arXiv subjects

Nobuchika Okada

Publications and source records attributed to Nobuchika Okada.

At least 145 records · Page 8Linked to original sources

arXiv2016

125 GeV Higgs boson mass and muon g-2 in 5D MSSM

In the MSSM, the tension between the observed Higgs boson mass and the experimental result of the muon $g-2$ measurement requires a large mass splitting between stops and smuons/charginos/neutralinos. We consider a 5-dimensional (5D) framework of the MSSM with the Randall-Sundrum warped background metric, and show that such a mass hierarchy is naturally achieved in terms of geometry. In our setup, the supersymmetry is broken at the ultraviolet (UV) brane, while all the MSSM multiplets reside in the 5D bulk. An appropriate choice of the bulk mass parameters for the MSSM matter multiplets can naturally realize the sparticle mass hierarchy desired to resolve the tension. Gravitino is localized at the UV brane and hence becomes very heavy, while the gauginos spreading over the bulk acquire their masses suppressed by the 5th dimensional volume. As a result, the LSP neutralino is a candidate for the dark matter as usual in the MSSM. In addition to reproducing the SM-like Higgs boson mass of around 125 GeV and the measured value of the muon $g-2$, we consider a variety of phenomenological constraints, and present the benchmark particle mass spectra which can be explored at the LHC Run-2 in the near future.

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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

Proton Decay Prediction in 5D Gauge-Higgs Unification

The Higgs boson mass and top quark mass imply that the Higgs quartic coupling vanishes around the scale of $10^9 - 10^{13}$ GeV, depending on the precise value of the top quark mass. The vanishing quartic coupling can be naturally addressed if the Higgs field originates from a 5-dimensional gauge field and the 5th dimension is compactified at the scale of the vanishing Higgs quartic coupling, which is a scenario based on gauge-Higgs unification. We present a general prediction of the scenario on the proton decay process $p \to π^0 e^+$. In many gauge-Higgs unification models, the 1st generation fermions are localized towards an orbifold fixed point in order to realize the realistic Yukawa couplings. Hence, four-fermion operators responsible for the proton decay can appear with a suppression of the 5-dimensional Planck scale (not the 4-dimensional Planck scale). Since the 5-dimensional Planck scale is connected to the compactification scale, we have a correlation between the proton partial decay width and the top quark mass. We show that the future Hyper-Kamiokande experiment may discover the proton decay if the top quark pole mass is larger than about 172.5 GeV.

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arXiv2016

Vacuum stability and naturalness in type-II seesaw

We study the vacuum stability and perturbativity conditions in the minimal type-II seesaw model. These conditions give characteristic constraints to model parameters. In the model, there is a $SU(2)_L$ triplet scalar field, which could cause a large Higgs mass correction. From the naturalness point of view, heavy Higgs masses should be lower than $350\,{\rm GeV}$, which can be testable by the LHC Run-II results. Due to effects of the triplet scalar field, branching ratios of the Higgs decay ($h\to γγ, Zγ$) deviate from the standard model, and large parameter region is excluded by the recent ATLAS and CMS combined analysis of $h\to γγ$. Our result of the signal strength for $h\to γγ$ is $R_{γγ} \lesssim 1.1$, but its deviation is too small to observe at the LHC experiment.

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arXiv2016

Classically conformal U(1)' extended standard model, electroweak vacuum stability, and LHC Run-2 bounds

