arXiv subjects

Hiroyuki Ishida

Publications and source records attributed to Hiroyuki Ishida.

At least 55 records · Page 3Linked to original sources

arXiv2015

A new dynamics of electroweak symmetry breaking with classically scale invariance

We propose a new dynamics of the electroweak symmetry breaking in a classically scale invariant version of the standard model. The scale invariance is broken by the condensations of additional fermions under a strong coupling dynamics. The electroweak symmetry breaking is triggered by negative mass squared of the elementary Higgs doublet, which is dynamically generated through the bosonic seesaw mechanism. We introduce a real pseudo-scalar singlet field interacting with additional fermions and Higgs doublet in order to avoid massless Nambu-Goldstone bosons from the chiral symmetry breaking in a strong coupling sector. We investigate the mass spectra and decay rates of these pseudo-Nambu-Goldstone bosons, and show they can decay fast enough without cosmological problems. We further evaluate the energy dependences of the couplings between elementary fields perturbatively, and find that our model is the first one which realizes the flatland scenario with the dimensional transmutation by the strong coupling dynamics. Similarly to the conventional flatland model with Coleman-Weinberg mechanism, the electroweak vacuum in our model is meta-stable.

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arXiv2015

What triggers $θ_{13}$ discrepancy between Daya Bay and T2K?

There seems to be a non-negligible discrepancy between Daya Bay and T2K experimental results of a neutrino mixing angle, $θ_{13}$ . We show the discrepancy can be simply passed away even in the standard three neutrinos framework by taking the same value of $Δm^2_{32}$ with a normal neutrino mass hierarchy and Dirac CP phase $δ= -π/ 2$, which is favorable in T2K experiment. For numerical analyses of neutrino oscillation probabilities, there are five independent parameters, $Δm_{21}^2$, $Δm_{32}^2$, $θ_{12}$, $θ_{23}$, and $δ$. We reevaluate $θ_{13}$ by scatter plots within $1 σ$ range of five-dimensional parameters space suggested by PDG, T2K, and Daya Bay. We project onto $δ$-$\sin^2 2 θ_{13}$ plane and find an overlap area of $θ_{13}$ suggested by T2K and Daya Bay. In the normal hierarchy case, the largest overlap area is given by parameters set suggested by T2K, and the overlap area becomes small in order of parameters sets suggested by PDG and Daya Bay. In the inverted hierarchy case, there is a tiny overlap area only in parameters set suggested by T2K. We also show several sample points of four parameters set, $Δm_{21}^2$, $Δm_{32}^2$, $θ_{12}$, and $θ_{23}$, where the dissonance of $θ_{13}$ is really disappeared with $δ= -π/2$.

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arXiv2015

Hierarchy problem, gauge coupling unification at the Planck scale, and vacuum stability

From the point of view of the gauge hierarchy problem, introducing an intermediate scale in addition to TeV scale and the Planck scale ($M_{\rm Pl} = 2.4 \times 10^{18}\,{\rm GeV}$) is unfavorable. In that way, a gauge coupling unification (GCU) is expected to be realized at $M_{\rm Pl}$. We explore possibilities of GCU at $M_{\rm Pl}$ by adding a few extra particles with TeV scale mass into the standard model (SM). When extra particles are fermions and scalars (only fermions) with the same mass, the GCU at $M_{\rm Pl}$ can (not) be realized. On the other hand, when extra fermions have different masses, the GCU can be realized around $\sqrt{8 π} M_{\rm Pl}$ without extra scalars. This simple SM extension has two advantages that a vacuum becomes stable up to $M_{\rm Pl}$ ($\sqrt{8 π} M_{\rm Pl}$) and a proton lifetime becomes much longer than an experimental bound.

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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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arXiv2015

Higgs inflation and Higgs portal dark matter with right-handed neutrinos

We investigate the Higgs inflation and the Higgs portal dark matter with the right-handed neutrino. The dark matter and the right-handed neutrino in the Higgs inflation play important roles in explaining the recent experimental results of the Higgs and top masses, and the cosmic microwave background by BICEP2 at the same time. This inflation model predicts $805~{\rm GeV} \lesssim m_{\rm DM} \lesssim1220~{\rm GeV}$ for the DM mass, $1.05 \times10^{14}~{\rm GeV} \lesssim M_R \lesssim 2.04 \times10^{14}~{\rm GeV}$ for the right-handed neutrino mass, and $8.42 \lesssim ξ\lesssim 12.4$ for the non-minimal coupling within $m_H=125.6 \pm 0.35~{\rm GeV}$ for the Higgs and $M_t=173.34 \pm 0.76~{\rm GeV}$ for the top masses.

