arXiv subjects

Hiroyuki Ishida

Publications and source records attributed to Hiroyuki Ishida.

At least 37 records · Page 2Linked to original sources

arXiv2020

Unified Interpretation of Scalegenesis in Conformally Extended Standard Models

We present a universal interpretation for a class of conformal extended standard models including Higgs portal interactions realized in low-energy effective theories. The scale generation mechanism in this class (scalegenesis) arises along the (nearly) conformal/flat direction for the scale symmetry breaking, where the electroweak-symmetry breaking structure is achieved in a similar way to the standard model's. A dynamical origin for the Higgs portal coupling can provide the discriminator for the low-energy ``universality class'', to be probed in forthcoming collider experiments.

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arXiv2020

A Walking Dilaton Inflation

We propose an inflationary scenario based on a many-flavor hidden QCD with eight flavors, which realizes the almost scale-invariant (walking) gauge dynamics. The theory predicts two types of composite (pseudo) Nambu-Goldstone bosons, the pions and the lightest scalar (dilaton) associated with the spontaneous chiral symmetry breaking and its simultaneous violation of the approximate scale invariance. The dilaton acts as an inflaton, where the inflaton potential is induced by the nonperturbative-scale anomaly linked with the underlying theory. The inflaton potential parameters are highly constrained by the walking nature, which are evaluated by straightforward nonperturbative analyses including lattice simulations. Due to the pseudo Nambu-Goldstone boson's natures and the intrinsic property for the chiral symmetry breaking in the walking gauge dynamics, the inflaton coupled to the pions naturally undergoes the small field inflation consistently with all the cosmological and astrophysical constraints presently placed by Planck 2018 data. When the theory is vector-likely coupled to the standard model in part in a way to realize a dynamical electroweak symmetry breaking, the reheating temperature is determined by the pion decays to electroweak gauge bosons. The proposed inflationary scenario would provide a dynamical origin for the small field inflation as well as the light pions as a smoking-gun to be probed by future experiments.

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arXiv2020

Determining Strong Phase of $a_1$ Meson Decay Amplitude using $W \to ντ(\to νa_1(\toπ^\mp π^\mp π^\pm))$ Process

To measure the helicity of a spin-1 meson from the triple vector product of the three-momenta of its decay products, one needs information about the strong phase of the decay amplitude. In this paper, taking $a_1(1260)$ meson as an example, we present a method to extract information about the strong phase from the triple vector product of the pion momenta in $W \to ντ(\to νa_1(\toπ^\mp π^\mp π^\pm))$ process, where the $a_1$ helicity is known a priori from electroweak theory. This process is advantageous in that highly-boosted $a_1^-$ mesons from $τ_L^-$ decays have nearly maximal helicity asymmetry and thus most reflect the strong phase. We revisit the theoretical calculation of the $a_1$ meson helicity in $W \to ντ(\to νa_1)$ process. We formulate the differential decay rate of polarized $a_1$ mesons in a manner convenient for the study of the $a_1$ meson helicity asymmetry. Finally, we present the method for extracting information about the strong phase, and assess its feasibility at the LHC.

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arXiv2018

Sneutrino Dark Matter via pseudoscalar X-funnel meets Inverse Seesaw

In this paper we study sneutrino dark matter in a recently proposed supersymmetric electroweak-scale inverse seesaw model, in which the majority of the sneutrino dark matter particle is a mixture of the right-handed sneutrino $\tilde{N}^c$ and the singlet field $\tilde{S}$. The scalar field $X$ responsible for the generation of neutrino masses can simultaneously play a crucial role for sneutrino annihilation in the early Universe via the pseudoscalar mediator $A_X$ into neutrinos. We focus here on the dominant annihilation channels and provide all the formulas together with analytic estimates in order to identify the relevant parameters. Furthermore, we show that the direct detection scattering cross section is many orders of magnitude below the current limits, and estimate the indirect detection annihilation rate, which is only a few orders of magnitude below the current limits.

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arXiv2018

Signals of New Gauge Bosons in Gauged Two Higgs Doublet Model

Recently a gauged two Higgs doublet model, in which the two Higgs doublets are embedded into the fundamental representation of an extra local $SU(2)_H$ group, is constructed. Both the new gauge bosons $Z^\prime$ and $W^{\prime (p,m)}$ are electrically neutral. While $Z^\prime$ can be singly produced at colliders, $W^{\prime (p,m)}$, which is heavier, must be pair produced. We explore the constraints of $Z^\prime$ using the current Drell-Yan type data from the Large Hadron Collider. Anticipating optimistically that $Z^\prime$ can be discovered via the clean Drell-Yan type signals at high luminosity upgrade of the collider, we explore the detectability of extra heavy fermions in the model via the two leptons/jets plus missing transverse energy signals from the exotic decay modes of $Z^\prime$. For the $W^{\prime (p,m)}$ pair production in a future 100 TeV proton-proton collider, we demonstrate certain kinematical distributions for the two/four leptons plus missing energy signals have distinguishable features from the Standard Model background. In addition, comparisons of these kinematical distributions between the gauged two Higgs doublet model and the littlest Higgs model with T-parity, the latter of which can give rise to the same signals with competitive if not larger cross sections, are also presented.

