arXiv subjects

Hiroshi Okada

Publications and source records attributed to Hiroshi Okada.

At least 199 records · Page 11Linked to original sources

arXiv2015

Flavour Dependent Gauged Radiative Neutrino Mass Model

We propose a one-loop induced radiative neutrino mass model with anomaly free flavour dependent gauge symmetry: $μ$ minus $τ$ symmetry $U(1)_{μ-τ}$. A neutrino mass matrix satisfying current experimental data can be obtained by introducing a weak isospin singlet scalar boson that breaks $U(1)_{μ-τ}$ symmetry, an inert doublet scalar field, and three right-handed neutrinos in addition to the fields in the standard model. We find that a characteristic structure appears in the neutrino mass matrix: two-zero texture form which predicts three non-zero neutrino masses and three non-zero CP-phases from five well measured experimental inputs of two squared mass differences and three mixing angles. Furthermore, it is clarified that only the inverted mass hierarchy is allowed in our model. In a favored parameter set from the neutrino sector, the discrepancy in the muon anomalous magnetic moment between the experimental data and the the standard model prediction can be explained by the additional neutral gauge boson loop contribution with mass of order 100 MeV and new gauge coupling of order $10^{-3}$.

hep-ph↗

arXiv2014

Quark mixing from Delta(6N^2) family symmetry

We consider a direct approach to quark mixing based on the discrete family symmetry Delta (6N^2) in which the Cabibbo angle is determined by a residual Z_2 times Z_2 subgroup to be $|V_{us}|=0.222521$, for $N$ being a multiple of 7. We propose a particular model in which unequal smaller quark mixing angles and CP phases may occur without breaking the residual Z_2 times Z_2 symmetry. We perform a numerical analysis of the model for $N=14$, where small Z_2 times Z_2 breaking effects of order 3% are allowed by model, allowing perfect agreement within the uncertainties of the experimentally determined best fit quark mixing values.

hep-ph↗

arXiv2014

X-ray line in Radiative Neutrino Model with Global $U(1)$ Symmetry

We study a three loop induced radiative neutrino model with global $U(1)$ symmetry at TeV scale, in which we consider two component dark matter particles. We discuss the possibility to explain the X-ray line signal at about 3.55 keV recently reported by XMN-Newton X-ray observatory using data of various galaxy clusters and Andromeda galaxy. Subsequently, we also discuss to show that sizable muon anomalous magnetic moment, a discrepancy of the effective number of neutrino species $ΔN_{\rm eff}\approx$ 0.39, and scattering cross section detected by direct detection searches can be derived.

hep-ph↗

arXiv2014

3.55 keV X-ray Line Signal from Excited Dark Matter in Radiative Neutrino Model

We study an exciting dark matter scenario in a radiative neutrino model to explain the X-ray line signal at $3.55$ keV recently reported by XMN-Newton X-ray observatory using data of various galaxy clusters and Andromeda galaxy. We show that the required large cross section for the up-scattering process to explain the X-ray line can be obtained via the resonance of the pseudo-scalar. Moreover this model can be compatible with the thermal production of dark matter and the constraint from the direct detection experiment.

hep-ph↗

arXiv2014

Electron/Muon Specific Two Higgs Doublet Model

We discuss two Higgs doublet models with a softly-broken discrete $\mathbb{S}_3$ symmery, where the mass matrix for charged-leptons is predicted as the diagonal form in the weak eigenbasis of lepton fields. Similar to an introduction of $\mathbb{Z}_2$ symmetry, the tree level flavor changing neutral current can be forbidden by imposing the $\mathbb{S}_3$ symmetry to the model. Under the $\mathbb{S}_3$ symmetry, there are four types of Yukawa interactions depending on the $\mathbb{S}_3$ charge assignment to right-handed fermions. We find that extra Higgs bosons can be muon and electron specific in one of four types of the Yukawa interaction. This property does not appear in any other two Higgs doublet models with a softly-broken ${\mathbb Z}_2$ symmetry. We discuss the phenomenology of the muon and electron specific Higgs bosons at the Large Hadron Collider; namely we evaluate allowed parameter regions from the current Higgs boson search data and discovery potential of such a Higgs boson at the 14 TeV run.

