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Hiroshi Okada

Publications and source records attributed to Hiroshi Okada.

At least 109 records · Page 6Linked to original sources

Dark matter physics in dark $SU(2)$ gauge symmetry with non-Abelian kinetic mixing

We investigate a model of dark sector based on non-Abelian $SU(2)_D$ gauge symmetry. This dark gauge symmetry is broken into discrete $Z_2$ via vacuum expectation values of two real triplet scalars, and an $SU(2)_D$ doublet Dirac fermion becomes $Z_2-$odd particles whose lighter component makes stable dark matter candidate. The standard model and dark sector can be connected via the scalar mixing and the gauge kinetic mixing generated by higher dimensional operators. We then discuss relic density of dark matter and implications to collider physics in the model. The most unique signatures of this model at the LHC would be the dark scalar ($Φ_1^{(')}$) productions where it subsequently decays into : (1) a fermionic dark matter ($χ_l$) and a heavy dark fermion ($χ_h$) pair, $Φ_1^{(')} \to \bar χ_l χ_h(\bar χ_h χ_l) $, followed by $χ_h$ decays into $χ_l$ and non-Abelian dark gauge bosons ($X_i$'s) which decays into SM fermion pair $\bar f_{SM} f_{SM}$ resulting in the reaction $p p \rightarrow Φ_1^{(')} \rightarrow \bar χ_h χ_l (\bar χ_l χ_h) \to f_{SM} \bar f_{SM} χ_l \bar χ_l $, (2) a pair of $X_i$'s followed by $X_i$ decays into a DM pair or the SM fermions resulting in the reaction, $p p \rightarrow Φ_1^{(')} \rightarrow X_i X_i \rightarrow \bar χ_l χ_l f_{SM} \bar f_{SM}$ or even number of $f_{SM} \bar{f}_{SM}$ pairs.

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Axion and neutrino mass from a hidden gauge symmetry model

We propose a model of dark sector described by gauged hidden $U(1)_H$ symmetry in which neutrino masses are generated at one-loop level and axion is induced by assigning Peccei-Quinn charge to fermions in dark sector relevantly. Then our scenario connects exotic fermion mass generation, neutrino mass matrix and axion through scalar fields associated with $U(1)_H$ and Peccei-Quinn symmetry breaking. We investigate neutrino mass formula, lepton flavor violation, anomalous magnetic moment of muon, dark matter relic density and axion couplings, which are originated from interactions among our dark sector and the standard model particles.

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Quark and lepton flavors with common modulus $τ$ in $A_4$ modular symmetry

We study quark and lepton mass matrices with the common modulus $τ$ in the $A_4$ modular symmetry. The viable quark mass matrices are composed of modular forms of weights $2$, $4$ and $6$. It is remarked that the modulus $τ$ is close to $i$, which is a fixed point in the fundamental region of SL$(2,Z)$, and the CP symmetry is not violated. Indeed, the observed CP violation is reproduced at $τ$ which is deviated a little bit from $τ=i$. The charged lepton mass matrix is also given by using modular forms of weights $2$, $4$ and $6$, where five cases have been examined. The neutrino mass matrix is generated in terms of the modular forms of weight $4$ through the Weinberg operator. Lepton mass matrices are also consistent with the observed mixing angles at $τ$ close to $i$ for NH of neutrino masses. Allowed regions of $τ$ of quarks and leptons overlap each other for all cases of the charged lepton mass matrix. However, the sum of neutrino masses is crucial to test the common $τ$ for quarks and leptons. The minimal sum of neutrino masses $\sum m_i$ is $140$meV at the common $τ$. The inverted hierarchy of neutrino masses is unfavorable in our framework. It is emphasized that our result suggests the residual symmetry $\mathbb{Z}_2^{S}=\{ I, S \}$ in the quark and lepton mass matrices.

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Cosmic-Ray Signatures of Dark Matter from a Flavor Dependent Gauge Symmetry Model with Neutrino Mass Mechanism

We propose an extension to the Standard Model accommodating two families of Dirac neutral fermions and Majorana fermions under additional ${U(1)_{e-μ} \times Z_3\times Z_2}$ symmetries where ${U(1)_{e-μ}}$ is a flavor dependent gauge symmetry related to the first and second family of the lepton sector, which features a two-loop induced neutrino mass model. The two families are favored by minimally reproducing the current neutrino oscillation data and two mass difference squares and canceling the gauge anomalies at the same time. As a result, we have a prediction for neutrino masses. The lightest Dirac neutral fermion is a dark matter candidate with tree-level interaction restricted to electron, muon and neutrinos, which makes it difficult to detect in direct dark matter search as well as indirect search focusing on the $τ$-channel, such as through $γ$-rays. It may however be probed by search for dark matter signatures in electron and positron cosmic rays, and allows interpretation of a structure appearing in the CALET electron+positron spectrum around 350-400 GeV as its signature, with a boost factor $\approx$40 Breit-Wigner enhancement of the annihilation cross section.

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A modular $A_4$ symmetric scotogenic model

We propose a minimal extention of the Standard Model where neutrino masses are generated radiatively at one-loop level via Scotogenic scanario. The model is augmented with $A_4$ modular symmetry as a scotogenic and flavor symmetry. With minimal number of parameters, the model makes predictions for neutrino oscillation data, Majorana and Dirac phases, dark matter characteristics, and neutrinoless double beta decay.

