arXiv subjects

Nobuchika Okada

Publications and source records attributed to Nobuchika Okada.

At least 163 records · Page 9Linked to original sources

arXiv2015

The Observational Status of Simple Inflationary Models: an Update

We provide an update on five relatively well motivated inflationary models in which the inflaton is a Standard Model singlet scalar field. These include i) the textbook quadratic and quartic potential models but with additional couplings of the inflaton to fermions and bosons, which enable reheating and also modify the naive predictions for the scalar spectral index $n_s$ and $r$, ii) models with Higgs and Coleman-Weinberg potentials, and finally iii) a quartic potential model with non-minimal coupling of the inflaton to gravity. For $n_s$ values close to 0.96, as determined by the WMAP9 and Planck experiments, most of the considered models predict $r\gtrsim0.02$. The running of the scalar spectral index, quantified by $|\rm{d} n_s/\rm{d}\ln k|$, is predicted in these models to be of order $10^{-4}$--$10^{-3}$.

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arXiv2015

Classically conformal U(1)$^\prime$ extended Standard Model and Higgs vacuum stability

We consider the minimal U(1)$^\prime$ extension of the Standard Model (SM) with conformal invariance at the classical level, where in addition to the SM particle contents, three generations of right-handed neutrinos and a U(1)$^\prime$ Higgs field are introduced. In the presence of the three right-handed neutrinos, which are responsible for the seesaw mechanism, this model is free from all the gauge and gravitational anomalies. The U(1)$^\prime$ gauge symmetry is radiatively broken via the Coleman-Weinberg mechanism, by which the U(1)$^\prime$ gauge boson ($Z^\prime$ boson) mass as well as the Majorana mass for the right-handed neutrinos are generated. The radiative U(1)$^\prime$ symmetry breaking also induces a negative mass squared for the SM Higgs doublet to trigger the electroweak symmetry breaking. In this context, we investigate a possibility to solve the SM Higgs vacuum instability problem. The model includes only three free parameters (U(1)$^\prime$ charge of the SM Higgs doublet, U(1)$^\prime$ gauge coupling and $Z^\prime$ boson mass), for which we perform parameter scan, and identify a parameter region resolving the SM Higgs vacuum instability. We also examine naturalness of the model. The heavy states associated with the U(1)$^\prime$ symmetry breaking contribute to the SM Higgs self-energy. We find an upper bound on $Z^\prime$ boson mass, $m_{Z^\prime} \lesssim 6$ TeV, in order to avoid a fine-tuning severer than 10 % level. The $Z^\prime$ boson in this mass range can be discovered at the LHC Run-2 in the near future.

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arXiv2015

Simple brane-world inflationary models: an update

In the light of the Planck 2015 results, we update simple inflationary models based on the quadratic, quartic, Higgs and Coleman-Weinberg potentials in the context of the Randall-Sundrum brane-world cosmology. Brane-world cosmological effect alters the inflationary predictions of the spectral index ($n_s$) and the tensor-to-scalar ratio ($r$) from those obtained in the standard cosmology. In particular, the tensor-to-scalar ratio is enhanced in the presence of the 5th dimension. In order to maintain the consistency with the Planck 2015 results for the inflationary predictions in the standard cosmology, we find a lower bound on the five-dimensional Planck mass ($M_5$). On the other hand, the inflationary predictions laying outside of the Planck allowed region can be pushed into the allowed region by the brane-world cosmological effect with a suitable choice of $M_5$.

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arXiv2015

Simple inflationary models in Gauss-Bonnet brane-world cosmology

In light of the recent Planck 2015 results for the measurement of the CMB anisotropy, we study simple inflationary models in the context of the Gauss-Bonnet brane-world cosmology. The brane-world cosmological effect modifies the power spectra of scalar and tensor perturbations generated by inflation and causes a dramatic change for the inflationary predictions of the spectral index ($n_s$) and the tensor-to-scalar ratio ($r$) from those obtained in the standard cosmology. In particular, the power spectrum of tensor perturbation is suppressed due to the Gauss-Bonnet brane-world cosmological effect, which is in sharp contrast with inflationary scenario in the Randall-Sundrum brane-world cosmology where the power spectrum is enhanced. Hence, these two brane-world cosmological scenarios are distinguishable. With the dramatic change of the inflationary predictions, the inflationary scenario in the Gauss-Bonnet brane-world cosmology can be tested by more precise measurements of $n_s$ and future observations of the CMB $B$-mode polarization.