We consider the minimal U(1)' extension of the standard model (SM) with the classically conformal invariance, where an anomaly-free U(1)' gauge symmetry is introduced along with three generations of right-handed neutrinos and a U(1)' Higgs field. Since the classically conformal symmetry forbids all dimensional parameters in the model, the U(1)' gauge symmetry is broken by the Coleman-Weinberg mechanism, generating the mass terms of the U(1)' gauge boson ($Z$' boson) and the right-handed neutrinos. Through a mixing quartic coupling between the U(1)' Higgs field and the SM Higgs doublet field, the radiative U(1)' gauge symmetry breaking also triggers the breaking of the electroweak symmetry. In this model context, we first investigate the electroweak vacuum instability problem in the SM. Employing the renormalization group equations at the two-loop level and the central values for the world average masses of the top quark ($m_t=173.34$ GeV) and the Higgs boson ($m_h=125.09$ GeV), we perform parameter scans to identify the parameter region for resolving the electroweak vacuum instability problem. Next we interpret the recent ATLAS and CMS search limits at the LHC Run-2 for the sequential $Z$' boson to constrain the parameter region in our model. Combining the constraints from the electroweak vacuum stability and the LHC Run-2 results, we find a bound on the $Z$' boson mass as $m_{Z'} \gtrsim 3.5$ TeV. We also calculate self-energy corrections to the SM Higgs doublet field through the heavy states, the right-handed neutrinos and the $Z$' boson, and find the naturalness bound as $m_{Z'} \lesssim 7$ TeV, in order to reproduce the right electroweak scale for the fine-tuning level better than 10\%. The resultant mass range of $3.5$ TeV $\lesssim m_{Z'} \lesssim 7$ TeV will be explored at the LHC Run-2 in the near future.

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arXiv2016

Higgs Phenomenology in the Minimal $SU(3)_L\times U(1)_X$ Model

We investigate the phenomenology of a model based on the $SU(3)_c\times SU(3)_L\times U(1)_X$ gauge theory, the so-called 331 model. In particular, we focus on the Higgs sector of the model which is composed of three $SU(3)_L$ triplet Higgs fields, and this corresponds to the minimal form to realize phenomenologically acceptable scenario. After the spontaneous symmetry breaking $SU(3)_L\times U(1)_X\to SU(2)_L\times U(1)_Y$, our Higgs sector effectively becomes that with two $SU(2)_L$ doublet scalar fields, in which the first and the second generation quarks couple to the different Higgs doublet from that couples to the third generation quarks. This structure causes the flavour changing neutral current mediated by Higgs bosons at the tree level. By taking an alignment limit of the mass matrix for the CP-even Higgs bosons, which is naturally realized in the case with the breaking scale of $SU(3)_L\times U(1)_X$ to be much larger than that of $SU(2)_L\times U(1)_Y$, we can avoid current constraints from flavour experiments such as the $B^0$-$\bar{B}^0$ mixing even for the Higgs bosons masses being ${\cal O}(100)$ GeV. In this allowed parameter space, we clarify that a characteristic deviation in quark Yukawa couplings of the standard model-like Higgs boson is predicted, which has a different pattern from that seen in two Higgs doublet models with a softly-broken $Z_2$ symmetry. We also find that the flavour violating decay modes of the extra Higgs boson, e.g., $H/A \to tc$ and $H^\pm \to ts$ can be dominant, and they yield the important signature to distinguish our model from the two Higgs doublet models.

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arXiv2016

Anomalous Top and Bottom Yukawa Couplings and LHC Run 1 Data

In some extensions of the Standard Model, Yukawa couplings of the physical Higgs boson can be deviated from those in the Standard Model. We study a possibility whether or not such anomalous Yukawa couplings are consistent with the LHC Run 1 data. It is found that sizable deviations of top and bottom (and tau) Yukawa couplings from the Standard Model predictions can nicely fit the data. New physics beyond the Standard Model can be revealed through more precise measurements of such anomalous Yukawa couplings at the LHC Run 2 in the near future. We also discuss a simple setup which can leads to anomalous Yukawa couplings.

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arXiv2016

Radiative Seesaw in Minimal 3-3-1 Model

We study the neutrino sector in a minimal $SU(3)_L\times U(1)_X$ model, in which its mass is generated at one-loop level with the charged lepton mass, and hence there exists a strong correlation between the charged-lepton mass and the neutrino mass. We identify the parameter region of this model to satisfy the current neutrino oscillation data as well as the constraints on lepton flavor violating processes. We also discuss a possibility to explain the muon anomalous magnetic moment.

