arXiv subjects

Takaaki Nomura

Publications and source records attributed to Takaaki Nomura.

At least 109 records · Page 6Linked to original sources

arXiv2019

An inverse seesaw model with global $U(1)_H$ symmetry

We propose an inverse seesaw model based on hidden global symmetry $U(1)_H$ in which we realize tiny neutrino masses with rather natural manner taking into account relevant experimental bounds. The small Majorana mass for inverse seesaw mechanism is induced via small vacuum expectation value of a triplet scalar field whose Yukawa interactions with standard model fermions are controlled by $U(1)_H$. We discuss the phenomenology of the exotic particles present in the model including the Goldstone boson coming from breaking of the global symmetry, and explore testability at the Large Hadron Collider experiments.

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arXiv2019

One-loop neutrino mass model without any additional symmetries

We propose a radiative seesaw model at one-loop level, introducing fields with large multiplet of $SU(2)_L$. Thanks to large representations, any additional symmetries are not needed. In this framework, we formulate lepton and new fermion sector such as mass matrices, LFVs, and muon $g-2$. Furthermore, we show our cut-off scale via RGEs of $SU(2)_L$, and numerical analysis at a benchmark point included in dark matter candidate. Finally, we brief discuss possibility to detect at colliders.

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arXiv2019

Top-quark flavor-changing $tqZ$ couplings and rare $ΔF=1$ processes

We model-independently study the impacts of anomalous $tqZ$ couplings ($q=u,c$), which lead to the $t\to q Z$ decays, on low energy flavor physics. It is found that the $tuZ$-coupling effect can significantly affect the rare $K$ and $B$ decays, whereas the $tcZ$-coupling effect is small. Using the ATLAS's branching ratio (BR) upper bound of $BR(t\to uZ) < 1.7\times 10^{-4}$, the influence of the anomalous $tuZ$-coupling on the rare decays can be found as follows: (a) The contribution to the Kaon direct CP violation can be up to $Re(ε'/ε) \lesssim 0.8 \times 10^{-3}$; (b) $BR(K^+\to π^+ ν\bar ν) \lesssim 12 \times 10^{-11}$ and $BR(K_L \to π^0 ν\bar ν)\lesssim 7.9 \times 10^{-11}$; (c) the BR for $K_S \to μ^+ μ^-$ including the long-distance effect can be enhanced by $11\%$ with respect to the standard model result, and (d) $BR(B_d \to μ^+ μ^-) \lesssim 1.97 \times 10^{-10}$. In addition, although $Re(ε'/ε)$ cannot be synchronously enhanced with $BR(K_L\to π^0 ν\barν)$ and $BR(K_S\to μ^+ μ^-)$ in the same region of the CP-violating phase, the values of $Re(ε'/ε)$, $BR(K^+ \to π^+ ν\barν)$, and $BR(B_d \to μ^+ μ^-)$ can be simultaneously increased.

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arXiv2019

A modular $A_4$ symmetric model of dark matter and neutrino

We propose a model based on modular $A_4$ symmetry containing a dark matter candidate, realizing radiatively induced neutrino mass at one-loop level. One finds that stability of dark matter candidate can be assured by nonzero value of modular weight and heavy neutral fermion mass hierarchies, which include dark matter under the $A_4$ triplet, are uniquely determined; $M_X\ll M_2<M_3$. Therefore we clearly identify single dark matter field which can discriminate from the other models with $A_4$ modular symmetry. Then we discuss several phenomenological aspects and show predictions on the lepton sector.Especially, we find $0.56\lesssim\sin^2θ_{23}\lesssim0.624$, which could have an advantage of this narrow region and be also discriminated from the other modular $A_4$ models.