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arXiv2015

Higgs Inflation, Seesaw Physics and Fermion Dark Matter

We present an inflationary model in which the Standard Model Higgs doublet field with non-minimal coupling to gravity drives inflation, and the effective Higgs potential is stabilized by new physics which includes a dark matter particle and right-handed neutrinos for the seesaw mechanism. All of the new particles are fermions, so that the Higgs doublet is the unique inflaton candidate. With central values for the masses of the top quark and the Higgs boson, the renormalization group improved Higgs potential is employed to yield the scalar spectral index $n_s \simeq 0.968$, the tensor-to-scalar ratio $r \simeq 0.003$, and the running of the spectral index $α=dn_s/d \ln k \simeq -5.2 \times 10^{-4}$ for the number of e-folds $N_0=60$ ($n_s \simeq 0.962$, $r \simeq 0.004$, and $α\simeq -7.5 \times 10^{-4}$ for $N_0=50$). The fairly low value of $r \simeq 0.003$ predicted in this class of models means that the ongoing space and land based experiments are not expected to observe gravity waves generated during inflation. [Dedicated to the memory of Dr. Paul Weber (1947 - 2015). Paul was an exceptional human being and a very special friend who will be sorely missed.]

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arXiv2014

Galactic Center gamma-ray excess from two-Higgs-doublet-portal dark matter

We consider a simple extension of the type-II two-Higgs-doublet model by introducing a real scalar as a candidate for dark matter in the present Universe. The main annihilation mode of the dark matter particle with a mass of around $31-40$ GeV is into a $b\bar{b}$ pair, and this annihilation mode suitably explains the observed excess of the gamma-ray flux from the Galactic Center. We identify the parameter region of the model that can fit the gamma-ray excess and satisfy phenomenological constraints, such as the observed dark matter relic density and the null results of direct dark matter search experiments. Most of the parameter region is found to be within the search reach of future direct dark matter detection experiments.

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arXiv2014

Simple brane-world inflationary models in light of BICEP2

Motivated by the recent CMB $B$-mode observation announced by the BICEP2 collaboration, we study simple inflationary models in the Randall-Sundrum brane-world cosmology. Brane-world cosmological effects alter the inflationary predictions of the spectral index ($n_s$) and the tensor-to-scalar ratio ($r$) from those obtained in the standard cosmology. In particular, the tensor-to-scalar ratio is enhanced in the presence of the 5th dimension, and simple inflationary models which predict small $r$ values in the standard cosmology can yield $r$ values being compatible with the BICEP2 result, $r=0.2^{+0.07}_{-0.05}$. Confirmation of the BICEP2 result and more precise measurements of $n_s$ and $r$ in the near future allow us to constrain the 5-dimensional Planck mass ($M_5$) of the brane-world scenario. We also discuss the post inflationary scenario, namely, reheating of the universe through inflaton decay to the Standard Model particles. When we require the renormalizability of the inflationary models, the inflaton only couples with the Higgs doublet among the Standard Model fields, through which reheating process occurs. If $M_5$ is as low as $10^6$ GeV, the mass of inflaton becomes of ${\cal O}$(100 GeV), providing the inflationary predictions being consistent with the BICEP2 and Planck results. Such a light inflaton has some impacts on phenomenology at the electroweak scale through the coupling with the Higgs doublet.

