arXiv subjects

Takaaki Nomura

Publications and source records attributed to Takaaki Nomura.

At least 19 recordsLinked to original sources

arXiv2026

Probing the Axion--Photon--Dark Photon Interaction with LHC Mono-Photons and Fermi-LAT Gamma-Ray Lines

We investigate the axion--photon--dark photon interaction at the LHC and in gamma-ray line searches. By recasting the CMS mono-photon data, we derive a $95\%$ CL exclusion reaching $g_{aγ'γ}\simeq4.8\times10^{-4}\ {\rm GeV}^{-1}$ for dark photon masses $m_{γ'}\lesssim100$ GeV, with sensitivity extending to $m_{γ'}\simeq2.3$ TeV, where the effective theory approaches its validity limit. This improves upon the existing LEP mono-photon constraint by a factor of $3$ or more, depending on the dark photon mass. The projected HL-LHC sensitivity improves by a further factor of $\sim2.7$ and extends the reach to $m_{γ'}\simeq3$ TeV. The same interaction also induces $s$-wave annihilation of axions into $γγ$. If the axion constitutes dark matter, Fermi-LAT gamma-ray line data become more constraining than the LHC for $m_a\gtrsim160$ GeV at $m_{γ'}=300$ GeV, where the on-shell collider signal is kinematically suppressed. Combining collider and gamma-ray line constraints excludes a thermal relic axion across the axion mass range considered for this benchmark.

hep-ph↗

arXiv2026

Triple $Z'$ signatures at $Z$ factories

We discuss triple $Z'$ boson signatures via the decay chain of $Z \to Z' ϕ\to Z' Z' Z'$, with a new light scalar $ϕ$, at future Z factories such as CEPC and FCC-ee. These new bosons $ϕ$ and $Z'$ naturally appear in models with a new $U(1)$ gauge symmetry which is spontaneously broken and introduced in various new physics scenarios. The branching ratio of $Z \to Z' ϕ\to Z' Z' Z'$ can be larger than $10^{-12}$, which gives $O(1)$ events at Tera-Z experiments, when a product of $g_X^{}$ (new gauge coupling) and $ζ$ ($Z$-$Z'$ mixing) is larger than around $10^{-6}$. We find that the search for $Z \to Z'Z'Z'$ can significantly improve the current bound on a kinetic mixing parameter $ε$ in the dark photon case, where $ε\gtrsim 10^{-6}$ with $g_X^{}={\cal O}(1)$ can be explored at Tera-Z experiments. We also show that a sufficiently large number of events with multi-lepton plus hadronic jets can be obtained in benchmark points, which cannot be realized by the usual decay of Z in the standard model.

hep-ph↗

arXiv2026

Effect of inelastic scalar dark matter in hidden $U(1)$ scenario after the LZ nuclear recoil

We study a hidden $U(1)_H$ extension of the Standard Model (SM) introducing two SM singlet complex scalar fields where one of them can be considered as a viable inelastic Dark Matter (DM) candidate due to its $U(1)_H$ charge being half of the charge of the other one which develops a vacuum expectation value. First we estimate a parameter region satisfying observed DM relic abundance through DM annihilation and further use these parameters to study inelastic DM-nucleus scattering to reproduce the recent high nuclear recoil energy event found by LZ collaboration. These interactions are mediated by a dark photon $(A^\prime)$ which is evolved after the spontaneous breaking of $U(1)_H$ symmetry and having a kinetic mixing with the SM photon. Hence we estimate bounds on the kinetic mixing as a function of $A^\prime$ mass and finally compare it with existing experimental bounds from different beam-dump and collider experiments. These parameters could be probed by lifetime and energy frontier experiments in future.

hep-ph↗

arXiv2026

Scotogenic Model with Non-invertible Symmetry

In the present work, the scotogenic model is constructed applying non-invertible $Z_M$ symmetries. The stability of dark matter and the scotogenic structure of the neutrino mass matrix is achieved via a novel application of non-invertible symmetry. The Scotogenic model with non-invertible symmetry is given with minimalistic content giving a one-zero structure of the neutrino mass matrix, and numerical analysis of the lepton mixing angles and physics are presented. Other relevant constraints are also studied.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv2026

Predictions of Modular Symmetry Fixed Points on Neutrino Masses, Mixing, and Leptogenesis

