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Hiroshi Okada

Publications and source records attributed to Hiroshi Okada.

At least 73 records · Page 4Linked to original sources
arXiv2022

A multi-charged particle model with local $U(1)_{μ-τ}$ to explain muon $g-2$, flavor physics, and possible collider signature

We consider a model with multi-charged particles including vector-like fermions and a charged scalar under a local $U(1)_{μ- τ}$ symmetry. We search for allowed parameter region explaining muon anomalous magnetic moment (muon $g-2$) and $b \to s \ell^+ \ell^-$ anomalies, satisfying constraints from the lepton flavor violations, $Z$ boson decays, meson anti-meson mixing and collider experiments. Carrying out numerical analysis, we explore the typical size of the muon $g-2$ and Wilson coefficients to explain $b \to s \ell^+ \ell^-$ anomalies in our model when all other experimental constraints are satisfied. We then discuss the collider physics of the multicharged vectorlike fermions, considering some benchmark points in the allowed parameter space.

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arXiv2022

Natural mass hierarchy among three heavy Majorana neutrinos for resonant leptogenesis under modular $A_4$ symmetry

It is clear that matter is dominant in the Universe compared to antimatter. We call this problem baryon asymmetry. The baryon asymmetry is experimentally determined by both cosmic microwave background and big bang nucleosynthesis measurements. To resolve the baryon number asymmetry of the Universe as well as neutrino oscillations, we study a radiative seesaw model in a modular $A_4$ symmetry. Degenerate heavy Majorana neutrino masses can be naturally realized in an appropriate assignments under modular $A_4$ with large imaginary part of modulus $τ$, and it can induce measured baryon number via resonant leptogenesis that is valid in around TeV scale energy theory. We also find that the dominant contribution to the CP asymmetry arises from Re[$τ$] through our numerical analysis satisfying the neutrino oscillation data.

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arXiv2022

A radiatively induced inverse seesaw model with hidden $U(1)$ gauge symmetry

We propose an inverse seesaw scenario under hidden $U(1)$ gauge symmetry, having rather natural hierarchies among neutral fermion mass scales. The hierarchies are derived from theory and experimental constraints. The theory requires Majorana exotic masses have to be induced at one-loop level. The experimental side requests that the Dirac mass terms has to be highly suppressed to satisfy the constraints from the lepton flavor violations(LFVs) such as $μ\to eγ$. In order to induce such a small Majorana exotic masses we introduce exotic fermions and bosons that also provide us additional intriguing phenomenologies such as muon anomalous magnetic moment, dark matter candidate as well as LFVs and deviations in leptonic $Z$ decays. We analyze these phenomenologies including neutrino oscillation data numerically, and show allowed region. Finally, we discuss the DM candidate in both the cases of fermion and scalar boson, where we focus on rather lighter range that is equal or less than 10 GeV. The dominant contributions originate from interactions of hidden gauge sector, and we show allowed ranges for both cases.

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arXiv2022

Radiative neutrino masses from modular $A_4$ symmetry and supersymmetry breaking

We investigate a modular $A_4$ invariant two-loop neutrino mass model in a supersymmetric framework, where we introduce new fields as minimum as possible, expecting contributions of superpartners to the neutrino masses. We successfully reproduce the neutrino oscillation data thanks to the superpartner contributions in case of normal hierarchy, and predict several observables such as phases and neutrino masses, concentrating on three specific regions at nearby fixed points of modulus $τ=i,ω,i\times \infty$, where $ω\equiv e^{2πi/3}$. These points are statistically favored in flux compactifications of the string theory. We show several results in each the points by performing global $χ^2$ analysis, and demonstrate benchmark points with the minimum $χ^2$.

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arXiv2022

A radiative seesaw model in a supersymmetric modular $A_4$ group

We propose a supersymmetric radiative seesaw model with modular $A_4$ symmetry. Thanks to contributions of supersymmetric partners to one-loop diagrams generating neutrino masses, we successfully fit neutrino data and obtain predictions in case of normal hierarchy in a minimal framework that would not be realized in a non-supersymmetric model. We show a several predictive figures and demonstrate a best fit benchmark point through $χ^2$

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arXiv2021

Linear seesaw model with a modular $S_4$ flavor symmetry

We discuss a linear seesaw model with as minimum field content as possible, introducing a modular $S_4$ with the help of gauged $U(1)_{B-L}$ symmetries. Due to rank two neutrino mass matrix, we have a vanishing neutrino mass eigenvalue, and only the normal mass hierarchy of neutrinos is favored through the modular $S_4$ symmetry. In our numerical $Δχ^2$ analysis, we especially find rather sharp prediction on sum of neutrino masses to be around $60$ meV in addition to the other predictions.