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arXiv2014

Direct Bounds on Electroweak Scale Pseudo-Dirac Neutrinos from $\sqrt s=8$ TeV LHC Data

Seesaw models with a small lepton number breaking can naturally accommodate electroweak-scale pseudo-Dirac neutrinos with a sizable mixing with the active neutrinos, while satisfying the light neutrino oscillation data. Due to the smallness of the lepton number breaking parameter, the 'smoking gun' collider signature of same-sign dileptons is suppressed, and the heavy neutrinos in these models would manifest at the LHC dominantly through lepton number conserving trilepton final states. Using the recent CMS results for anomalous production of multilepton events at $\sqrt{s}$=8 TeV LHC with an integrated luminosity of $19.5~{\rm fb}^{-1}$, we derive direct upper bounds on the light-heavy neutrino mixing parameter as a function of the heavy Dirac neutrino mass. These limits extend the collider sensitivity to higher heavy neutrino masses up to about 500 GeV. In the lower mass range, our limits are comparable to the existing indirect constraints derived from Higgs and electroweak precision data.

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arXiv2014

TeV scale seesaw from supersymmetric Higgs-lepton inflation and BICEP2

We discuss the physics resulting from the supersymmetric Higgs-lepton inflation model and the recent CMB B-mode observation by the BICEP2 experiment. The tensor-to-scalar ratio r=0.20+0.07-0.05 of the primordial fluctuations indicated by the CMB B-mode polarization is consistent with the prediction of this inflationary model for natural parameter values. A salient feature of the model is that it predicts the seesaw mass scale M from the amplitude of the tensor mode fluctuations. It is found that the 68% (95%) confidence level (CL) constraints from the BICEP2 experiment give 927 GeV < M < 1.62 TeV (751 GeV < M < 2.37 TeV) for 50 e-foldings and 391 GeV < M < 795 GeV (355 GeV < M < 1.10 TeV) for 60 e-foldings. In the type I seesaw case, the right-handed neutrinos in this mass range are elusive in collider experiments due to the small mixing angle. In the type III seesaw, in contrast, the heavy leptons will be within the reach of future experiments. We point out that a significant portion of the parameter region corresponding to the 68% CL of the BICEP2 experiment will be covered by the Large Hadron Collider experiments at 14 TeV.

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arXiv2014

Supersymmetric B-L inflation near the conformal coupling

We investigate a novel scenario of cosmological inflation in a gauged $B-L$ extended minimal supersymmetric Standard Model with R-symmetry. We use a noncanonical Kähler potential and a superpotential, both preserving the R-symmetry to construct a model of slow-roll inflation. The model is controlled by two real parameters: the nonminimal coupling $ξ$ that originates from the Kähler potential, and the breaking scale $v$ of the $U(1)_{B-L}$ symmetry. We compute the spectrum of the cosmological microwave background radiation and show that the prediction of the model fits well the recent Planck satellite observation for a wide range of the parameter space. We also find that the typical reheating temperature of the model is low enough to avoid the gravitino problem but nevertheless allows sufficient production of the baryon asymmetry if we take into account the effect of resonance enhancement. The model is free from cosmic strings that impose stringent constraints on generic $U(1)_{B-L}$ based scenarios, as in our scenario the $U(1)_{B-L}$ symmetry is broken from the onset.

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arXiv2014

Neutrino Mass and Dark Matter in light of recent AMS-02 results

We study a simple extension of the Standard Model supplemented by an electroweak triplet scalar field to accommodate small neutrino masses by the type-II seesaw mechanism, while an additional singlet scalar field can play the role of cold dark matter (DM) in our Universe. This DM candidate is leptophilic for a wide range of model parameter space, and the lepton flux due to its annihilation carries information about the neutrino mass hierarchy. Using the recently released high precision data on positron fraction and flux from the AMS-02 experiment, we examine the DM interpretation of the observed positron excess in our model for two kinematically distinct scenarios with the DM and triplet scalar masses (a) non-degenerate ($m_{\rm DM}\gg m_Δ$), and (b) quasi-degenerate ($m_{\rm DM} \simeq m_Δ$). We find that a good fit to the AMS-02 data can be obtained in both cases (a) and (b) with a normal hierarchy of neutrino masses, while the inverted hierarchy case is somewhat disfavored. Although we require a larger boost factor for the normal hierarchy case, this is still consistent with the current upper limits derived from Fermi-LAT and IceCube data for case (a). Moreover, the absence of an excess anti-proton flux as suggested by PAMELA data sets an indirect upper limit on the DM-nucleon spin-independent elastic scattering cross section which is stronger than the existing DM direct detection bound from LUX in the AMS-02 preferred DM mass range.

