arXiv subjects

Hiroshi Okada

Publications and source records attributed to Hiroshi Okada.

At least 163 records · Page 9Linked to original sources

arXiv2017

Gauged Lepton Symmetry Model

We study a model with $U(1)$ gauged lepton number symmetry in which the active neutrino masses are generated at two-loop level through the spontaneous breaking of the lepton number symmetry. To cancel gauge anomalies some exotic leptons are introduced that also play an important role in generating neutrino masses and the lightest neutral one can be a dark matter candidate. We discuss a phenomenology of the neutral fermion sector including dark matter, lepton flavor violations, and an extra neutral gauge boson $Z'$ that can be tested by $e^+ e^-$ collision at the International Linear Collider (ILC).

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arXiv2017

Explaining $B\to K^{(*)}\ell^+ \ell^-$ anomaly by radiatively induced coupling in $U(1)_{μ-τ}$ gauge symmetry

We propose an extension of the standard model (SM) with $U(1)_{μ-τ}$ gauge symmetry, extra vectorlike quark doublets $Q'_a$ and singlet scalar $χ$, both of which are charged under $U(1)_{μ-τ}$ and carry {odd} dark $Z_2$ parity. Then assuming that $χ$ is the dark matter (DM) of the universe and imposing various constraints from dark matter search, flavor physics and collider search for $Q'_a$, one can show that radiative corrections to $b\rightarrow s Z^{'*} \rightarrow s l^+ l^- $ involving $Q'_a$ and $χ$ can induce $ΔC_9 \sim -1$ which can resolve the LHCb anomalies related with $B\to K^{(*)} \ell^+ \ell^-$. Therefore both DM and $B$ physics anomalies could be accommodated in the model.

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arXiv2017

A four-loop Radiative Seesaw Model

We propose a new type of radiative neutrino model with global hidden $U(1)$ symmetry, in which neutrino masses are induced at the four loop level. Then we discuss the muon anomalous magnetic moment to solve the discrepancy between observation and the standard model prediction, and estimate the relic density of a fermion or boson DM candidate in the model. We also discuss the diphoton resonance $R$ by considering a process $pp \to R \to γγ$ as a possible signal of our model.

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arXiv2017

A Three-loop Neutrino Model with Leptoquark Triplet Scalars

We propose a three-loop neutrino mass model with a few leptoquark scalars in $SU(2)_L$-triplet form, through which we can explain the anomaly of $B \to K^{(*)} μ^+ μ^-$, a sizable muon $g-2$ and a bosonic dark matter candidate, and at the same time satisfying all the constraints from lepton flavor violations. We perform global numerical analyses and show the allowed regions, in which we find somewhat restricted parameter space, such as the mass of dark matter candidate and various components of the Yukawa couplings in the model.

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arXiv2017

Accommodation of the Dirac Phase in the Krauss-Nasri-Trodden Model

We investigate one of the radiative models, Kraus-Nasri-Trodden model, with the maximal value of Dirac CP violating phase, $δ$ is -π/2 (or equivalently 3π/2), which is preferred in not only recent long baseline experiments but also the global fit of neutrino oscillation data. We show that our predicted region of the μ-e conversion rate can be searched in the future experiments without conflicting lepton-flavor violation and dark matter constraints.

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arXiv2017

A Three-loop Neutrino Mass Model with a Colored Triplet Scalar

We study a variation of the Krauss-Nasri-Trodden (KNT) model with a colored triplet scalar field and a colored singlet scalar field, in which we discuss the anomaly coming from $b\to sμ\barμ$, fitting to the muon anomalous magnetic moment and the relic density of the Majorana-type dark matter candidate, as well as satisfying various constraints such as lepton-flavor violations and flavor-changing neutral currents. Also, we discuss the direct constraints from the collider searches and the possibilities of detecting the new fields at the LHC.

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arXiv2017

Radiative Neutrino Mass in Alternative Left-Right Model

We propose a radiative seesaw model in alternative left-right model without any bidoublet scalar fields, in which all the fermion masses in the standard model are generated through a canonical seesaw mechanism at the tree level. On the other hand the observed neutrino masses are generated at two-loop level. In this paper we focus on the neutrino sector and show how to induce the active neutrino masses. Then we discuss the observed neutrino oscillation, constraints from lepton flavor violations, new sources of muon anomalous magnetic moment, a long-lived dark matter candidate with keV scale mass, and collider physics.

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arXiv2016

A testable radiative neutrino mass model without additional symmetries and Explanation for the $b \to s \ell^+ \ell^-$ anomaly

We propose a one-loop radiative Majorana-type neutrino-mass matrix without any kind of additional symmetries by introducing two leptoquark-like bosons only. In this scenario, we show that the anomaly appearing in the process $b\to s\ell\bar\ell$ can be explained without any conflicts against various constraints such as lepton-flavor violations, flavor-changing neutral currents, oblique parameters $ΔS,\ ΔT$, and the Drell-Yan process. We make the predictions for the flavor-violating lepton-pair production ($eμ$, $eτ$, and $μτ$) at the LHC, as well as the cross sections for pair production of these leptoquark-like bosons.

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arXiv2016

Two-loop Induced Majorana Neutrino Mass in a Radiatively Induced Quark and Lepton Mass Model

A two-loop induced radiative neutrino model is proposed as an extension of our previous work in which the first and second generation standard model fermion masses are generated at one-loop level in both quark and lepton sectors. Then we discuss current neutrino oscillation data, lepton flavor violations, muon anomalous magnetic moment, and a bosonic dark matter candidate, for both the normal and inverted neutrino mass hierarchy. Our numerical analysis shows that less hierarchical Yukawa coupling constants can fit the experimental data with TeV scale dark matter.

