deviating from the conventional scale-invariant spectrum. Such a distinct GW spectrum could be detectable by next-generation GW interferometer missions, such as U-DECIGO. In our framework, the GW spectrum depends on the $U(1)_X$ gauge coupling and the mass of the $U(1)_X$ gauge boson ($Z^\prime$). As a result, future GW observations and $Z^\prime$ boson resonance searches at high-energy collider experiments are complementary to one another.

astro-ph.CO↗

arXiv2025

Experimentally distinguishable origin for electroweak symmetry breaking

We consider a classically conformal $U(1)$ gauge extension of the Standard Model (SM), in which the $U(1)$ gauge symmetry is radiatively broken by the Coleman-Weinberg mechanism. This breaking triggers the electroweak (EW) symmetry breaking through a mixed quartic coupling between the $U(1)$ Higgs field and the SM Higgs doublet. For two Higgs boson mass eigenstates after the symmetry breaking, $h_1$ (SM-like Higgs boson) and $h_2$ (SM singlet-like Higgs boson), we calculate the Higgs boson trilinear coupling ($g_{h_{1} h_{2} h_{2}}$) in the model by setting the Higgs boson mass spectrum to be $M_{h_1} > 2 M_{h_2}$. For a common Higgs mass spectrum and mixing angle between two Higgs fields, we find that $g_{h_{1} h_{2} h_{2}}$ in the classically conformal model is highly suppressed compared to that calculated for the conventional Higgs potential, where the $U(1)$ and EW symmetry breaking originate from the negative squared masses for the Higgs fields at the tree-level. Thus, this coupling suppression is a striking nature of the radiative origin of EW symmetry breaking. We then consider how to distinguish this origin at the proposed International Linear Collider (ILC) via precise measurements of anomalous SM Higgs boson couplings and the search for anomalous SM Higgs boson decay $h_1 \rightarrow h_2 h_2$ followed by $h_2 \to b \bar{b}$. We conclude that once the anomalous couplings are measured at the ILC, the observation of the anomalous Higgs boson decay is promising in the conventional Higgs potential, while this decay process is highly suppressed and undetectable for the classically conformal model.

hep-ph↗

arXiv2025

Split supersymmetry and hybrid inflation in light of Atacama Cosmology Telescope DR6 data

Inspired by the recent measurement of the scalar spectral index, $n_s = 0.9743 \pm 0.0034$, by the Atacama Cosmology Telescope (ACT) DR6 data, we present an update on the split supersymmetry hybrid inflation model, also known as $μ$-term hybrid inflation. The model employs a canonical Kähler potential but incorporates an additional renormalizable term in the superpotential $W$, which yields the MSSM $μ$-term following supersymmetry breaking. This additional term in $W$ is responsible for a high reheat temperature, $T_r \gtrsim 10^{12}$ GeV, and consequently the necessity of split supersymmetry in this class of models. The predicted scalar spectral index is in excellent agreement with the ACT measurement and $r$, the tensor to scalar ratio, is estimated to be less than or of order $10^{-2} -10^{-3}$. For the running of the scalar spectral index we find $|dn_s/d \ln k| = O(10^{-4})$. With $T_r \gtrsim 10^{12}$ GeV, leptogenesis is readily implemented in this class of models. A wino-like LSP with mass of around 2 TeV is a plausible dark matter candidate.

hep-ph↗

arXiv2025

Discovery potential of a long-lived partner of inelastic dark matter at MATHUSLA in $U(1)_{X_3}$ extension of the standard model

We investigate the discovery potential at the MATHUSLA experiment of a long-lived particle (LLP), which is the heavier state of inelastic scalar dark matter (DM) in third generation-philic $U(1)$ ($U(1)_{X_3}$) extension of the standard model. Since the heavier state and DM state form the complex scalar charged under the $U(1)_{X_3}$, it is natural that the heavier state $P$ is almost degenerate with the DM state and hence long-lived. We find that third generation-philic right-handed $U(1)$, $U(1)_{R_3}$, model is the most interesting, because third generation-philic models are less constrained by the current experimental results and right-handed $U(1)$ interactions leave visible final decay products without produing neutrinos. For a benchmark of the model parameters consistent with the current phenomenological constraints, we find that the travel distance of the LLP can be $\mathcal{O}(100)$ m and the LLP production cross section at the 14 TeV LHC can be $\mathcal{O}(10)$ fb. Thus, we conclude that the LLP can be discovered at the MATHUSLA with a sufficiently large number of LLP decay events inside the MATHUSLA detector.