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arXiv2016

R-parity Conserving Minimal SUSY $B-L$ Model

We propose a simple gauged U(1)$_{B-L}$ extension of the minimal supersymmetric Standard Model (MSSM), where R-parity is conserved as usual in the MSSM. The global $B-L$ (baryon number minus lepton number) symmetry in the MSSM is gauged and three MSSM gauge-singlet chiral multiplets with a unit $B-L$ charge are introduced, ensuring the model free from gauge and gravitational anomalies. We assign an odd R-parity for two of the new chiral multiplets and hence they are identified with the right-handed neutrino superfields, while an even R-parity is assigned to the other one ($Φ$). The scalar component of $Φ$ plays the role of a Higgs field to break the U(1)$_{B-L}$ symmetry through its negative mass squared, which is radiatively generated by the renormalization group running of soft supersymmetry (SUSY) breaking parameters from a high energy. This radiative U(1)$_{B-L}$ symmetry breaking leads to its breaking scale being at the TeV naturally. Because of our novel R-parity assignment, three light neutrinos are Dirac particles with one massless state. Since R-parity is conserved, the lightest neutralino is a prime candidate of the dark matter as usual. In our model, the lightest eigenstate of the mixture of the $B-L$ gaugino and the fermionic component of $Φ$ appears as a new dark matter candidate. We investigate phenomenology of this dark matter particle. We also discuss collider phenomenology of our model. In particular, the $B-L$ gauge boson ($Z'$), once discovered at the Large Hadron Collider, can be a probe to determine the number of (right-handed) Dirac neutrinos with its invisible decay width, in sharp contrast with the conventional $B-L$ extension of the SM or MSSM where the right-handed neutrinos are heavy Majorana particles and decay to the SM leptons.

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arXiv2016

Bosonic seesaw mechanism in a classically conformal extension of the Standard Model

We suggest the so-called bosonic seesaw mechanism in the context of a classically conformal $U(1)_{B-L}$ extension of the Standard Model with two Higgs doublet fields. The $U(1)_{B-L}$ symmetry is radiatively broken via the Coleman-Weinberg mechanism, which also generates the mass terms for the two Higgs doublets through quartic Higgs couplings. Their masses are all positive but, nevertheless, the electroweak symmetry breaking is realized by the bosonic seesaw mechanism. Analyzing the renormalization group evolutions for all model couplings, we find that a large hierarchy among the quartic Higgs couplings, which is crucial for the bosonic seesaw mechanism to work, is dramatically reduced toward high energies. Therefore, the bosonic seesaw is naturally realized with only a mild hierarchy, if some fundamental theory, which provides the origin of the classically conformal invariance, completes our model at some high energy, for example, the Planck scale. We identify the regions of model parameters which satisfy the perturbativity of the running couplings and the electroweak vacuum stability as well as the naturalness of the electroweak scale.

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arXiv2016

Radiative Breaking of the Minimal Supersymmetric Left-Right Model

We study a variation to the SUSY Left-Right symmetric model based on the gauge group $SU(3)_c\times SU(2)_L\times SU(2)_R\times U(1)_{BL}$. Beyond the quark and lepton superfields we only introduce a second Higgs bidoublet to produce realistic fermion mass matrices. This model does not include any $SU(2)_R$ triplets. We calculate renormalization group evolutions of soft SUSY parameters at the one-loop level down to low energy. We find that an $SU(2)_R$ slepton doublet acquires a negative mass squared at low energies, so that the breaking of $SU(2)_R\times U(1)_{BL}\rightarrow U(1)_Y$ is realized by a non-zero vacuum expectation value of a right-handed sneutrino. Small neutrino masses are produced through neutrino mixings with gauginos. Mass limits on the $SU(2)_R\times U(1)_{BL}$ sector are obtained by direct search results at the LHC as well as lepton-gaugino mixing bounds from the LEP precision data.

