arXiv subjects

Nobuchika Okada

Publications and source records attributed to Nobuchika Okada.

At least 127 records · Page 7Linked to original sources

arXiv2018

SU(5)$\times$U(1)$_X$ grand unification with minimal seesaw and $Z^\prime$-portal dark matter

We propose a grand unified SU(5)$\times$U(1)$_X$ model, where the standard SU(5) grand unified theory is supplemented by minimal seesaw and a right-handed neutrino dark matter with an introduction of a global $Z_2$-parity. In the presence of three right-handed neutrinos (RHNs), the model is free from all gauge and mixed-gravitational anomalies. The SU(5) symmetry is broken into the Standard Model (SM) gauge group at $M_{\rm GUT} \simeq 4 \times 10^{16}$ GeV in the standard manner, while the U(1)$_X$ symmetry breaking occurs at the TeV scale, which generates the TeV-scale mass of the U(1)$_X$ gauge boson ($Z^\prime$ boson) and the three Majorana RHNs. A unique $Z_2$-odd RHN is stable and serves as the dark matter (DM) in the present Universe, while the remaining two RHNs work to generate the SM neutrino masses through the minimal seesaw. We investigate the $Z^\prime$-portal RHN DM scenario in this model context, and find that the constraints from the DM relic abundance and the search results for a $Z^\prime$ boson resonance at the Large Hadron Collider (LHC) are complementary to narrow down the allowed parameter region, which will be fully covered by the future LHC experiments (for the $Z^\prime$ boson mass $<$ 5 TeV). We also briefly discuss the successful implementation of Baryogenesis and cosmological inflation scenarios in the present model.

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arXiv2018

Non-minimal quartic inflation in classically conformal U(1)$_X$ extended Standard Model

We propose quartic inflation with non-minimal gravitational coupling in the context of the classically conformal U(1)_X extension of the SM. In this model, the U(1)_X gauge symmetry is radiatively broken through the Coleman-Weinberg (CW) mechanism, by which the U(1)_X gauge boson (Z' boson) and the right-handed neutrinos (RHNs) acquire their masses. We consider their masses in the range of O(10 GeV)-O(10 TeV), which are accessible to high energy collider experiments. The radiative U(1)_X gauge symmetry breaking also generates a negative mass squared for the SM Higgs doublet, and the electroweak symmetry breaking occurs subsequently. We identify the U(1)_X Higgs field with inflaton and calculate the inflationary predictions. Due to the CW mechanism, the inflaton quartic coupling during inflation, which determines the inflationary predictions, is correlated to the U(1)_X gauge coupling. With this correlation, we investigate complementarities between the inflationary predictions and the current constraint from the Z' boson resonance search at the LHC Run-2 as well as the prospect of the search for the Z' boson and the RHNs at the future collider experiments. The radiative U(1)_X gauge symmetry breaking also generates a negative mass squared for the SM Higgs doublet, and the electroweak symmetry breaking occurs subsequently. We identify the U(1)_X Higgs field with inflaton and calculate the inflationary predictions. Due to the Coleman-Weinberg mechanism, the inflaton quartic coupling during inflation, which determines the inflationary predictions, is correlated to the U(1)_X gauge coupling. With this correlation, we investigate complementarities between the inflationary predictions and the current constraint from the Z' boson resonance search at the LHC Run-2 as well as the prospect of the search for the Z' boson and the RHNs at the future collider experiments.

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arXiv2018

Scalar dark matter, Type II Seesaw and the DAMPE cosmic ray $e^+ + e^-$ excess

The DArk Matter Particle Explorer (DAMPE) has reported a measurement of the flux of high energy cosmic ray electrons plus positrons (CREs) in the energy range between $25$ GeV and $4.6$ TeV. With unprecedented high energy resolution, the DAMPE data exhibit an excess of the CREs flux at an energy of around $1.4$ TeV. In this letter, we discuss how the observed excess can be understood in a minimal framework where the Standard Model (SM) is supplemented by a stable SM singlet scalar as dark matter (DM) and type II seesaw for generating the neutrino mass matrix. In our framework, a pair of DM particles annihilates into a pair of the SM SU(2) triplet scalars ($Δ$s) in type II seesaw, and the subsequent $Δ$ decays create the primary source of the excessive CREs around $1.4$ TeV. The lepton flavor structure of the primary source of CREs has a direct relationship with the neutrino oscillation data. We find that the DM interpretation of the DAMPE excess determines the pattern of neutrino mass spectrum to be the inverted hierarchy type, taking into account the constraints from the Fermi-LAT observations of dwarf spheroidal galaxies.