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arXiv2015

Higgs Inflation and Scalar Dark Matter with Right-handed Neutrinos

We investigate the possibility of Higgs inflation with an extended standard model by right-handed neutrinos and scalar dark matter. We find that the masses of the dark matter and one of the right-handed neutrinos should stand around TeV and $10^{-14}$ GeV scale respectively even if the magnitude of tensor-to-scalar ratio indicates sufficiently small value.

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arXiv2015

Search for new physics via photon polarization of $b \rightarrow s γ$

We suggest a discriminant analysis of new physics beyond the standard model through a detection of photon polarization in a radiative B meson decay. This analysis is investigated in SUSY SU(5) GUT with right-handed neutrino and left-right symmetric models. New physics search via CP asymmetry in the same process are also evaluated in each model for comparison. We show that new physics can be found via detecting the photon polarization in a parameter space of TeV energy scale.

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arXiv2015

$ν_R$ dark matter-philic Higgs for 3.5 keV X-ray signal

We suggest a new model of 7 keV right-handed neutrino dark matter inspired by a recent observation of 3.5 keV X-ray line signal in the XMM-Newton observatory. It is difficult to derive the tiny masses with a suitable left-right mixing of the neutrino in a framework of ordinary simple type-I seesaw mechanism. We introduce a new Higgs boson, a dark matter-philic Higgs boson, in which the smallness of its vacuum expectation value can be achieved. We investigate suitable parameter regions where the observed dark matter properties are satisfied. We find that the vacuum expectation value of dark matter-philic Higgs boson should be about 0.17 GeV.

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arXiv2014

Vanishing Higgs potential at the Planck scale in a singlet extension of the standard model

We discuss the realization of a vanishing effective Higgs potential at the Planck scale, which is required by the multiple-point criticality principle (MPCP), in the standard model with singlet scalar dark matter and a right-handed neutrino. We find the scalar dark matter and the right-handed neutrino play crucial roles for realization of the MPCP, where a neutrino Yukawa becomes effective above the Majorana mass of the right-handed neutrino. Once the top mass is fixed, the MPCP at the (reduced) Planck scale and the suitable dark matter relic abundance determine the dark matter mass, $m_S$, and the Majorana mass of the right-handed neutrino, $M_R$, as $8.5~(8.0)\times10^2~{\rm GeV}\leq m_S\leq1.4~(1.2)\times10^3~{\rm GeV}$ and $6.3~(5.5)\times10^{13}~{\rm GeV}\leq M_R\leq1.6~(1.2)\times10^{14}~{\rm GeV}$ within current experimental values of the Higgs and top masses. This scenario is consistent with current dark matter direct search experiments, and will be checked by future experiments such as LUX with further exposure and/or the XENON1T.

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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

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.

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arXiv2014

U(1)$_{B-L}$ Symmetry Restoration and Effective Neutrino Species

The U(1)$_{B-L}$ symmetry could be restored during inflation, since the BICEP2 results suggest a GUT-scale inflation with the Hubble parameter, $H_{\rm inf} \simeq 10^{14}$ GeV, close to the U(1)$_{B-L}$ breaking scale. We consider a scenario in which the ${B-L}$ Higgs field dominates the Universe after inflation, and mainly decays into the U(1)$_{B-L}$ gauge bosons, whose subsequent decays reheat the Universe. Interestingly, if one of the right-handed neutrinos is extremely light and behaves as dark radiation or hot dark matter, its abundance is determined by the $B-L$ charge assignment and the relativistic degree of freedom in plasma. We find that $ΔN_{\rm eff}$ takes discrete values between $0.188$ and $0.220$ in the standard model plus three right-handed neutrinos, depending on whether the decay into heavier right-handed neutrinos are kinematically accessible or not. In the fiveness U(1)$_{\bf 5}$ case, we find that $ΔN_{\rm eff}$ takes discrete values between $0.313$ and $0.423$. The tension between BICEP2 and {\it Planck} can be partially relaxed by dark radiation.

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arXiv2014

7 keV sterile neutrino dark matter from split flavor mechanism

The recently discovered X-ray line at about $3.5\,$keV can be explained by sterile neutrino dark matter with mass, $m_s \simeq 7\,$keV, and the mixing, $\sin^2 2θ\sim 10^{-10}$. Such sterile neutrino is more long-lived than estimated based on the seesaw formula, which strongly suggests an extra flavor structure in the seesaw sector. We show that one can explain both the small mass and the longevity based on the split flavor mechanism where the breaking of flavor symmetry is tied to the breaking of the $B-L$ symmetry. In a supersymmetric case we find that the $7\,$keV sterile neutrino implies the gravitino mass about $100\,$TeV.