hep-ph↗

arXiv2014

Effects of Long-lived 10 MeV Scale Sterile Neutrino on Primordial Elemental Abundances and Effective Neutrino Number

The primordial lithium abundance inferred from observations of metal-poor stars is ~3 times smaller than the theoretical value in standard big bang nucleosynthesis (BBN) model. We assume a simple model including a sterile neutrino nu_H with mass of O(10) MeV which decays long after BBN. We then investigate cosmological effects of a sterile neutrino decay. We formulate the injection spectrum of nonthermal photons induced by electrons and positrons generated at the nu_H decay, as a function of the nu_H mass and the photon temperature. We then consistently solve (1) the cosmic thermal history, (2) nonthermal nucleosynthesis induced by the nonthermal photons, (3) the baryon-to-photon ratio eta, and (4) the effective neutrino number N_eff. Amounts of energy injection at the nu_H decay are constrained from limits on primordial D and 7Li abundances, the N_eff value, and the cosmic microwave background energy spectrum. We find that 7Be is photodisintegrated and the Li problem is partially solved for the lifetime 10^4-10^5 s and the mass >~ 14 MeV. 7Be destruction by more than a factor of 3 is not possible because of an associated D over-destruction. In the parameter region, the eta value is decreased slightly, while the N_eff value is increased by a factor of <~ 1. In this study, errors in photodisintegration cross sections of 7Be(g, a)3He and 7Li(g, a)3H that have propagated through literatures are corrected. It is then found that the new photodisintegration rates are 2.3 to 2.5 times smaller than the old rates, so that efficiencies of 7Be and 7Li photodisintegration are significantly smaller.

astro-ph.CO↗

arXiv2014

Inert Extension of the Zee-Babu Model

We propose a two-loop induced Zee-Babu type neutrino mass model at the TeV scale. Although there is no dark matter candidate in the original Zee-Babu model, that is contained in our model by introducing an unbroken discrete $Z_2$ symmetry. The discrepancy between the experimental value of the muon anomalous magnetic moment (muon $g-2$) and its prediction in the standard model can be explained by contributions from additional vector-like charged-leptons which are necessary to give non-zero neutrino masses. The mass of vector-like leptons to be slightly above 300 GeV is favored and allowed from the muon $g-2$ and the current LHC data. We find that from the structure of neutrino mass matrix, doubly-charged scalar bosons in our model can mainly decay into the same-sign and same-flavour dilepton plus missing transverse momentum. By measuring an excess of these events at the LHC, our model can be distinguished from the other models including doubly-charged scalar bosons.

hep-ph↗

arXiv2014

3.55 keV X-ray Line Interpretation in Radiative Neutrino Model

We discuss the 3.55 keV X-ray line anomaly reported by XMN-Newton X-ray observatory using data of various galaxy clusters and Andromeda galaxy in a radiative neutrino model, in which the mixing between the active neutrino and the dark matter is generated at two-loop level after the spontaneous breaking of $Z_2$ symmetry. It might provide us a natural explanation of its tiny mixing ${\cal O}(10^{-10})$, which is observed by their experiments. Such an Abelian discrete symmetry plays a crucial role in differentiating the TeV scale Majorana field from our dark matter, whose mass is expect to be around 7.1 keV.

hep-ph↗

arXiv2014

Two Loop Neutrino Model and Dark Matter Particles with Global $B-L$ Symmetry

We study a two loop induced seesaw model with global $U(1)_{B-L}$ symmetry, in which we consider two component dark matter particles. The dark matter properties are investigated together with some phenomenological constraints such as electroweak precision test, neutrino masses and mixing and lepton flavor violation. In particular, the mixing angle between the Standard Model like Higgs and an extra Higgs is extremely restricted by the direct detection experiment of dark matter. We also discuss the contribution of Goldstone boson to the effective number of neutrino species $ΔN_{\rm eff}\approx0.39$ which has been reported by several experiments.