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arXiv2018

125 GeV Higgs boson mass from 5D gauge-Higgs unification

In the context of a simple gauge-Higgs unification (GHU) scenario based on the gauge group SU(3)$\times$U(1)$^\prime$ in a 5-dimensional flat space-time, we investigate a possibility to reproduce the observed Higgs boson mass of around 125 GeV. We introduce bulk fermion multiplets with a bulk mass and a (half) periodic boundary condition. In our analysis, we adopt a low energy effective theoretical approach of the GHU scenario, where the running Higgs quartic coupling is required to vanish at the compactification scale. Under this "gauge-Higgs condition," we investigate the renormalization group evolution of the Higgs quartic coupling and find a relation between the bulk mass and the compactification scale so as to reproduce the 125 GeV Higgs boson mass. Through quantum corrections at the one-loop level, the bulk fermions contribute to the Higgs boson production and decay processes and deviate the Higgs boson signal strengths at the Large Hadron Collider (LHC) experiments from the Standard Model (SM) predictions. Employing the current experimental data which show the the Higgs boson signal strengths for a variety of Higgs decay modes are consistent with the SM predictions, we obtain lower mass bounds on the lightest mode of the bulk fermions.

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arXiv2017

Fermionic Minimal Dark Matter in 5D Gauge-Higgs Unification

We propose a Minimal Dark Matter (MDM) scenario in the context of a simple gauge-Higgs Unification (GHU) model based on the gauge group SU(3) x U(1)' in 5-dimensional Minkowski space with a compactification of the 5th dimension on S^1/Z_2 orbifold. A pair of vector-like SU(3) multiplet fermions in a higher-dimensional representation is introduced in the bulk, and the DM particle is identified with the lightest mass eigenstate among the components in the multiplets. In the original model description, the DM particle communicates with the Standard Model (SM) particles only through the bulk gauge interaction, and hence our model is the GHU version of the MDM scenario. There are two typical realizations of the DM particle in 4-dimensional effective theory: (i) the DM particle is mostly composed of the SM SU(2)_L multiplets, or (ii) the DM is mostly composed of the SM SU(2)_L singlets. Since the case (i) is very similar to the original MDM scenario, we focus on the case (ii), which is a realization of the Higgs-portal DM scenario in the context of the GHU model. We identify an allowed parameter region to be consistent with the current experimental constraints, which will be fully covered by the direct dark matter detection experiments in the near future. In the presence of the bulk multiplet fermions in higher-dimensional $SU(3)$ representations, we reproduce the 125 GeV Higgs boson mass through the renormalization group evolution of Higgs quartic coupling with the compactification scale of 10-100 TeV.

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arXiv2017

Right-handed neutrino dark matter in the classically conformal U(1)' extended Standard Model