In recently proposed framework of non-holomorphic modular symmetry introduces the concept of negative and zero modular weight of Yukawa couplings. These Yukawa couplings are function of complex modulus $τ$, which is responsible for the CP asymmetry produced during leptogenesis. In this work, we restrict the $τ$ on the fixed points of modular symmetry rather than its fundamental domain in such manner Yukawa couplings are also get fixed. We have adopt this framework and propose a type III seesaw mechanism. The model is tested against neutrino oscillation data through a $χ^2$ analysis using NuFIT~6.1. To test the stability of these predictions, we also analyze regions near each fixed point by introducing a deviation $τ\rightarrow τ_{\rm fixed}(1 + εe^{iϕ})$ with $ε\in (0,0.1)$ and $ϕ\in (-π,π)$. Our results show that certain fixed points, along with their nearby regions, are capable of producing viable neutrino phenomenology while also generating the observed baryon asymmetry of the Universe.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv2026

Dark matter and dark radiation from chiral $U(1)$ gauge symmetry

We consider a simple model of a dark sector with a chiral $U(1)$ gauge symmetry. The anomaly-free condition requires at least five chiral fermions. Some of the fermions acquire masses through a vacuum expectation value of a Higgs field, and they are stable due to an accidental symmetry. This makes them dark matter candidates. If the dark sector was once in thermal equilibrium with the Standard Model and dark radiation constraints are included, two-component dark matter may be needed since the number of massless fermions is restricted. When the Dirac fermion is the main component of dark matter, the kinetic mixing should be around $10^{-6}$: a larger value is restricted by direct detection limits, while a smaller value prevents thermal freeze-out. If the main dark matter component is a Majorana fermion, the kinetic mixing can be larger. Still, a sub-component of Dirac fermion could produce a detectable signal in future direct detection experiments. We also discuss the possibility of testing an invisible dark photon at future lepton collider experiments, taking into account cosmological constraints.

hep-ph↗

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.

hep-ph↗

arXiv2026

Seesaw-I and II Hybrid $T^{\prime}$ Symmetric Neutrino Masses with Mass Selection Rule

The present manuscript studies a recently proposed new neutrino mixing scheme with a neutrino mass selection rule, $m_ν^{13} + m_ν^{22} + m_ν^{31} = 0$, among a neutrino mass matrix elements. The neutrino mass matrix texture is achieved by means of $T^{\prime}$ flavor discrete symmetry extension of the Standard Model gauge group. The model realizing the neutrino mixing pattern consists of a combination of type-I and II seesaw mechanisms in the minimal possible extension of the Standard Model. Stringent predictions are obtained for normal and inverted neutrino mass orderings. Some important predictions include $α_2 \approx 2π$ (best fit), $m_{\text{lightest}} \approx 3 - 4 \times 10^{-4}\,\text{eV}$, and $m_{ee}^{0νββ} \approx 3 \times 10^{-3}\,\text{eV}$ for normal neutrino mass ordering; $110^\circ < α_1 < 170^\circ$, $40(220)^\circ < α_2 < 60(240)^\circ$, $240^\circ < δ_{\text{Dirac}} < 310^\circ$, and $m_{\text{lightest}} \approx 2 \times 10^{-5} - 7 \times 10^{-3}\,\text{eV}$ for inverted neutrino mass ordering.

hep-ph↗

arXiv2025

Are 2HDMs with a gauged $U(1)$ symmetry alive?

We investigate the phenomenology of 2 Higgs doublet models (2HDMs) with a new $U(1)$ gauge symmetry, $U(1)_X$, by which flavor changing neutral currents are forbidden at tree level. As an important consequence of the spontaneous breaking of both the $U(1)_X$ and electroweak symmetries by electroweak vacuum expectation values, upper limits appear on masses of an additional gauge boson $Z'$ and extra Higgs bosons which are less than the TeV scale. In addition, the standard model (SM) like Higgs boson $h$ and a heavier Higgs boson $H$ mainly decay into a pair of $Z'$ which induces four lepton final states. These new decay modes cannot be suppressed by taking no $Z$-$Z'$ mixing and/or the Higgs alignment limit. We find that the minimum setup of these 2HDMs has been excluded by current data for four lepton searches at LHC. Such severe constraints can, however, be avoided by introducing a pair of vector-like fermions $χ$ which are singlet under the SM symmetry but charged under $U(1)_X$, and can be a candidate of dark matter. Thanks to the existence of $χ$, $Z'$ can mainly decay into $χ\barχ$ instead of SM leptons. As benchmark models, we consider the $U(1)_H$ and $U(1)_R$ models realized by fixing specific $U(1)_X$ charges, and find regions of parameter space allowed by theoretical and current experimental constraints. We clarify that $m_H \in [160, 220]$ GeV and $\tan β\in [3, 4.4]$ are allowed in the $U(1)_H$ model, while $m_H \in [160, 380]$ GeV and $\tan β\in [1.6, 4.4]$ are allowed in the $U(1)_R$ model. In both the models, the $Z'$ mass is constrained to be $100~\text{GeV} \lesssim m_{Z'} \lesssim 110$ GeV. Such a quite limited parameter space can further be explored at future collider experiments, e.g., High-Luminosity LHC and lepton colliders.