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Scotogenic neutrino mass with large $SU(2)_L$ multiplet fields

We construct a scotogenic neutrino mass model introducing large $SU(2)_L$ multiplet fields without adding an extra symmetry. We have introduced extra scalar fields such as a septet, quintet and quartet where we make the vacuum expectation value of quartet scalar to be zero while septet and quintet develop non-zero ones. Then the neutrino mass is generated at one-loop level by introducing quintet fermion. We analyze the neutrino mass matrix taking constraints from lepton flavor violation into account and discuss collider physics regarding charged fermions from large multiplet fields. We have analysed the production and the decays of the quintet fermions, as well as the discovery reach at 14 TeV and 27 TeV LHC.

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Modular $S_3$ symmetric radiative seesaw model

We propose a one-loop induced radiative seesaw model applying a modular $S_3$ flavor symmetry, which is known as the minimal non-Abelian discrete group. In this scenario, dark matter (DM) candidate is correlated with neutrinos and lepton flavor violations (LFVs). We show several predictions of mixings and phases satisfying LFVs, observed relic density, and neutrino oscillation data.

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Seesaw model with hidden $SU(2)_H \times U(1)_X$ gauge symmetry

We propose a seesaw model with a hidden gauge symmetry $SU(2)_H \times U(1)_X$ where two types of standard model singlet fermions in realizing a seesaw mechanism are organized into $SU(2)_H$ doublet. Then we formulate scalar and gauge sector, neutrino mass matrix and lepton flavor violations. In our gauge sector, $Z$-$Z'$ mixing appears after spontaneous symmetry breaking and we investigate constraint from $ρ$-parameter. In addition we discuss $Z'$ production at the large hadron collider via $Z$-$Z'$ mixing, where $Z'$ tends to dominantly decay into heavy neutrinos.

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An Inverse Seesaw model with $A_4$-modular symmetry

We discuss an inverse seesaw model based on right-handed fermion specific $U(1)$ gauge symmetry and $A_4$-modular symmetry. These symmetries forbid unnecessary terms and restrict structures of Yukawa interactions which are relevant to inverse seesaw mechanism. Then we can obtain some predictions in neutrino sector such as Dirac-CP phase and sum of neutrino mass, which are shown by our numerical analysis. Besides the relation among masses of heavy pseudo-Dirac neutrino can be obtained since it is also restricted by the modular symmetry. We also discuss implications to lepton flavor violation and collider physics in our model.

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A neutrino mass model with hidden $U(1)$ gauge symmetry

We propose a realisation of inverse seesaw model controlled by hidden $U(1)$ gauge symmetry, and discuss the impact of a bosonic dark matter (DM) candidate by imposing a $Z_2$ parity. We present the detail of scalar spectra and apply the Casas-Ibarra parametrisation to fit the neutrino oscillation data. For this model, the allowed region is extracted to explain the observed relic density and the muon $g-2$ discrepancy, satisfying flavor constraints with DM involved. We interpret the DM annihilation into $\bar f f $ including all SM charged fermions and investigate the direct detection to place the bound on DM-Higgs coupling. Finally the LHC DM production is explored in light of charged lepton pair signature plus missing transverse energy.

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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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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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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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A radiative seesaw model in modular $A_4$ symmetry

We study one-loop induced radiative seesaw model applying a modular $A_4$ flavor symmetry, in which the neutrino mass matrix is achieved by two different Yukawa couplings one of which also contributes to positive value of muon anomalous magnetic moment as well as lepton flavor violations. Thanks to the specific mass matrix via $A_4$ symmetry and its modular weight, we find several predictions for lepton sector through our numerical analysis.

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A simple model for explaining muon-related anomalies and dark matter

We propose a model to explain several muon-related experimental anomalies and the abundance of dark matter. We introduce an vector-like exotic lepton that form an iso-doublet and three right-handed Majorana fermions as an iso-singlet. A real/complex scalar field is added as a dark matter candidate. We impose a global $U(1)_ μ$ symmetry under which fields associated with the SM muon are charged. To stabilize the dark matter, we impose a $Z_2$ (or $Z_3$) symmetry under which the exotic lepton doublet and the real (or complex) scalar field are charged. We find that the model can simultaneously explain the muon anomalous magnetic dipole moment and the dark matter relic density in no conflict with any lepton flavor-violating/conserving observables, with some details depending upon whether the scalar field is real or complex. Besides, we extend the framework to the quark sector in a way similar to the lepton sector, and find that the recent anomalies associated with the $b \to s μ^+ μ^-$ transition can also be accommodated while satisfying constraints such as the $B_{s(d)} \to μ^+ μ^-$ decays and neutral meson mixings.

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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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Resolving $B$-meson anomalies by flavor-dependent gauged symmetries $\displaystyle \prod_{i=1}^3U(1)_{B_i-L_i}$

We propose a model with flavor dependent gauged symmetries of $\displaystyle \prod_{i=1}^3 U(1)_{B_i-L_i}$ with $i$ the family indices. After formulating the renormalizable Yukawa Lagrangian, Higgs potential and kinetic term, we study the lepton sector based on a successful two-zero texture without introducing extra scalar bosons to avoid the dangerous Goldstone bosons. In particular, we discuss the muon related phenomenologies via additional neutral gauge bosons. In our numerical analysis, we explore the allowed parameter space, in which the anomaly of $B \to K^* \bar μμ$ can be explained.

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