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arXiv2019

Searching for scalar boson decaying into light $Z'$ boson at collider experiments in $U(1)_{L_μ-L_τ}$ model

We study a model with $U(1)_{L_μ- L_τ}$ gauge symmetry and discuss collider searches for a scalar boson, which breaks $U(1)_{L_μ- L_τ}$ symmetry spontaneously, decaying into light $Z'$ gauge boson. In this model, the new gauge boson, $Z'$, with a mass lighter than $\mathcal{O}(100)$ MeV, plays a role in explaining the anomalous magnetic moment of muon via one-loop contribution. For the gauge boson to have such a low mass, the scalar boson, $ϕ$ with $\mathcal{O}(100)$ GeV mass appears associated with the symmetry breaking. We investigate experimental constraints on $U(1)_{L_μ- L_τ}$ gauge coupling, kinetic mixing, and mixing between the SM Higgs and $ϕ$. Then collider search is discussed considering $ϕ$ production followed by decay process $ϕ\to Z' Z'$ at the large hadron collider and the international linear collider. We also estimate discovery significance at the linear collider taking into account relevant kinematical cut effects.

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arXiv2019

Influence of an inert charged Higgs boson on the muon $g-2$ and radiative neutrino masses in a scotogenic model

A simple extension of Ma's approach in a scotogenic model is studied for the purpose of simultaneously interpreting the neutrino data and the excess of muon anomalous magnetic moment (muon $g-2$). The feasible minimal extension is to add an $Z_2$-odd vector-like lepton doublet to the Ma's model. It is found that in addition to the neutrino data, the strict constraints on the relevant parameters are from the electroweak oblique parameters and the induced lepton-flavor violation processes, such as $\ell_i \to \ell_j γ$ and $\ell_i \to \ell^-_j \ell^-_j \ell^+_j$. Performing parameter scan, we numerically demonstrate that when the constraint conditions are satisfied, the muon $g-2$ of $O(10^{-9})$ can be achieved, where it can be expected that with a $5σ$ observation, the Muon $g-2$ experiment at Fermilab can observe $a_μ\approx 13.31 \times 10^{-10}$ when the current experiment and the SM errors are reduced by a factor of 4 and 2, respectively. Moreover, the branching ratio of the $τ\to μγ$ decay can match the Belle II sensitivity of $O(10^{-9})$ with an integrated luminosity of 50 ab$^{-1}$.

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arXiv2019

Zee Model with Flavor Dependent Global $U(1)$ Symmetry

We study a simple extension of the Zee model, in which a discrete $Z_2$ symmetry imposed in the original model is replaced by a global $U(1)$ symmetry retaining the same particle content. Due to the $U(1)$ symmetry with flavor dependent charge assignments, the lepton sector has an additional source of flavor violating Yukawa interactions with a controllable structure, while the quark sector does not at tree level. We show that current neutrino oscillation data can be explained under constraints from lepton flavor violating decays of charged leptons in a successful charge assignment of the $U(1)$ symmetry. In such scenario, we find a characteristic pattern of lepton flavor violating decays of additional Higgs bosons, which can be a smoking gun signature at collider experiments.

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arXiv2019

Left-handed color-sextet diquark in the Kaon system

We investigate whether a color-sextet scalar diquark (${\bf H}_6$) coupling to the left-handed quarks contributes to the $ΔS=2$ process. It is found that the box diagrams mediated by $W$ and ${\bf H}_6$ bosons have no contributions to $ΔS=2$ when the limit of $m_t=0$ is used, and the flavor mixing matrices for diagonalizing quark mass matrices are introduced at the same time. When the heavy top-quark mass effects are taken into account, it is found that in addition to the $W-{\bf H}_6$ box diagrams significantly contributing to $ΔS=2$, their effects can be as large as those from the ${\bf H}_6-{\bf H}_6$ box diagrams. Using the parameters that are constrained by the $K^0-\bar K^0$ mixing parameter $ΔM_K$ and the Kaon indirect CP violation $ε_K$, we find that the left-handed color-sextet diquark can lead to the Kaon direct CP violation being $Re(ε'/ε) \sim 0.4 \times 10^{-3}$. In the chosen scheme, although the diquark contribution to $K_L\to π^0 ν\barν$ is small, the branching ratio of $K^+ \to π^+ ν\barν$ can reach the current experimental upper bound.