hep-ph↗

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.

hep-ph↗

arXiv2025

Neutrino mass model at a three-loop level from a non-holomorphic modular $A_4$ symmetry

We study a three-loop induced neutrino mass scenario from a non-holomorphic modular $A_4$ flavor symmetry and reach the minimum scenario leading predictions of the lepton masses, mixing angles, and Dirac and Majorana phases, which are shown through the chi square analyses. In addition, we discuss the lepton flavor violations, muon anomalous magnetic moment, lepton universality, and relic density of the dark matter candidate. And, we find that we need to extend our model if we satisfy the observed relic density of dark matter within the limit of perturbation where it can be done by adding one singlet scalar boson without changing predictions in neutrino sector.

hep-ph↗

arXiv2025

Lepton number violation from Higgs/$Z$ decays into a pair of right-handed neutrinos

We explore signatures of Lepton Number Violation (LNV) from decays of the discovered Higgs ($h$) and Z bosons into a pair of Right-Handed Neutrinos (RHNs). Due to the Majorana nature of RHNs, the final state can be the same-sign dilepton plus jets leading to 2 units of LNV. As a simple but plausible scenario, we investigate such a signal in models with a new $U(1)_X$ gauge symmetry which naturally introduces three RHNs for gauge anomaly cancellation, and is spontaneously broken down by a vacuum expectation value of an isospin singlet scalar field ($ϕ$). In this scenario, $h$ and the Z boson can decay into a pair of RHNs via the $h$-$ϕ$ mixing and the $Z$-$Z'$ mixing with $Z'$ being a new massive gauge boson, respectively. Estimating the LNV signal and corresponding backgrounds for the $\ell^{\pm} \ell^{\pm} 4j$ final states, we find bounds on the $h$-$ϕ$ mixing and the $Z$-$Z^\prime$ mixing as a function of a mass of RHNs at Higgs factories, e.g., at High-Luminosity LHC with $\sqrt{s}=$ 14 TeV, future $e^-e^+$ colliders at $\sqrt{s}=$ 250 GeV, muon colliders at $\sqrt{s}=$ 125 GeV as well as at Z factories, e.g., CEPC and FCC-ee at $\sqrt{s}=$ 91.2 GeV. We also discuss limits on the scalar mixing at $e^-μ^+$ and $μ^+ μ^+$ collisions ($μ$TRISTAN) with $\sqrt{s}=$346 GeV/775 GeV and 2 TeV, respectively.

hep-ph↗

arXiv2025

Testing tree level TeV scale seesaw scenarios in $μ$TRISTAN

We investigate TeV scale seesaw scenarios at $μ^+ e^-$ and $μ^+ μ^+$ colliders in the $μ$TRISTAN experiment. In minimal type-I seesaw scenario we consider two generations of Standard Model (SM) singlet heavy Majorana type Right Handed Neutrinos (RHNs) which couples with SM gauge bosons through light-heavy neutrino mixing. We discuss the prospects of probing heavy neutrinos via the processes such as $μ^+e^-\to νN_i\to e^+ j jν$ or $μ^- j jν$ for $\sqrt{s}=346$~GeV and $1\text{ ab}^{-1}$ luminosity. Studying these process, we estimate limits on the light-heavy neutrino mixing angles as a function of heavy neutrino mass, which could be two orders of magnitude stronger than electroweak precision data. Further, we study the effect of doubly charged scalar boson $(H^{++})$ from the type-II seesaw scenario in $μ^+ μ^+$ collision at $\sqrt{s}=2$ TeV. In this case we consider $μ^+ μ^+ \to \ell_i^+ \ell_j^+$ and $μ^+ μ^+ \to H^{++} Z/ γ$ processes followed by the same sign dilepton decay of $H^{++}$. We find that events involving $e^+ e^+$ among these final states can probe the neutrino mass ordering in $μ$TRISTAN experiment at 5$σ$ significance. In addition to that we study the production of positively charged triplet fermion in $μ$TRISTAN following $μ^+ μ^+ \to μ^+ Σ^+$ process where $Σ^+$ decays into $μ^+ jj$ mode through $Z$ boson exchange. Considering a triplet at 1 TeV and studying SM backgrounds we estimate the discovery potential of $μ^+ μ^+ jj$ signal at $μ$TRISTAN with respect to projected luminosity.

hep-ph↗