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arXiv2016

Testing the 2-TeV Resonance with Trileptons

The CMS collaboration has reported a 2.8$σ$ excess in the search of the SU(2)$_R$ gauge bosons decaying through right-handed neutrinos into the two electron plus two jets ($eejj$) final states. This can be explained if the SU(2)$_R$ charged gauge bosons $W_R^\pm$ have a mass of around 2 TeV and a right-handed neutrino with a mass of ${\cal O}(1)$ TeV mainly decays to electron. Indeed, recent results in several other experiments, especially that from the ATLAS diboson resonance search, also indicate signatures of such a 2 TeV gauge boson. However, a lack of the same-sign electron events in the CMS $eejj$ search challenges the interpretation of the right-handed neutrino as a Majorana fermion. Taking this situation into account, in this paper, we consider a possibility of explaining the CMS $eejj$ excess based on the $SU(2)_L\otimes SU(2)_R\otimes U(1)_{B-L}$ gauge theory with pseudo-Dirac neutrinos. We find that both the CMS excess events and the ATLAS diboson anomaly can actually be explained in this framework without conflicting with the current experimental bounds. This setup in general allows sizable left-right mixing in both the charged gauge boson and neutrino sectors, which enables us to probe this model through the trilepton plus missing-energy search at the LHC. It turns out that the number of events in this channel predicted in our model is in good agreement with that observed by the CMS collaboration. We also discuss prospects for testing this model at the LHC Run-II experiments.

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arXiv2015

Off-Shell Supersymmetry

Supersymmetry does not dictate the way we should quantize the fields in the supermultiplets, and so we have the freedom to quantize the Standard Model (SM) particles and their superpartners differently. We propose a generalized quantization scheme under which a particle can only appear off-shell, while its contributions to quantum corrections are exactly the same as those in the usual quantum field theory. We apply this quantization scheme solely to the sparticles in the $R$-parity preserving Minimal Supersymmetric Standard Model (MSSM). Thus sparticles can only appear off-shell. They could be light but would completely escape the direct detection at any experiments such as the LHC. However, our theory still retains the same desirable features of the usual MSSM at the quantum level. For instance, the gauge hierarchy problem is solved and the three MSSM gauge couplings are unified in the usual way. Although direct detection of sparticles is impossible, their existence can be revealed by precise measurements of some observables (such as the running QCD coupling) that may receive quantum corrections from them and have sizable deviations from the SM predictions. Also the experimental constraints from the indirect sparticle search are still applicable.

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arXiv2015

Improved bounds on the heavy neutrino productions at the LHC

The Majorana neutrino in type-I seesaw and the pseudo-Dirac neutrinos in the inverse seesaw can have sizable mixings with the light neutrinos in the standard model (SM), through which the heavy neutrinos can be produced at the Large Hadron Collider (LHC). In producing the heavy neutrinos we study a variety of initial states such as quark-quark, quark-gluon and gluon-gluon as well as photon mediated processes. For the Majorana heavy neutrino production we consider same-sign di-lepton plus di-jet as the signal events. Using the recent ATLAS and CMS data at $\sqrt{s}=$ 8 TeV with 20.3 fb$^{-1}$ and 19.7 fb$^{-1}$ luminosities, respectively, we obtain direct upper bounds on the light-heavy neutrino mixing angles. For the pseudo-Dirac heavy neutrino production we consider the final sate with tri-lepton plus missing energy as the signal events. Using the recent anomalous multi-lepton search by CMS at $\sqrt{s}=$ 8 TeV with 19.5 fb$^{-1}$ luminosity we obtain upper bounds on the mixing angles. Taking the varieties of initial states into account, the previously obtained upper bounds on the mixing angles have been improved. We scale our results at the 8 TeV LHC to obtain a prospective search reach at the 14 TeV LHC with high luminosities.