We consider the dark matter (DM) scenario in the context of the classically conformal U(1)' extended standard model (SM), with three right-handed neutrinos (RHNs) and the U(1)' Higgs field. The model is free from all the U(1)' gauge and gravitational anomalies in the presence of the three RHNs. We introduce a $Z_2$-parity in the model, under which an odd-parity is assigned to one RHN, while all the other particles is assigned to be $Z_2$-even, and hence the $Z_2$-odd RHN serves as a DM candidate. In this model, the U(1)' gauge symmetry is radiatively broken through the Coleman-Weinberg mechanism, by which the electroweak symmetry breaking is triggered. There are three free parameters in our model, the U(1)' charge of the SM Higgs doublet ($x_H$), the new U(1)' gauge coupling ($g_X$), and the U(1)' gauge boson ($Z'$) mass ($m_{Z'}$), which are severely constrained in order to solve the electroweak vacuum instability problem, and satisfy the LHC Run-2 bounds from the search for $Z'$ boson resonance. In addition to these constraints, we investigate the RHN DM physics. Because of the nature of classical conformality, we find that a RHN DM pair mainly annihilates into the SM particles through the $Z'$ boson exchange. This is the so-called $Z'$-portal DM scenario. Combining the electroweak vacuum stability condition, the LHC Run-2 bounds, and the cosmological constraint from the observed DM relic density, we find that all constrains complementarily work to narrow down the allowed parameter regions, and, especially, exclude $m_{Z'} \lesssim 3.5$ TeV. For the obtained allowed regions, we calculate the spin-independent cross section of the RHN DM with nucleons. We find that the resultant cross section well below the current experimental upper bounds.

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arXiv2017

Bounds on heavy Majorana neutrinos in type-I seesaw and implications for collider searches

The neutrino masses and flavor mixings, which are missing in the Standard Model (SM), can be naturally incorporated in the type-I seesaw extension of the SM with heavy Majorana neutrinos being singlet under the SM gauge group. If the heavy Majorana neutrinos are around the electroweak scale and their mixings with the SM neutrinos are sizable, they can be produced at high energy colliders, leaving characteristic signatures with lepton-number violations. Employing the general parametrization for the neutrino Dirac mass matrix in the minimal seesaw scenario, we perform a parameter scan and identify allowed regions to satisfy a variety of experimental constraints from the neutrino oscillation data, the electroweak precision measurements and the lepton-flavor violating processes. We find that the resultant mixing parameters between the heavy neutrinos and the SM neutrinos are more severely constrained than those obtained from the current search for heavy Majorana neutrinos at the LHC. Such parameter regions can be explored at the High-Luminosity LHC and a 100 TeV pp-collider in the future.

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arXiv2017

Fermion Dark Matter in Gauge-Higgs Unification

We propose a Majorana fermion dark matter in the context of a simple gauge-Higgs Unification (GHU) scenario based on the gauge group $SU(3)$ x $U(1)'$ in 5-dimensional Minkowski space with a compactification of the 5th dimension on $S^1/Z_2$ orbifold. The dark matter particle is identified with the lightest mode in $SU(3)$ triplet fermions additionally introduced in the 5-dimensional bulk. We find an allowed parameter region for the dark matter mass around a half of the Standard Model Higgs boson mass, which is consistent with the observed dark matter density and the constraint from the LUX 2016 result for the direct dark matter search. The entire allowed region will be covered by, for example, the LUX-ZEPLIN dark matter experiment in the near future. We also show that in the presence of the bulk $SU(3)$ triplet fermions the 125 GeV Higgs boson mass is reproduced through the renormalization group evolution of Higgs quartic coupling with the compactification scale of around $10^8$ GeV.

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arXiv2017

Thermal Inflation with Flaton Chemical Potential

Thermal inflation driven by a scalar field called "flaton" is a possible scenario to solve the cosmological moduli problem. We study a model of thermal inflation with a flaton chemical potential. In the presence of the chemical potential, a negative mass squared of the flaton, which is necessary to terminate the thermal inflation, is naturally induced. We identify the allowed parameter region for the chemical potential ($μ$) and the flaton self-coupling constant to solve the cosmological moduli problem and satisfy theoretical consistencies. In general, the chemical potential is a free parameter and it can be taken to be much larger than the typical scale of soft supersymmetry breaking parameters of $\mathcal{O} (1)$ TeV. For $μ\gtrsim 10^8$ GeV, we find that the reheating temperature after the thermal inflation can be high enough for the thermal leptogenesis scenario to be operative. This is in sharp contrast to the standard thermal inflation scenario, in which the reheating temperature is quite low and a special mechanism is necessary for generating sufficient amount of baryon asymmetry in the Universe after thermal inflation.