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arXiv2021

Neutrino mass model with a modular $S_4$ symmetry

We propose a predictive lepton model under a modular $S_4$ symmetry, where the neutrino mass matrix arises from a radiative seesaw at one-loop level. The tree-level mass matrix is forbidden by well-assigned modular weights, which also play an important role in stabilizing dark matter candidate due to a remnant $Z_2$ symmetry even after breaking the modular symmetry. Supposing three families of the Majorana neutrinos, right-handed charged-leptons and left-handed charged-leptons to be embedded respectively into singlet, doublet, and triplet under $S_4$, we obtain the predictive mass matrices in the normal hierarchy. Then, we show our numerical results such as phases, mixings, and neutrino masses, applying $χ^2$ analysis. We also demonstrate two sample points, imposing on minimizing $χ^2$ and best fit value of $δ_{CP}^\ell$ of $195^\circ$.

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arXiv2021

Dark matter stability at fixed points in a modular $A_4$ symmetry

We propose a new mechanism of stabilization for dark matter under fixed points of modular $A_4$ symmetry, thanks to recovering a residual symmetry for each of fixed points at $τ=ω,\ i, i\times\infty$. We find that a model at $τ=i$ successfully reproduces the neutrino oscillation data without conflict of the DM stability.

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arXiv2021

Dirac Radiative Neutrino Mass with Modular Symmetry and Leptogenesis

Minimalistic Dirac radiative neutrino mass model based on modular symmetry is proposed. We predict maximum number of observables possible including neutrino mass splittings, neutrino mass scale, lepton mixing angles, and Dirac phases in the leptonic sector with minimum number of input parameters possible. Model is capable of accommodating multicomponent dark matter, thanks to the $\emph{R}-$parity and accidental scotogenic $\mathbb{Z}_2$ discrete symmetry. Furthermore, even-though neutrinos are Dirac in our model, matter-antimatter asymmetry of the Universe is achieved via neutrinogenesis mechanism. Phenomenology of the dark sector including various dark matter candidates is briefly discussed.

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arXiv2021

Muon $g-2$, $B\to K^{(*)}μ^+ μ^-$ anomalies, and leptophilic dark matter in $U(1)_{μ-τ}$ gauge symmetry

We propose a new class of $U(1)_{μ-τ}$ gauged model that can explain recent flavor anomalies such as muon $g-2$, $b\to sμ^+ μ^-$, as well as include a scalar dark matter candidate while satisfying all the phenomenological constraints. For this purpose, we add new vectorlike quarks and leptons as well as two inert singlet scalars, one leptophilic and the other leptophobic. In our model $b\to sμ^+ μ^-$ anomalies can be explained by loop induced interactions among the SM and exotic quarks and $Z'$ gauge boson. In our numerical analysis, we show allowed region of our model to be narrow, and it would be tested soon, for example by searching for 4 muons and dimuon + missing transverse momentum from $pp \to μ^+ μ^- Z'$ and $pp \to ν_{μ,τ} \bar ν_{μ,τ} Z'$ followed by $Z' \to μ^+ μ^- , ν_{μ,τ} \bar ν_{μ,τ} $ at the LHC.

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arXiv2021

Zee-Babu model in modular $A_4$ symmetry

We study a Zee-Babu model in a modular $A_4$ flavor symmetry, in which we search for several predictions such as phases, sum of neutrino masses, and neutrinoless double beta decay, satisfying neutrino oscillation data in a minimum framework of the charge assignments of modular weight. We perform $Δχ^2$ analysis to get our results and find $τ$ is localized nearby at one of the fixed points of $i\times \infty$ for both of normal and inverted mass hierarchies.

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arXiv2021

Muon $g-2$ in $U(1)_{μ-τ}$ Symmetric Gauged Radiative Neutrino Mass Model

We explore muon anomalous magnetic moment (muon $g-2$) in a scotogenic neutrino model with a gauged lepton number symmetry $U(1)_{μ-τ}$. In this model, a dominant muon $g-2$ contribution comes not from an additional gauge sector but from a Yukawa sector. In our numerical $Δχ^2$ analysis, we show that our model is in favor of normal hierarchy with some features. We demonstrate one benchmark point, satisfying muon $g-2$ at the best fit value $25.1\times10^{-10}$.

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arXiv2021

Modular $A_4$ symmetry and light dark matter with gauged $U(1)_{B-L}$

We propose a radiative seesaw model with a light dark matter candidate (DM) under modular $A_4$ and gauged $U(1)_{B-L}$ symmetries, in which neutrino masses are generated via one-loop level, and we have a bosonic DM candidate $\sim 0.01-50$ GeV. DM is stabilized by nonzero modular weight and a remnant $Z_2$ symmetry of $U(1)_{B-L}$ symmetry. The naturalness of tiny DM mass is indirectly realized by the radiatively induced mass of neutral fermions that interact with our DM candidate. Thus, DM mass does not have to exceed the neutral fermion masses, whose scale is assumed to be 50 GeV. Taking several benchmark points of DM mass in DM analysis, we show the numerical analysis for the neutrino oscillation data in the normal and inverted hierarchy cases and find several features for both cases.