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arXiv2015

$μ$-Term Hybrid Inflation and Split Supersymmetry

We consider $μ$-term hybrid inflation which, in its minimal format with gravity mediated supersymmetry breaking, leads to split supersymmetry. The MSSM $μ$-term in this framework is larger than the gravitino mass $m_G$, and successful inflation requires $m_G$ (and hence also $|μ|$) $\gtrsim 5 \times 10^7$ GeV, such that the gravitino decays before the LSP neutralino freezes out. Assuming universal scalar masses of the same order as $m_G$, this leads to split supersymmetry. The LSP wino with mass $\simeq$ 2 TeV is a plausible dark matter candidate, the gluino may be accessible at the LHC, and the MSSM parameter $\tan β\simeq 1.7$ in order to be compatible with the measured Higgs boson mass. The tensor-to-scalar ratio $r$, a canonical measure of gravity waves, can be as high as $0.001$.

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arXiv2015

Running Non-Minimal Inflation with Stabilized Inflaton Potential

In the context of the Higgs model involving gauge and Yukawa interactions with the spontaneous gauge symmetry breaking, we consider $λϕ^4$ inflation with non-minimal gravitational coupling, where the Higgs field is identified as inflaton. Since the inflaton quartic coupling is very small, once quantum corrections through the gauge and Yukawa interactions are taken into account, the inflaton effective potential most likely becomes unstable. In order to avoid this problem, we need to impose stability conditions on the effective inflaton potential, which lead to not only non-trivial relations amongst the particle mass spectrum of the model, but also correlations between the inflationary predictions and the mass spectrum. For concrete discussion, we investigate the minimal $B-L$ extension of the Standard Model with identification of the $B-L$ Higgs field as inflaton. The stability conditions for the inflaton effective potential fix the mass ratio amongst the $B-L$ gauge boson, the right-handed neutrinos and the inflaton. This mass ratio also correlates with the inflationary predictions. In other words, if the $B-L$ gauge boson and the right-handed neutrinos are discovered in future, their observed mass ratio provides constraints on the inflationary predictions.

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arXiv2015

Towards LHC Physics with Non-local Standard Model

We take a few steps towards constructing a string-inspired nonlocal extension of the Standard Model. We start by illustrating how quantum loop calculations can be performed in nonlocal scalar field theory. In particular, we show the potential to address the hierarchy problem in the nonlocal framework. Next, we construct a nonlocal abelian gauge model and derive modifications of the gauge interaction vertex and field propagators. We apply the modifications to a toy version of the nonlocal Standard Model and investigate collider phenomenology. We find the lower bound on the scale of non-locality from the 8 TeV LHC data to be $2.5-3$ TeV.

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arXiv2015

Electroweak symmetry breaking through bosonic seesaw mechanism in a classically conformal extension of the Standard Model

We suggest the so-called bosonic seesaw mechanism in the context of a classically conformal $U(1)_{B-L}$ extension of the Standard Model with two Higgs doublet fields. The $U(1)_{B-L}$ symmetry is radiatively broken via the Coleman-Weinberg mechanism, which also generates the mass terms for the two Higgs doublets through quartic Higgs couplings. Their masses are all positive but, nevertheless, the electroweak symmetry breaking is realized by the bosonic seesaw mechanism. We analyze the renormalization group evolutions for all model couplings, and find that a large hierarchy among the quartic Higgs couplings, which is crucial for the bosonic seesaw mechanism to work, is dramatically reduced toward high energies. Therefore, the bosonic seesaw is naturally realized with only a mild hierarchy, if some fundamental theory, which provides the origin of the classically conformal invariance, completes our model at some high energy, for example, the Planck scale. The requirements for the perturbativity of the running couplings and the electroweak vacuum stability in the renormalization group analysis as well as for the naturalness of the electroweak scale, we have identified the regions of model parameters. For example, the scale of the $U(1)_{B-L}$ gauge symmetry breaking is constrained to be $\lesssim 100$ TeV, which corresponds to the extra heavy Higgs boson masses to be $\lesssim 2$ TeV. Such heavy Higgs bosons can be tested at the Large Hadron Collider in the near future.

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