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arXiv2017

Anomalous Higgs Yukawa Couplings and Recent LHC Data

Very recently, the CMS collaboration has reported a search for the production for a Standard Model (SM) Higgs boson in association with a top quark pair ($t bar{t} H$) at the LHC Run-2 and a best fit $t bar{t} H$ yield of $1.5 pm 0.5$ times the SM prediction with an observed significance of $3.3 sigma$. We study a possibility of whether or not this observed deviation can be explained by anomalous Higgs Yukawa couplings with the top and the bottom quarks, along with the LHC Run-1 data for the Higgs boson properties. We find that anomalous top and bottom Yukawa couplings with about $0-20$% and $10-40$% reductions from their SM values, respectively, can simultaneously fit the recent CMS result and the LHC Run-1 data.

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arXiv2017

Inflection-point inflation in hyper-charge oriented U(1)$_X$ model

Inflection-point inflation is an interesting possibility to realize a successful slow-roll inflation when inflation is driven by a single scalar field with its value during inflation below the Planck mass ($ϕ_I \lesssim M_{Pl}$). In order for a renormalization group (RG) improved effective $λϕ^4$ potential to develop an inflection-point, the running quartic coupling $λ(ϕ)$ must exhibit a minimum with an almost vanishing value in its RG evolution, namely $λ(ϕ_I) \simeq 0$ and $β_λ(ϕ_I) \simeq 0$, where $β_λ$ is the beta-function of the quartic coupling. In this paper, we consider the inflection-point inflation in the context of the minimal gauged U(1)$_X$ extended Standard Model (SM), which is a generalization of the minimal U(1)$_{B-L}$ model, and is constructed as a linear combination of the SM U(1)$_Y$ and U(1)$_{B-L}$ gauge symmetries. We identify the U(1)$_X$ Higgs field with the inflaton field. For a successful inflection-point inflation to be consistent with the current cosmological observations, the mass ratios among the U(1)$_X$ gauge boson, the right-handed neutrinos and the U(1)$_X$ Higgs boson are fixed. Focusing on the case that the extra U(1)$_X$ gauge symmetry is mostly aligned along the SM U(1)$_Y$ direction, we investigate a consistency between the inflationary predictions and the latest LHC Run-2 results on the search for a narrow resonance with the di-lepton final state. %In addition, the inflection-point inflation provides a unique prediction for the running of the spectral index $α\simeq - 2.7 \times %10^{-3}\left(\frac{60}{N}\right)^2$ ($N$ is the e-folding number), which can be tested in the near future.

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arXiv2017

Inflection-point Higgs Inflation

Inflection-point inflation is an interesting possibility to realize a successful slow-roll inflation when inflation is driven by a single scalar field with its initial value below the Planck mass ($ϕ_I \lesssim M_{Pl}$). In order for a renormalization group (RG) improved effective $λϕ^4$ potential to develop an inflection-point, the quartic coupling $λ(ϕ)$ must exhibit a minimum with an almost vanishing value in its RG evolution, namely $λ(ϕ_I) \simeq 0$ and $β_λ(ϕ_I) \simeq 0$, where $β_λ$ is the beta-function of the quartic coupling. As an example, we consider the minimal gauged $B-L$ extended Standard Model at the TeV scale, where we identify the $B-L$ Higgs field as the inflaton field. For a successful inflection-point inflation, which is consistent with the current cosmological observations, the mass ratios among the $Z^{\prime}$ gauge boson, the right-handed neutrinos and the $B-L$ Higgs boson are fixed. Our scenario can be tested in the future collider experiments such as the High-Luminosity LHC and the SHiP experiments. In addition, the inflection-point inflation provides a unique prediction for the running of the spectral index $α\simeq - 2.7 \times 10^{-3}\left(\frac{60}{N}\right)^2$ ($N$ is the $e$-folding number), which can be tested in the near future.