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arXiv2017

Initial condition for baryogenesis via neutrino oscillation

We consider a baryogenesis scenario via the oscillation of right-handed neutrinos with Majorana masses of the order of GeV, which are also responsible for neutrino masses by the seesaw mechanism. We study how the initial condition alters the prediction of the present baryon asymmetry by this mechanism. It is usually assumed that the abundance of right-handed neutrinos is zero after the reheating of the inflationary universe and they are produced in scattering processes by the renomalizable Yukawa interaction. However, the higher-dimensional operator with right-handed neutrinos may provide an additional production which is most effective at the reheating epoch. It is shown that such an initial abundance of right-handed neutrinos can significantly modify the prediction when the strong washout of the asymmetry is absent. This leads to the parameter space of the model for the successful baryogenesis being enlarged.

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arXiv2017

A supersymmetric electroweak scale seesaw model

In this paper we propose a novel supersymmetric inverse seesaw model which has only one additional $Z_6$ symmetry. The field content is minimal to get a viable neutrino spectrum at tree-level. Interestingly, the inverse seesaw scale in our model is related to the scale of electroweak symmetry breaking. Due to that origin we are less biased about hierarchies and discuss three different types of the inverse seesaw mechanism with different phenomenologies. We can successfully reproduce neutrino masses and mixing and our model is consistent with current bounds on neutrinoless double beta decay, non-unitarity of the PMNS matrix and charged lepton flavor violation.

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arXiv2017

Bosonic-Seesaw Portal Dark Matter

We discuss a new type of Higgs-portal dark matter (DM)-production mechanism, called bosonic-seesaw portal (BSP) scenario. The BS provides the dynamical origin of the electroweak symmetry breaking, triggered by mixing between the elementary Higgs and a composite Higgs generated by a new-color strong dynamics, hypercolor (HC). At the HC strong coupling scale, the classical-scale invariance assumed in the model is dynamically broken as well as the "chiral" symmetry present in the HC sector. In addition to the composite Higgs, HC baryons emerge to potentially be stable because of the unbroken HC baryon number symmetry. Hence the lightest HC baryon can be a DM candidate. Of interest in the present scenario is that HC pions can be as heavy as the HC baryon due to the possibly enhanced-explicit "chiral"-breaking effect triggered after the BS mechanism, so the HC baryon pair cannot annihilate into HC pions. As in the standard setup of freeze-in scenario, it is assumed that the DM was never in the thermal equilibrium, which ends up with no thermal abundance. It is then the non-thermal BSP process that crucially comes into the game below the HC scale: the HC baryon significantly couples to the standard-model Higgs via the BS mechanism, and can non-thermally be produced from the thermal plasma below the HC scale, which turns out to allow the TeV mass scale for the composite baryonic DM, much smaller than the generic bound placed in the conventional thermal freeze-out scenario, to account for the observed relic abundance. Thus the DM can closely be related to the mechanism of the electroweak symmetry breaking.

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arXiv2017

Scalegenesis via dynamically induced multiple seesaws

We propose a model which accounts for the dynamical origin of the electroweak symmetry breaking (EWSB), directly linking to the mass generation of dark matter (DM) candidate and active neutrinos. The standard model (SM) is weakly charged under the $U(1)_{\rm B-L}$ gauge symmetry, in conjunction with newly introduced three right-handed Majorana neutrinos and the $U(1)_{\rm B-L}$ Higgs. The model is built on the classical scale invariance, that is dynamically broken by a new strongly coupled sector, what is called the hypercolor (HC) sector, which is also weakly coupled to the $U(1)_{\rm B-L}$ gauge. At the HC strong scale, the simultaneous breaking of the EW and $U(1)_{\rm B-L}$ gauge symmetries is triggered by dynamically induced multiple seesaw mechanisms, namely bosonic seesaw mechanisms. Thus, all the origins of masses are provided singly by the HC dynamics: that is what we call the dynamical scalegenesis. We also find that an HC baryon, with mass on the order of a few TeV, can be stabilized by the HC baryon number and the $U(1)_{\rm B-L}$ charge, so identified as a DM candidate. The relic abundance of the HC-baryon DM can be produced dominantly via the bosonic-seesaw portal process, and the HC-baryon DM can be measured through the large magnetic moment coupling generated from the HC dynamics, or the $U(1)_{\rm B-L}$-gauge boson portal in direct detection experiments.