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arXiv2019

$b\rightarrow s μ^+ μ^-$ anomalies and related phenomenology in $U(1)_{B_3 - x_μL_μ- x_τL_τ}$ flavor gauge models

We propose a generation dependent lepton/baryon gauge symmetry, $U(1)_{B_3 - x_μL_μ- x_τL_τ} \equiv U(1)_X$ (with $x_μ+ x_τ=1$ for anomaly cancellation), as a possible solution for the $b \rightarrow s μ^+ μ^-$ anomalies. By introducing two Higgs doublet fields, we can reproduce the observed CKM matrix, and generate flavor changing $Z'$ interactions in the quark sector. Thus one can explain observed anomalies in $b \to s \ell^+ \ell^-$ decay with the lepton non-universal $U(1)_X$ charge assignments. We show the minimal setup explaining $b \to s \ell^+ \ell^-$ anomalies, neutrino masses and mixings and dark matter candidate, taking into account experimental constraints of flavor physics such as charged lepton flavor violations and the $B_s$--$\bar B_s$ mixing. Finally we discuss collider physics focusing on $Z'$ production at the Large Hadron Collider and relic density of our dark matter candidate.

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arXiv2019

Constraining a general U(1)$^\prime$ inverse seesaw model from vacuum stability, dark matter and collider

We consider a class of gauged $U(1)$ extensions of the Standard Model (SM), where the light neutrino masses are generated by an inverse seesaw mechanism. In addition to the three right handed neutrinos, we add three singlet fermions and demand an extra $Z_2$ symmetry under which, the third generations of both of the neutral fermions are odd, which in turn gives us a stable dark matter candidate. We express the $U(1)$ charges of all the fermions in terms of the U(1) charges of the standard model Higgs and the new complex scalar. We study the bounds on the parameters of the model from vacuum stability, perturbative unitarity, dark matter relic density and direct detection constraints. We also obtain the collider constraints on the $Z'$ mass and the $U(1)'$ gauge coupling. Finally we compare all the bounds on the $Z'$ mass versus the $U(1)'$ gauge coupling plane.

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arXiv2019

A radiative neutrino mass model with hidden gauge symmetry inducing semi-annihilating dark matter

We propose two-loop neutrino mass model with gauged hidden $U(1)$ symmetry, and discuss a Majorana type of dark matter candidate that has semi-annihilation processes in the relic density as well as lepton flavor violations and muon anomalous magnetic moment. Also, we demonstrate global analysis to satisfy neutrino oscillation data, lepton flavor violations, and relic density of dark matter candidate and show that semi-annihilation modes play a crucial role in finding observed relic density.

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arXiv2019

One-loop neutrino mass model with $SU(2)_L$ multiplet fields

We propose a one-loop neutrino mass model with several $SU(2)_L$ multiplet fermions and scalar fields in which the inert feature of a scalar to realize the one-loop neutrino mass can be achieved by the cancellation among Higgs couplings thanks to non-trivial terms in the Higgs potential and to present it in a simpler way. Then we discuss our typical cut-off scale by computing renormalization group equation for $SU(2)_L$ gauge coupling, lepton flavor violations, muon anomalous magnetic moment, possibility of dark matter candidate, neutrino mass matrix satisfying the neutrino oscillation data. Finally, we search for our allowed parameter region to satisfy all the constraints, and discuss a possibility of detecting new charged particles at the large hadron collider.

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arXiv2019

Inverse seesaw model with large $SU(2)_L$ multiplets and natural mass hierarchy

We propose an inverse seesaw model with large $SU(2)_L$ multiplet fields which realizes natural mass hierarchies among neutral fermions. Here, lighter neutral fermion mass matrices are induced via two suppression mechanisms; one is small vacuum expectation value of $SU(2)_L$ triplet required by rho parameter constraint and the other is generation of Majorana mass term of extra singlet fermions at one-loop level. To realize the loop masses, we impose $Z_2$ symmetry which also guarantees stability of a dark matter candidate. Furthermore, we discuss anomalous magnetic moment and collider physics from interactions of large multiplet fields.