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arXiv2014

Gamma ray emission in Fermi bubbles and Higgs portal dark matter

It has been recently pointed out that the excess of the gamma ray spectrum in the Fermi Bubbles at low latitude can be well explained by the annihilation of dark matter particles. The best-fit candidate corresponds to the annihilation of a dark matter with mass of around $62$ GeV into $b \bar{b}$ with the cross section, $σv \simeq 3.3 \times 10^{-26}$ cm$^3$/s, or the annihilation of a dark matter with mass of around $10$ GeV into a tau lepton pair with the cross section, $σv \simeq 5.6 \times 10^{-27}$ cm$^3$/s. We point out that the Higgs portal dark matter models are perfectly compatible with this interpretation of the dark matter annihilation, satisfying other phenomenological constraints. We also show that the parameter region which reproduces the bet-fit values can be partly explored by the future direct dark matter search at the XENON1T.

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arXiv2013

Higgs-lepton inflation in the supersymmetric minimal seesaw model

We investigate a scenario of cosmological inflation realised along a flat direction of the minimal seesaw model embedded in supergravity with a noncanonical R-parity violating Kähler potential. It is shown that with appropriate seesaw parameters the model is consistent with the present observation of the cosmological microwave background (CMB) as well as with the neutrino oscillation data. It is also shown that the baryon asymmetry of the Universe can be generated through leptogenesis. The model favours supersymmetry breaking with the gravitino as the lightest superparticle, and thus indicates the gravitino dark matter scenario. An interesting feature of this model is that the seesaw parameters are constrained by the CMB spectra. The 2-$σ$ constraints from the 9-year WMAP data yield a mild lower bound on the seesaw mass scale $\gtrsim$ TeV. We expect that the observation by the Planck satellite will soon provide more stringent constraints. The phenomenological and cosmological implications of the R-parity violation are also discussed.

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arXiv2013

Observable Gravity Waves From U(1)_${B-L}$ Higgs and Coleman-Weinberg Inflation

We present a realistic non-supersymmetric inflation model based on a gauged U(1)$_{B-L}$ symmetry and a tree-level Higgs potential. The inflaton is identified with the scalar field which spontaneously breaks U(1)$_{B-L}$, and we include radiative corrections à la Coleman-Weinberg in the inflaton potential. If the scalar spectral index $n_s$ lies close to 0.96, as indicated by the recent Planck and WMAP 9-yr measurements, the tensor-to-scalar ratio $r$, a canonical measure for gravity waves, exceeds 0.01. Thus, according to this model, gravity waves should be found in the near future. In this case, the quantity $|dn_s/d \ln k|$ lies in the range $0.004-0.005$. Successful baryogenesis can be realized in this class of models either via thermal or non-thermal leptogenesis.