hep-ph↗

arXiv2025

Analytic formulation of Leptogenesis with neutrino oscillation data employing the general parametrization for neutrino mass matrix

The observed neutrino oscillations and baryon asymmetry, unexplained by the Standard Model (SM), can both be accounted for by extending the SM to include Majorana right-handed neutrinos (RHNs). Tiny neutrino masses naturally arise through the Type-I seesaw mechanism, which involves lepton number violation. Meanwhile, the baryon asymmetry can be generated via leptogenesis, where the out-of-equilibrium decay of RHNs produces a lepton asymmetry that is partially converted into a baryon asymmetry through sphaleron processes. The Dirac Yukawa couplings play the crucial role for both Type-I seesaw and leptogenesis. In this work, we derive an analytic expression for the CP asymmetry parameter in a general parametrization. Focusing on a hierarchical RHN mass spectrum, we evaluate the lowest mass of the lightest RHN that reproduce both neutrino oscillation data and the observed baryon asymmetry. We study the case with two and three generations of RHNs for both thermal and non-thermal leptogenesis scenarios. Besides the standard Type-I seesaw involving SM Higgs doublet, we also examine the Type-I seesaw with a new neutrinophillic Higgs doublet. In this case for non-thermal leptogenesis, the minimum value for the lightest RHN mass can be as low as sphaleron decoupling temperature.

hep-ph↗

arXiv2025

Possible origin of the KM3-230213A neutrino event from dark matter decay

We study the possibility of the highest energy neutrino event with 220 PeV energy, detected recently by the KM3NeT experiment to be originating from heavy dark matter (DM) decay. Considering a heavy right handed neutrino (RHN) DM for illustrative purpose, we show that DM mass of 440 PeV, can explain the observed flux. The required DM lifetime to produce the best-fit value of the neutrino flux saturates the existing gamma-ray bounds. Due to the large uncertainty in the flux, it is possible to explain the KM3NeT event from RHN DM decay at $3σ$ confidence level (CL) while being in agreement with gamma-ray bounds and non-observation of similar events at IceCube. While we consider a gauged $B-L$ scenario where DM relic can be generated due to other interactions, we also briefly discuss some alternate DM possibilities where the gamma-ray bounds can be alleviated compared to the minimal RHN DM discussed here.

hep-ph↗

arXiv2025

Testing neutrino mass hierarchy under type-II seesaw scenario in $U(1)_X$ from colliders

The origin of tiny neutrino mass is a long standing unsolved puzzle of the Standard Model (SM), which allows us to consider scenarios beyond the Standard Model (BSM) in a variety of ways. One of them being a gauge extension of the SM may be realized as in the form of an anomaly free, general $U(1)_X$ extension of the SM, where an $SU(2)_L$ triplet scalar with a $U(1)_X$ charge is introduced to have Dirac Yukawa couplings with the SM lepton doublets. Once the triplet scalar developes a Vacuum Expectation Value (VEV), light neutrinos acquire their tiny Majorana masses. Hence, the decay modes of the triplet scalar has a direct connection to the neutrino oscillation data for different neutrino mass hierarchies. After the breaking of the $U(1)_X$ gauge symmetry, a neutral $U(1)_X$ gauge boson $(Z^\prime)$ acquires mass, which interacts differently with the left and right handed SM fermions. Satisfying the recent LHC bounds on the triplet scalar and $Z^\prime$ boson productions, we study the pair production of the triplet scalar at LHC, 100 TeV proton proton collider FCC, $e^-e^+$ and $μ^-μ^+$ colliders followed by its decay into dominant dilepton modes whose flavor structure depend on the neutrino mass hierarchy. Generating the SM backgrounds, we study the possible signal significance of four lepton final states from the triplet scalar pair production. We also compare our results with the purely SM gauge mediated triplet scalar pair production followed by four lepton final states, which could be significant only in $μ^- μ^+$ collider.