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arXiv2017

Electroweak vacuum stability in classically conformal B-L extension of the Standard Model

We consider the minimal U(1)$_{B-L}$ extension of the Standard Model (SM) with the classically conformal invariance, where an anomaly free U(1)$_{B-L}$ gauge symmetry is introduced along with three generations of right-handed neutrinos and a U(1)$_{B-L}$ Higgs field. Because of the classically conformal symmetry, all dimensional parameters are forbidden. The $B-L$ gauge symmetry is radiatively broken through the Coleman-Weinberg mechanism, generating the mass for the $U(1)_{B-L}$ gauge boson ($Z^\prime$ boson) and the right-handed neutrinos. Through a small negative coupling between the SM Higgs doublet and the $B-L$ Higgs field, the negative mass term for the SM Higgs doublet is generated and the electroweak symmetry is broken. In this model context, we investigate the electroweak vacuum instability problem in the SM. It is known that in the classically conformal U(1)$_{B-L}$ extension of the SM, the electroweak vacuum remains unstable in the renormalization group analysis at the one-loop level. In this paper, we extend the analysis to the two-loop level, and perform parameter scans. We identify a parameter region which not only solve the vacuum instability problem, but also satisfy the recent ATLAS and CMS bounds from search for $Z^\prime$ boson resonance at the LHC Run-2. Considering self-energy corrections to the SM Higgs doublet through the right-handed neutrinos and the $Z^\prime$ boson, we derive the naturalness bound on the model parameters to realize the electroweak scale without fine-tunings.

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arXiv2017

Sparticle spectroscopy of the minimal SO(10) model

The supersymmetric (SUSY) minimal SO(10) model is a well-motivated grand unified theory, where the Standard Model (SM) fermions have Yukawa couplings with only one ${\bf 10}$-plet and one $\overline{\bf 126}$-plet Higgs fields and it is highly non-trivial if the realistic quark and lepton mass matrices can be reproduced in this context. It has been known that the best fit for all the SM fermion mass matrices is achieved by a vacuum expectation value of the $\overline{\bf 126}$-plet Higgs field being at the intermediate scale of around ${\cal O}(10^{13})$ GeV. Under the presence of the SO(10) symmetry breaking at the intermediate scale, the successful SM gauge coupling unification is at risk and likely to be spoiled. Recently, it has been shown that the low-energy fermion mass matrices, except for the down-quark mass predicted to be too low, are very well-fitted without the intermediate scale. In order to resolve the too-low down quark mass while keeping the other fittings intact, we consider SUSY threshold corrections to reproduce the right down quark mass. It turns out that this requires flavor-dependent soft parameters. Motivated by this fact, we calculate particle mass spectra at low energies with flavor-dependent sfermion masses at the grand unification scale. We present a benchmark particle mass spectrum which satisfies a variety of phenomenological constraints, in particular, the observed SM-like Higgs boson mass of around 125 GeV and the relic abundance of the neutralino dark matter as well as the experimental result of the muon anomalous magnetic moment. In the resultant mass spectrum, sleptons in the first and second generations, bino and winos are all light, and this scenario can be tested at the LHC Run-2 in the near future.

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arXiv2017

$Z^\prime_{BL}$ portal dark matter and LHC Run-2 results

We consider a concise dark matter scenario in the minimal gauged $B-L$ extension of the Standard Model (SM), where the global $B-L$ (baryon number minus lepton number) symmetry in the SM is gauged, and three generations of right-handed neutrinos and a $B-L$ Higgs field are introduced. Associated with the $B-L$ gauge symmetry breaking by a VEV of the $B-L$ Higgs field, the seesaw mechanism for generating the neutrino mass is automatically implemented after the electroweak symmetry breaking in the SM. In this model context, we introduce a $Z_2$-parity and assign an odd parity for one right-handed neutrino while even parities for the other fields. Therefore, the dark matter candidate is identified as the right-handed Majorana neutrino with odd $Z_2$ parity, keeping the minimality of the particle content intact. When the dark matter particle communicates with the SM particles mainly through the $B-L$ gauge boson ($Z^\prime_{BL}$ boson), its relic abundance is determined by only three free parameters, the $B-L$ gauge coupling ($α_{BL}$), the $Z^\prime_{BL}$ boson mass ($m_{Z^\prime}$) and the dark matter mass ($m_{DM}$). With the cosmological upper bound on the dark matter relic abundance we find a lower bound on $α_{BL}$ as a function of $m_{Z^\prime}$. On the other hand, we interpret the recent LHC Run-2 results on search for $Z^\prime$ boson resonance to an upper bound on $α_{BL}$ as a function of $m_{Z^\prime}$. Combining the two results we identify an allowed parameter region for this "$Z^\prime_{BL}$ portal" dark matter scenario, which turns out to be a narrow window with the lower mass bound of $m_{Z^\prime} > 2.5$ TeV.