hep-ph↗

arXiv2014

Radiative Generation of Lepton Masses with the $U(1)'$ Gauge Symmetry

We revisit our previous model proposed in Ref. \cite{Okada:2013iba}, in which lepton masses except the tauon mass are generated at the one-loop level in a TeV scale physics. Although in the previous work, rather large Yukawa couplings constants; i.e., greater than about 3, are required to reproduce the muon mass, we do not need to introduce such a large but ${\cal O}$(1) couplings. In our model, masses for neutrinos (charged-leptons) are generated by a dimension five effective operator with two isospin triplet (singlet and doublet) scalar fields. Thus, the mass hierarchy between neutrinos and charged-leptons can be naturally described by the difference in the number of vacuum expectation values (VEVs) of the triplet fields which must be much smaller than the VEV of the doublet field due to the constraint from the electroweak rho parameter. Furthermore, the discrepancy in the measured muon anomalous magnetic moment ($g-2$) from the prediction in the standard model are explained by one-loop contributions from vector-like extra charged-leptons which are necessary to obtain the radiative generation of the lepton masses. We study the decay property of the extra leptons by taking into account the masses of muon, neutrinos, muon $g-2$ and dark matter physics. We find that the extra leptons can mainly decay into the mono-muon, dark matter with or without $Z$ bosons in the favored parameter regions.

hep-ph↗

arXiv2014

Two loop Induced Dirac Neutrino Model and Dark Matters with Global $U(1)'$ Symmetry

We propose a two loop induced Dirac type neutrino model at TeV scale. Subsequently, three types of dark matter particles; fermion and two bosons, are naturally introduced. Here we discuss to analyze two possibilities; two component dark matter scenario (Dirac fermion and complex boson) and single dark matter one (another real boson), comparing to current experimental data such as Planck/WMAP and LUX. We briefly mention the possibility to explain the discrepancy of the effective number of neutrino species reported by several experiments.

hep-ph↗

arXiv2014

7 keV Dark Matter as X-ray Line Signal in Radiative Neutrino Model

We study a light dark matter in a radiative neutrino model to explain the X-ray line signal at about $3.5$ keV recently reported by XMN-Newton X-ray observatory using data of various galaxy clusters and Andromeda galaxy. The signal requires very tiny mixing between the dark matter and an active neutrino; $\sin^2 2θ\approx 10^{-10}$. It could suggest that such a light dark matter cannot contribute to the observed neutrino masses if we use the seesaw mechanism. In other words, neutrino masses might come a structure different from the dark matter. We propose a model in which Dirac type active neutrino masses are induced at one-loop level. On the other hand the mixing between active neutrino and dark matter are generated at two-loop level. As a result we can explain both the observed neutrino masses and the X-ray line signal from the dark matter decay with rather mild hierarchy of parameters in TeV scale.

hep-ph↗

arXiv2014

Radiative Lepton Model and Dark Matter with Global $U(1)'$ Symmetry

We propose a radiative lepton model, in which the charged lepton masses are generated at one-loop level, and the neutrino masses are induced at two-loop level. On the other hand, tau mass is derived at tree level since it is too heavy to generate radiatively. Then we discuss muon anomalous magnetic moment together with the constraint of lepton flavor violation. A large muon magnetic moment is derived due to the vector like charged fermions which are newly added to the standard model. In addition, considering a scalar dark matter in our model, a strong gamma-ray signal is produced by dark matter annihilation via internal bremsstrahlung. We can also obtain the effective neutrino number by the dark radiation of the Goldstone boson coming from the imposed global $U(1)'$ symmetry.

hep-ph↗

arXiv2014

Radiative Generation of the Lepton Mass

We propose a new mechanism where both Dirac masses for the charged-leptons and Majorana masses for neutrinos are generated via quantum levels. The charged-lepton masses are given by the vacuum expectation values (VEVs) of the Higgs doublet field and that of a triplet field. On the other hand, neutrino masses are generated by two VEVs of triplet Higgs fields. As a result, the hierarchy between the masses for charged-leptons and neutrinos can be explained by the triplet VEVs which have to be much smaller than the doublet VEV due to the constraint from the electroweak rho parameter. We construct a concrete model to realize this mechanism with discrete $\mathbb{Z}_2$ and $\mathbb{Z}_4$ symmetries, in which masses for neutrinos and those for the muon and electron are generated at the one-loop level. As a bonus in our model, the deviation in the measured muon $g-2$ from the standard model prediction can be explained by contributions of extra particle loops. Besides, the lightest $\mathbb{Z}_2$-odd neutral particle can be a dark matter candidate. The collider phenomenology is also discussed, especially focusing on doubly-charged scalar bosons which are necessary to introduce to occur our mechanism.