hep-ph↗

arXiv2025

Matter-Dark Matter Coincidence and Mirror World

Why matter and dark matter contents of the universe are of the same order of magnitude, is one of the puzzles of modern cosmology. At the face of it, this would seem to point towards a basic similarity between matter and dark matter, suggesting perhaps the widely discussed mirror world picture as an ideal setting for a discussion of this issue. Here we outline a new and simple mirror world scenario to explain this puzzle. Our model uses Affleck-Dine mechanism to generate baryon asymmetry and dark matter relic density leading to an asymmetric dark matter picture. We find that for a certain parameter range of the model, the mirror electron is the unique possibility for dark matter whereas in the complementary parameter range, the mirror baryons constitute the dark matter. In either case, the mirror photon must have mass in the MeV range for consistency with observations. For the case of mirror electron dark matter, the model predicts a lower bound on the amount of dark radiation i.e. $ΔN_{eff} \geq 0.007$.

hep-ph↗

arXiv2024

Truth, beauty, and goodness in grand unification: a machine learning approach

We investigate the flavour sector of the supersymmetric $SU(5)$ Grand Unified Theory (GUT) model using machine learning techniques. The minimal $SU(5)$ model is known to predict fermion masses that disagree with observed values in nature. There are two well-known approaches to address this issue: one involves introducing a 45-representation Higgs field, while the other employs a higher-dimensional operator involving the 24-representation GUT Higgs field. We compare these two approaches by numerically optimising a loss function, defined as the ratio of determinants of mass matrices. Our findings indicate that the 24-Higgs approach achieves the observed fermion masses with smaller modifications to the original minimal $SU(5)$ model.

hep-ph↗

arXiv2024

Aspects of Non-local QED and the Weak Gravity Conjecture

We use the Weak Gravity Conjecture (WGC) to investigate the impact of charge dequantization arising from non-local QED on the scale of non-locality of neutrinos. We find this scale to be <= 87 TeV, which could be probed in future colliders. We also investigate the electric force and potential in non-local QED and use them to reformulate the Weak Gravity Conjecture (WGC) in non-local QED.

hep-ph↗

arXiv2024

Non-perturbative Origin of the Electroweak Scale with Dyson-Schwinger: Fermionic Mass Gap and Higher-order Excitations

We study interaction of a fermion field $ψ$ with a scalar field $ϕ$ and analyze the spectrum of the theory obtained in this way. It is shown that due to non-perturbative dynamics in the hidden fermion 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 fermion sector can be transmuted to the EW sector, expressed in terms of fermion mass and the $ϕ$ self-quartic. We identify the suitable parameter space where the observed SM Higgs boson can be successfully generated .

hep-ph↗

arXiv2024

The Generalized Scalar Weak Gravity Conjecture and its Implications

We propose a generalized formulation of the Scalar Weak Gravity Conjecture (SWGC) based on the analogy with the derivation of the Gauge Weak Gravity Conjecture (GWGC). We discuss some phenomenological implications of this Generalized SWGC, including the scale of New Physics (NP) when applied to the SM Higgs sector, and the bounds on the axion's couplings to fermions and the photon when applied to the axion. The Generalized SWGC constraints rule out most of the parameter space of axion-nucleon couplings, leaving only a tiny parameter space.

hep-ph↗

arXiv2024

Double Inflation in Classically Conformal $B-L$ Model

It has recently been shown in Ref. [1] that the double-inflation scenario based on the Coleman-Weinberg potential can successfully generate primordial black holes (PBHs) with the inflationary predictions consistent with the Planck measurements. These PBHs can play the role of dark matter in our universe. In this paper, we propose the classically conformal minimal $B-L$ model as an ultra-violet (UV) completion of the scenario. In our model, the $B-L$ Higgs field is identified with the inflaton and the electroweak symmetry breaking is triggered by the radiative $B-L$ symmetry breaking with the Coleman-Weinberg potential. We show that this UV completion leads to a viable cosmological history after the double inflaton: the universe is reheated via inflaton decay into right-handed neutrinos whose mass is determined consistently by a relation between the number of e-folds and reheating temperature. Using the general parameterization for neutrino Dirac Yukawa couplings through the seesaw mechanism and the neutrino oscillation data, we also show that the observed baryon asymmetry of the universe is successfully reproduced by either resonant leptogenesis or non-thermal leptogenesis. Based on the scalar power spectrum shown in Ref. [1], we evaluate the scalar induced gravitational wave spectrum, which can be tested by various proposed gravitational wave observatories like BBO, DECIGO etc.

astro-ph.CO↗