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arXiv2016

A Radiative Neutrino Model with $SU(2)_L$ Triplet Fields

We propose a loop induced neutrino mass model, in which we introduce several exotic fermions and bosons with $SU(2)_L$ multiplet, and discuss various phenomenologies such as lepton flavor violations, muon anomalous magnetic moment, nonstandard interacting neutrinoless double beta decay, relic density of dark matter, and the possibility of the spin independent direct detection searches, imposing the constraints of oblique parameters. And we show a benchmark point to satisfy all the constraints and discuss our predictions.

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arXiv2016

Explanation of $B \to K^{(*)} \ell^+ \ell^-$ and muon $g-2$, and implications at the LHC

More than $3σ$ deviations from the standard model are observed in the angular observable $P'_5$ of $B\to K^* μ^+ μ^-$ and muon $g-2$. To resolve these anomalies, we extend the standard model by adding two leptoquarks. It is found that the signal strength of the diphoton Higgs decay can exhibit a significant deviation from unity and is within the data errors. Although $\ell_i \to \ell_j γ$ puts severe bounds on some couplings, it is found that the excesses of $P'_5$ and muon $g-2$ can still be explained and can be accommodated to the measurement of $B_s\to μ^+ μ^-$ in this model. In addition, the leptoquark effects can also explain the LHCb measurement of $R_K = BR(B^+ \to K^+ μ^+μ^-)/BR(B^+ \to K^+ e^+e^-)=0.745^{+0.090}_{-0.074} \pm 0.036$, which shows a $2.6σ$ deviation from the standard model prediction.

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arXiv2016

LHC 750 GeV Diphoton excess in a radiative seesaw model

We investigate a possibility for explaining the recently announced 750\,GeV diphoton excess by the ATLAS and the CMS experiments at the CERN LHC in a model with multiple doubly charged particles, which was originally suggested for explaining tiny neutrino masses through a three-loop effect in a natural way. The enhanced radiatively generated effective coupling of a new singlet scalar $S$ with diphoton with multiple charged particles in the loop enlarges the production rate of $S$ in $pp\to S+X$ via photon fusion process and also the decay width $Γ(S\to γγ)$ even without assuming a tree level production mechanism. We provide detailed analysis on the cases with or without allowing the mixing between $S$ and the standard model Higgs doublet.

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arXiv2016

A Colored KNT Neutrino Model

We propose a radiative seesaw model at the three-loop level, in which quarks, leptons, leptoquark bosons, and a Majorana fermion of dark matter candidate are involved in the neutrino loop. Analyzing neutrino oscillation data including all possible constraints such as flavor changing neutral currents, lepton flavor violations, upper/lower bound on the mass of leptoquark from the collider physics, and the measured relic density of the dark matter, we show the allowed region to satisfy all the data/constraints.

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arXiv2016

Radiatively induced Quark and Lepton Mass Model

We propose a radiatively induced quark and lepton mass model in the first and second generation with extra $U(1)$ gauge symmetry and vector-like fermions. Then we analyze the allowed regions which simultaneously satisfy the FCNCs for the quark sector, LFVs including $μ-e$ conversion, the quark mass and mixing, and the lepton mass and mixing. Also we estimate the typical value for the $(g-2)_μ$ in our model.

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arXiv2016

Classically Conformal Radiative Neutrino Model with Gauged B-L Symmetry

We discuss a classically conformal radiative neutrino model with gauged B$-$L symmetry, in which the B$-$L symmetry breaking can occur through the Coleman-Weinberg mechanism. As a result, Majorana mass term is generated and EW symmetry breaking also occurs. We show some allowed parameters to satisfy several theoretical and experimental constraints. Theoretical constraints are inert conditions and Coleman-Weinberg condition. Experimental bounds are lepton flavor violation(especially μ-> e γ), the current bound on the $Z'$ mass at LHC, in additions to the neutrino oscillations.

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arXiv2016

Radiative Neutrino Model with Inert Triplet Scalar

We study a one-loop induced radiative neutrino model with an inert isospin triplet scalar field, in which we discuss current neutrino oscillation data, lepton flavor violations, muon anomalous magnetic moment, and a dark matter candidate. We show global analysis combining all the constraints, and briefly mention another possibility.

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arXiv2016

An Extended Colored Zee-Babu Model

We study the extended colored Zee-Babu model introducing a vector-like quark and singlet scalar. The active neutrino mass matrix and muon anomalous magnetic moment are analyzed, which can be fitted to experimental data satisfying the constraints from flavor changing neutral current. Then we discuss signature of our model via vector-like quark production. In addition, the diphoton excess can be explained with the contribution from vector-like quark

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arXiv2016

Confronting a New Three-loop Seesaw Model with the 750 GeV Diphoton Excess

We propose a new type of radiative neutrino model with a local hidden $U(1)$ symmetry, in which neutrino masses are induced at the three loop level, and discuss the muon anomalous magnetic moment, and dark matter candidates therein. By allowing the hypercharges larger than 3/2 for new fields that contribute to the neutrino masses and making them decay into the standard model fields appropriately, we introduce a lot of new particles with multiple electrical charges in a natural manner. As a by-product, we can accommodate the 750 GeV diphoton excess depending on the hypercharge quantum numbers of new fields responsible for the neutrino masses at the three loop level.

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