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arXiv2014

Effective Valence Quark Model and a Possible Dip in $d Br(B \to K \ell \bar{\ell})/dq^2$

In rare $B$ meson decays $B\rightarrow K \ell^+ \ell^-$, a possible contribution of $\ell^+\ell^-$ emission via photon from the "spectator" quark $q$ ($q=u,d$) in the $B$ meson $(q\bar{b})$ is investigated in addition to the conventional one $\bar{b} \rightarrow \bar{s} +γ\rightarrow \bar{s}+\ell^+ + \ell^-$. If such a contribution is sizable compared with the standard estimation of $B\rightarrow K \ell^+ \ell^-$, we will observe visible difference between $d Γ(B^0\rightarrow K^0 \ell^+ \ell^-)/d q^2$ and $d Γ(B^+\rightarrow K^+ \ell^+ \ell^-)/d q^2$ in $q^2$ dependence ($q^2 \equiv m_{\ell\ell}^2$). Besides, as a result of the interference between the conventional one and a new one, a dip appears in $d Γ(B\rightarrow K \ell^+ \ell^-)/d q^2$ at a small region of $q^2$. The interference effect in the $B^0$ decay will also be observed differently from that in the $B^+$ decay. The calculation is done based on a semi-classical approach, a valence quark model. In the present model, the photon emission from the spectator quark $q$, $d \rightarrow d+γ$ ($u \rightarrow u+γ$) is independent of the $b$-$s$ transition mechanism, and the characteristic results are due to a straightforward estimate of the quark propagator which cannot be incorporated into the factorization method. The model is not a valence quark "dominant" model, so that, for example, the valence quarks in the final state carry only $24\%$ of the energy-momentum of the kaon.

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arXiv2013

Partial mass-degeneracy and spontaneous CP violation in the lepton sector

Inspired by the small mass-squared difference measured in the solar neutrino oscillation experiments and by the testability, we suggest that a limit of the partial mass degeneracy, in which masses of the first two generation fermions are degenerate, may be a good starting point for understanding the observed fermion mass spectra and mixing patterns. The limit indicates the existence of a two-dimensional rotation symmetry, such as $O(2)$, $D_N$ and so on, in flavor space of the first two generations. We propose simple models for the lepton sector based on $D_N$ and show that the models can successfully reproduce the experimental data without imposing unnatural hierarchies among dimensionless couplings, although at least $10\%$ tuning is necessary in order to explain a large atmospheric mixing. It is especially found that the $Z_2$ subgroup of the $D_N$ symmetry plays an important role in understanding the smallness of the electron mass and $θ_{13}^{\rm PMNS}$. We also discuss testability of the models by the future neutrinoless-double-beta-decay experiments and cosmological observations.

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arXiv2013

CKM matrix and flavor symmetries

Following the way proposed recently by Hernandez and Smirnov, we seek possible residual symmetries in the quark sector with a focus on the von Dyck groups. We begin with two extreme cases in which both $θ_{13}$ and $θ_{23}$ or only $θ_{13}$ are set to zero. Then, cases where all the Cabibbo-Kobayashi-Maskawa parameters are allowed to take nonzero values are explored. The $Z_7$ symmetry is favorable to realize only the Cabibbo angle. On the other hand, larger groups are necessary in order to be consistent with all the mixing parameters. Possibilities of embedding the obtained residual symmetries into the $Δ(6N^2)$ series are also briefly discussed.

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arXiv2013

Longevity Problem of Sterile Neutrino Dark Matter

Sterile neutrino dark matter of mass O(1-10) keV decays into an active neutrino and an X-ray photon, and the non-observation of the corresponding X-ray line requires the sterile neutrino to be more long-lived than estimated based on the seesaw formula : the longevity problem. We show that, if one or more of the B-L Higgs fields are charged under a flavor symmetry (or discrete R symmetry), the split mass spectrum for the right-handed neutrinos as well as the required longevity is naturally realized. We provide several examples in which the predicted the X-ray flux is just below the current bound.

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arXiv2013

Partial Mass Degenerated Model and Spontaneous CP Violation in the Leptonic Sector

We have investigated a flavour model [1] which inspired by small squared-mass difference measured in solar neutrino oscillation experiments and observability in neutrinoless double beta decay experiments. In our model, the $1^{\rm st.}$ and $2^{\rm nd.}$ generations of fermions have a common mass at the leading order. Such limit may be a good starting point from the points of view of understanding the mixing patterns and mass spectra. In this limit, the mass matrices are respected an $O (2)$ symmetry on flavor space of the first two generations. For simplicity, we propose a model for lepton sector based on the $D_N$ group which is a discrete subgroup of $O (2)$. We show that our model can reproduce the experimental data without hierarchical couplings except for $5 \mathchar`- 10%$ tuning partially for the large neutrino mixing. Further, we show a novel relation between the tiny electron mass and the relatively large $θ_{13}^{\rm PMNS}$ via CP violation by the complex vacuum expectation values of the extra scalar fields.

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