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arXiv2019

A Linear seesaw model with hidden gauge symmetry

We propose a natural realization of linear seesaw model with hidden gauge symmetry in which $SU(2)_L$ triplet fermions, one extra Higgs singlet, doublet and quartet scalar are introduced. Small neutrino mass can be realized by two suppression factors that are small vacuum expectation value of quartet scalar and inverse of Dirac mass for triplet. After formulating neutrino mass matrix, we discuss collider phenomenology of the model focusing on signals from exotic charged particles production at the LHC.

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arXiv2019

Dark matter and $B$-meson anomalies in a flavor dependent gauge symmetry

A possibility of explaining the anomalies in the semileptonic $B$-meson decay $B \to K^{*} μ\barμ$ has been explored in the framework of the gauged $U(1)_{μ-τ}$ symmetry. Apart from the muon anomalous magnetic moment and neutrino sector, we formulate the model starting with a valid Lagrangian and consider the constraints from the neutral meson mixings, the bounds on direct detection and the relic density of the bosonic dark matter candidate augmented to collider constraints. We search the parameter space, which accommodates the size of the anomaly of the $B \rightarrow K^* μ\bar μ$ decay, to satisfy all experimental constraints. We found the allowed region on the plane of the dark matter and $Z'$ masses is a rather narrow compared to the previous analysis.

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arXiv2019

Radiative neutrino mass in an alternative $U(1)_{B-L}$ gauge symmetry

We propose a neutrino model in which neutrino masses are generated at one loop level and three right-handed fermions have non-trivial charges under $U(1)_{B-L}$ gauge symmetry in no conflict with anomaly cancellation. After the spontaneously symmetry breaking, a remnant $Z_2$ symmetry is induced and plays an role in assuring the stability of dark matter candidate.

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arXiv2019

An inverse seesaw model with natural hierarchy at TeV scale

We propose a new kind of inverse seesaw model without any additional symmetries. Instead of the symmetries, we introduce several fermions and bosons with higher $SU(2)_L$ representations. After formulating the Higgs sector and neutrino sector, we show that the cut-off energy, which is valid to our model, is at around TeV scale, by examining behavior of $SU(2)_L$ gauge coupling. Then we show the testability of our model at collider physics.

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arXiv2019

Neutrino mass in a gauged $L_μ- L_τ$ model

We study the origin of neutrino mass through lepton-number violation and spontaneous $U(1)_{L_μ-L_τ}$ symmetry breaking. To accomplish the purpose, we include one Higgs triplet, two singlet scalars, and two vector-like doublet leptons in the $U(1)_{L_μ-L_τ}$ gauge extension of the standard model. To completely determine the free parameters, we employ the Frampton-Glashow-Marfatia (FGM) two-zero texture neutrino mass matrix as a theoretical input. It is found that when some particular Yukawa couplings vanish, an FGM pattern can be achieved in the model. Besides the explanation of neutrino data, we find that the absolute value of neutrino mass $m_j$ can be obtained in the model, and their sum can satisfy the upper bound of the cosmological measurement with $\sum_j |m_j| < 0.12$ eV. The effective Majorana neutrino mass for neutrinoless double-beta decay is below the current upper limit and is obtained as $\langle m_{ββ} \rangle =(0.34,\, 2.3)\times 10^{-2}$ eV. In addition, the doubly charged Higgs $H^{\pm\pm}$ decaying to $μ^\pm τ^\pm$ final states can be induced from a dimension-6 operator and is not suppressed, and its branching ratio is compatible with the $H^{\pm \pm}\to W^\pm W^\pm$ decay when the vacuum expectation value of Higgs triplet is $O(0.01)$ GeV.

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