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arXiv2021

Explanations for anomalies of muon anomalous magnetic dipole moment, $b\to s μ\barμ$ and radiative neutrino masses in a leptoquark model

We propose a leptoquark model simultaneously to explain anomalies of muon anomalous magnetic dipole moment and $b\to s μ\barμ$ in light of experimental reports very recently. Here, we satisfy several stringent constraints such as $μ\to eγ$ and meson mixings. In addition, we find these leptoquarks also play an role in generating tiny neutrino masses at one-loop level without introducing any additional symmetries. We have numerical analysis and show how degrees our parameter space is restricted.

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arXiv2021

Towards unification of quark and lepton flavors in $A_4$ modular invariance

We study quark and lepton mass matrices in the $A_4$ modular symmetry towards the unification of the quark and lepton flavors. We adopt modular forms of weights $2$ and $6$ for quarks and charged leptons, while we use modular forms of weight $4$ for the neutrino mass matrix which is generated by the Weinberg operator. We obtain the successful quark mass matrices, in which the down-type quark mass matrix is constructed by modular forms of weight $2$, but the up-type quark mass matrix is constructed by modular forms of weight $6$. Two regions of $τ$ are consistent with observed CKM matrix elements. The one is close to $τ=i$ and the other is in the larger ${\rm Im }[τ]$. On the other hand, lepton mass matrices work well only at nearby $τ=i$, which overlaps with the one of the quark sector, for the normal hierarchy of neutrino masses. In the common $τ$ region for quarks and leptons, the predicted sum of neutrino masses is $87$--$120$meV taking account of its cosmological bound. Since both the Dirac CP phase $δ_{CP}^\ell$ and $\sin^2θ_{23}$ are correlated with the sum of neutrino masses, improving its cosmological bound provides crucial tests for our scheme as well as the precise measurement of $\sin^2θ_{23}$ and $δ_{CP}^\ell$. The effective neutrino mass of the $0νββ$ decay is $\langle m_{ee}\rangle=15$--$31$\,meV. It is remarked that the modulus $τ$ is fixed at nearby $τ=i$ in the fundamental domain of SL$(2,Z)$, which suggests the residual symmetry $Z_2$ in the quark and lepton mass matrices. The inverted hierarchy of neutrino masses is excluded by the cosmological bound of the sum of neutrino masses.

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arXiv2021

Modulus $τ$ linking leptonic CP violation to baryon asymmetry in $A_4$ modular invariant flavor model

We propose an $A_4$ modular invariant flavor model of leptons, in which both CP and modular symmetries are broken spontaneously by the vacuum expectation value of the modulus $τ$. The value of the modulus $τ$ is restricted by the observed lepton mixing angles and lepton masses for the normal hierarchy of neutrino masses at $3σ$ confidence level. The predictive Dirac CP phase $δ_{CP}$ is in the ranges $[0^\circ,50^\circ]$, $[170^\circ,175^\circ]$ and $[280^\circ,360^\circ]$ for ${\rm Re}\, [τ]<0$, and $[0^\circ,80^\circ]$, $[185^\circ,190^\circ]$ and $[310^\circ,360^\circ]$ for ${\rm Re}\,[τ]>0$ at $3σ$ confidence level. The sum of three neutrino masses is predicted in $[60, 84]$ meV, and the effective mass for the $0νββ$ decay is in [0.003, 3] meV. On the other hand, there is no allowed region of the modulus $τ$ for the inverted hierarchy of neutrino masses at $3σ$ confidence level. The modulus $τ$ links the Dirac CP phase to the cosmological baryon asymmetry (BAU) via the leptogenesis. Due to the strong wash-out effect, the predictive baryon asymmetry $Y_B$ can be at most the same order of the observed value. Then, the lightest right-handed neutrino mass is restricted in the range of $M_1 =[1.5,\,6.5] \times 10^{13}$ GeV. We find the correlation between the predictive $Y_B$ and the Dirac CP phase $δ_{CP}$. Only two predictive $δ_{CP}$ ranges, $[5^\circ,40^\circ]$ (${\rm Re}\,[τ]>0$) and $[320^\circ,355^\circ]$ (${\rm Re}\,[τ]<0$) are consistent with the BAU.

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arXiv2021

A Radiative Neutrino Mass Model in Dark Non-Abelian Gauge Symmetry

We discuss a model based on dark sector described by non-Abelian $SU(2)_D$ gauge symmetry where we introduce $SU(2)_L \times SU(2)_D$ bi-doublet vector-like leptons to generate active neutrino masses and kinetic mixing between $SU(2)_D$ and $U(1)_Y$ gauge fields at one-loop level. After spontaneous symmetry breaking of $SU(2)_D$, we have remnant $Z_4$ symmetry guaranteeing stability of dark matter candidates. We formulate neutrino mass matrix and related lepton flavor violating processes and discus dark matter physics estimating relic density. It is found that our model realize multicomponent dark matter scenario due to the $Z_4$ symmetry and relic density can be explained by gauge interactions with kinetic mixing effect.

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