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arXiv2017

$Z^\prime$-portal right-handed neutrino dark matter in the minimal U(1)$_X$ extended Standard Model

We consider a concise dark matter (DM) scenario in the context of a non-exotic U(1) extension of the Standard Model (SM), where a new U(1)$_X$ gauge symmetry is introduced along with three generation of right-handed neutrinos (RHNs) and an SM gauge singlet Higgs field. The model is a generalization of the minimal gauged U(1)$_{B-L}$ (baryon number minus lepton number) extension of the SM, in which the extra U(1)$_X$ gauge symmetry is expressed as a linear combination of the SM U(1)$_Y$ and U(1)$_{B-L}$ gauge symmetries. We introduce a $Z_2$-parity and assign an odd-parity only for one RHN among all particles, so that this $Z_2$-odd RHN plays a role of DM. The so-called minimal seesaw mechanism is implemented in this model with only two $Z_2$-even RHNs. In this context, we investigate physics of the RHN DM, focusing on the case that this DM particle communicates with the SM particles through the U(1)$_X$ gauge boson ($Z^\prime$ boson). This "$Z^\prime$-portal RHN DM" scenario is controlled by only three free parameters: the U(1)$_X$ gauge coupling ($α_X$), the $Z^\prime$ boson mass ($m_{Z^\prime}$), and the U(1)$_X$ charge of the SM Higgs doublet ($x_H$). We consider various phenomenological constraints to identify a phenomenologically viable parameter space. The most important constraints are the observed DM relic abundance and the latest LHC Run-2 results on the search for a narrow resonance with the di-lepton final state. We find that these are complementary with each other and narrow the allowed parameter region, leading to the lower mass bound of $m_{Z^\prime} \gtrsim 2.7$ TeV.

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arXiv2017

Multiple-point principle with a scalar singlet extension of the Standard Model

We suggest a scalar singlet extension of the standard model, in which the multiple-point principle (MPP) condition of a vanishing Higgs potential at the Planck scale is realized. Although there have been lots of attempts to realize the MPP at the Planck scale, the realization with keeping naturalness is quite difficult. Our model can easily achieve the MPP at the Planck scale without large Higgs mass corrections. It is worth noting that the electroweak symmetry can be radiatively broken in our model. In the naturalness point of view, the singlet scalar mass should be of ${\cal O}(1)\,{\rm TeV}$ or less. We also consider right-handed neutrino extension of the model for neutrino mass generation. The model does not affect the MPP scenario, and might keep the naturalness with the new particle mass scale beyond TeV, thanks to accidental cancellation of Higgs mass corrections.

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arXiv2016

Non-Minimal Quartic Inflation in Supersymmetric SO(10)

We describe how quartic ($λϕ^4$) inflation with non-minimal coupling to gravity is realized in realistic supersymmetric $SO(10)$ models. In a well-motivated example the $16-\overline{16}$ Higgs multiplets, which break $SO(10)$ to $SU(5)$ and yield masses for the right-handed neutrinos, provide the inflaton field $ϕ$. Thus, leptogenesis is a natural outcome in this class of $SO(10)$ models. Moreover, the adjoint (45-plet) Higgs also acquires a GUT scale value during inflation so that the monopole problem is evaded. The scalar spectral index $n_s$ is in good agreement with the observations and $r$, the tensor to scalar ratio, is predicted for realistic values of GUT parameters to be of order $10^{-3}-10^{-2}$.

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