hep-ph↗

arXiv2013

New Interpretation of the Recent Result of AMS-02 and Multi-component Decaying Dark Matters with non-Abelian Discrete Flavor Symmetry

Recently the AMS-02 experiment has released the data of positron fraction with much small statistical error. Because of the small error, it is no longer easy to fit the data with a single dark matter for a fixed diffusion model and dark matter profile. In this paper, we propose a new interpretation of the data that it originates from decay of two dark matter. This interpretation gives a rough threshold of the lighter DM component. When DM decays into leptons, the positron fraction in the cosmic ray depends on the flavor of the final states, and this is fixed by imposing non-Abelian discrete symmetry in our model. By assuming two gauge-singlet fermionic decaying DM particles, we show that a model with non-Abelian discrete flavor symmetry, e.g. $T_{13}$, can give a much better fitting to the AMS-02 data compared with single dark matter scenario. Few dimension six operators of universal leptonic decay of DM particles are allowed in our model since its decay operators are constrained by the $T_{13}$ symmetry. We also show that the lepton masses and mixings are consistent with current experimental data, due to the flavor symmetry.

hep-ph↗

arXiv2013

Hidden Sector Dark Matter with Global $U(1)_X$-symmetry and Fermi-LAT 130 GeV $γ$-ray Excess

We suggest a dark matter scenario which could contribute the possible anomaly observed by Fermi-LAT $γ$-ray space telescope. It is based on the model recently proposed by Weinberg. In our scenario the gamma-ray line signal comes from the fermionic dark matter ($M_{\rm DM}=214 $ GeV) annihilating into two light scalars with mass around 500 MeV which in turn decay into two neutral pions. Finally the pions can decay into two 130 GeV photons. The strong constraint from the direct detection leaves only the channel of the dark matter annihilation into two light scalars for both the relic density and the Fermi-LAT gamma-ray line signal. The resulting gamma-ray spectrum is rather broad and does not fit to the data perfectly, but the data also show there may be fluctuation in the spectrum. There is no associated $Z$-boson or Higgs boson production contrary to most other works where the signal comes from the loops of charged particles. The annihilation into the other SM particles are highly suppressed due to the small mixing from the direct detection. Future experiments with more data will give more clues on the possible scenarios.

hep-ph↗

arXiv2013

$T_7$ Flavor Model in Three Loop Seesaw and Higgs Phenomenology

We propose a new type of radiative seesaw model in which observed neutrino masses are generated through a three-loop level diagram in combination with tree-level type-II seesaw mechanism in a renormalizable theory. We introduce a Non-abelian flavor symmetry $T_7$ in order to constrain the form of Yukawa interactions and Higgs potential. Although several models based on a Non-abelian flavor symmetry predict the universal coupling constants among the standard model like Higgs boson and charged leptons, which is disfavored by the current LHC data, our model can avoid such a situation. We show a benchmark parameter set that is consistent with the current experimental data, and we discuss multi-muon events as a key collider signature to probe our model.

hep-ph↗

arXiv2013

Multicomponent dark matter particles in a two-loop neutrino model

We construct a loop induced seesaw model in a TeV scale theory with gauged U(1)_{B-L} symmetry. Light neutrino masses are generated at two-loop level and right-handed neutrinos also obtain their masses by one-loop effect. Multi-component Dark Matters (DMs) are included in our model due to the remnant discrete symmetry after the B-L symmetry breaking and the Z_2 parity which is originally imposed to the model. We investigate the multi-component DM properties, in which we have two fermionic DMs with different mass scales, O(10) GeV and O(100-1000) GeV. The former mass corresponds to the lightest right-handed neutrino mass induced by the loop effect, although the latter one to the SM gauge singlet fermion. We show each of the DM annihilation processes and compare to the the observation of relic abundance, together with the constraints of Lepton Flavor Violation (LFV) and active neutrino masses. Moreover we show that our model has some parameter region allowed by the direct detection result reported by XENON100, and it is possible to verify the model by the future XENON experiment.

hep-ph↗