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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

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

On neutrinoless double beta decay in the $ν$MSM

We consider the neutrinoless double beta ($0νββ$) decay in the so-called $ν$MSM, in which three right-handed neutrinos with masses below the electroweak scale are additionally introduced to the Standard Model. In this model there appear three heavy neutral leptons $N_1$, $N_2$, and $N_3$ corresponding to right-handed neutrinos. It has been known that the lightest one $N_1$ with keV mass, which is a candidate for dark matter, gives a negligible contribution to the $0 νββ$ decay. By contrast, the heavier ones $N_2$ and $N_3$, which are responsible to the seesaw mechanism of neutrino masses and baryogenesis, give the destructive contribution (compared with one from active neutrinos). This is because their mass degeneracy at high precision has been assumed, which is expected by analytical studies of baryogengesis. In this analysis, we find that the effective mass of the $0νββ$ decay becomes larger than one from active neutrinos due to the $N_2$ and $N_3$ constructive contribution when the mass difference becomes larger and the mass ordering of active neutrinos is inverted. Such a possibility will be explored by the current and near future experiments of the $0 νββ$ decay.

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arXiv2016

Lepton number violation by heavy Majorana neutrino in $B$ decays

Heavy Majorana neutrinos are predicted in addition to ordinary active neutrinos in the models with the seesaw mechanism. We investigate the lepton number violation (LNV) in $B$ decays induced by such a heavy neutrino $N$ with GeV-scale mass. Especially, we consider the decay channel $B^+ \to μ^+ \, N \to μ^+ μ^+ π^-$ and derive the sensitivity limits on the mixing angle $Θ_μ$ by the future search experiments at Belle II and in $e^+ e^-$ collisions at the Future Circular Collider (FCC-ee).

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arXiv2016

Naturalness and lepton number/flavor violation in inverse seesaw models

We introduce three right-handed neutrinos and three sterile neutrinos, and consider an inverse seesaw mechanism for neutrino mass generation. From naturalness point of view, their Majorana masses should be small, while it induces a large neutrino Yukawa coupling. Then, a neutrinoless double beta decay rate can be enhanced, and a sizable Higgs mass correction is inevitable. We find that the enhancement rate can be more than ten times compared with a standard prediction from light neutrino contribution alone, and an analytic form of heavy neutrino contributions to the Higgs mass correction. In addition, we numerically analyze the model, and find almost all parameter space of the model can be complementarily searched by future experiments of neutrinoless double beta decay and $μ\to e$ conversion.

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arXiv2016

Invisible Axion-Like Dark Matter from Electroweak Bosonic Seesaw

We explore a model based on the classically-scale invariant standard model (SM) with a strongly coupled vector-like dynamics, which is called hypercolor (HC). The scale symmetry is dynamically broken by the vector-like condensation at the TeV scale, so that the SM Higgs acquires the negative mass-squared by the bosonic seesaw mechanism to realize the electroweak symmetry breaking. An elementary pseudoscalar $S$ is introduced to give masses for the composite Nambu-Goldstone bosons (HC pions): the HC pion can be a good target to explore through a diphoton channel at the LHC. As the consequence of the bosonic seesaw, the fluctuating mode of $S$, which we call $s$, develops tiny couplings to the SM particles and is predicted to be very light. The $s$ predominantly decays to diphoton and can behave as an invisible axion-like dark matter. The mass of the $s$-dark matter is constrained by currently available cosmological and astrophysical limits to be $10^{-4} {\rm eV} \lesssim m_s \lesssim 1 \,{\rm eV}$. We find that the sufficient amount of relic abundance for the $s$-dark matter can be accumulated via the coherent oscillation. The detection potential in microwave cavity experiments is also addressed.

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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

Gauge coupling unification in a classically scale invariant model

There are a lot of works within a class of classically scale invariant model, which is motivated by solving the gauge hierarchy problem. In this context, the Higgs mass vanishes at the UV scale due to the classically scale invariance, and is generated via the Coleman-Weinberg mechanism. Since the mass generation should occur not so far from the electroweak scale, we extend the standard model only around the TeV scale. We construct a model which can achieve the gauge coupling unification at the UV scale. In the same way, the model can realize the vacuum stability, smallness of active neutrino masses, baryon asymmetry of the universe, and dark matter relic abundance. The model predicts the existence vector-like fermions charged under $SU(3)_C$ with masses lower than $1\,{\rm TeV}$, and the SM singlet Majorana dark matter with mass lower than $2.6\,{\rm TeV}$.

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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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