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arXiv2013

125 GeV Higgs Boson and TeV Scale Colored Fermions in Gauge-Higgs Unification

In the context of a simple gauge-Higgs unification scenario based on the gauge group SU(3) times U(1)' in a 5-dimensional flat spacetime, we investigate a possibility to realize the Higgs boson mass of 125 GeV with a compactification scale at the TeV. With the introduction of colored bulk fermions in a certain representation under the gauge group with a half-periodic boundary condition, we analyze the one-loop RGE of the Higgs quartic coupling with the gauge-Higgs condition and successfully obtain 125 GeV Higgs boson mass by adjusting a bulk mass for the fermions with a fixed compactification scale. The Kaluza-Klein modes of the colored fermions contribute to the Higgs-to-digluon effective coupling through the one-loop corrections. Since the contribution is destructive to the Standard Model one and reduces the Higgs boson production cross section, the recent LHC result gives a lower bound on the mass of the lightest Kaluza-Klein mode fermion. We find the mass of the lightest Kaluza-Klein mode fermion in the range of 2-3 TeV for 10%-5% reductions of the Higgs boson production cross section through the gluon fusion channel. Such Kaluza-Klein mode fermions can be discovered at the LHC run II with sqrt{s}=13-14 TeV, and LHC phenomenology for the Kaluza-Klein fermions is briefly discussed.

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arXiv2013

Inverse Seesaw Neutrino Signatures at LHC and ILC

We study the collider signature of pseudo-Dirac heavy neutrinos in the inverse seesaw scenario, where the heavy neutrinos with mass at the electroweak scale can have sizable mixings with the Standard Model neutrinos, while providing the tiny light neutrino masses by the inverse seesaw mechanism. Based on a simple, concrete model realizing the inverse seesaw, we fix the model parameters so as to reproduce the neutrino oscillation data and to satisfy other experimental constraints, assuming two typical flavor structures of the model and the different types of hierarchical light neutrino mass spectra. With the fixed parameters in this way, we analyze the heavy neutrino signal at the LHC through tri-lepton final state with large missing energy and at the ILC through a single lepton plus di-jet with large missing energy. We find that in some cases, the heavy neutrino signal can be observed with a large statistical significance via different flavor charged lepton final states. Therefore, we can not only discover the heavy neutrinos in the future but also obtain a clue to reveal the origin of the small neutrino mass and flavor mixing.

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arXiv2013

Simple fermionic dark matter models and Higgs boson couplings

We consider a simple extension of the Standard Model (SM) that incorporates a Majorana fermion dark matter and a charged scalar particle with a coupling to the SM leptons through renormalizable terms. Another renormalizable term involving the charged scalar and the Higgs boson gives rise to interactions between the dark matter and SM quarks at the one-loop level, which induce the elastic scatterings between dark matter and nucleus. The same term also affects the effective coupling of the Higgs boson to diphoton through a one-loop diagram with the charged scalar. Therefore, our model predicts a correlation between the spin-independent cross section for dark matter-nucleus elastic scatterings and a new contribution to the effective Higgs boson coupling to diphoton. When the spin-dependent cross section is large enough to be tested in future direct dark matter detection experiments, the Higgs-diphoton decay rate shows a sizable deviation from the SM prediction. We also consider the case where the fermion dark matter is a Dirac particle. Most of discussions is similar to the Majorana case, but we find that the magnetic dipole moment of the Dirac fermion dark matter is loop-induced and this interaction dominates the spin-independent cross section for dark matter-nucleus elastic scatterings. We find that the resultant cross section is about an order of magnitude below the current experimental bound and hence can be tested in the near future.

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arXiv2013

Isospin violating dark matter being asymmetric

The isospin violating dark matter (IVDM) scenario offers an interesting possibility to reconcile conflicting results among direct dark matter search experiments for a mass range around 10 GeV. We consider two simple renormalizable IVDM models with a complex scalar dark matter and a Dirac fermion dark matter, respectively, whose stability is ensured by the conservation of "dark matter number". Although both models successfully work as the IVDM scenario with destructive interference between effective couplings to proton and neutron, the dark matter annihilation cross section is found to exceed the cosmological/astrophysical upper bounds. Then, we propose a simple scenario to reconcile the IVDM scenario with the cosmological/astrophysical bounds, namely, the IVDM being asymmetric. Assuming a suitable amount of dark matter asymmetry has been generated in the early Universe, the annihilation cross section beyond the cosmological/astrophysical upper bound nicely works to dramatically reduce the anti-dark matter relic density and as a result, the constraints from dark matter indirect searches are avoided. We also discuss collider experimental constraints on the models and an implication to Higgs boson physics.

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