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Kei Yagyu

Publications and source records attributed to Kei Yagyu.

At least 109 records · Page 6Linked to original sources
arXiv2013

Higgs sectors with exotic scalar fields

The discovery of the Higgs boson like particle with the mass of around 126 GeV has given us a great clue to know what is the true Higgs sector. New physics models at the TeV scale often introduce Higgs sectors extended from the minimal form, so that the determination of the Higgs sector can be a probe of new physics models. In this talk, we focus on the Higgs sector with exotic representation fields whose isospin is larger than 1/2. We first discuss the general features of exotic Higgs sectors, and then we consider several concrete models to clarify them. The phenomenology of the Higgs triplet model is discussed as the simple but important example.

hep-ph↗
arXiv2013

Higgs boson decays to $γγ$ and $Z γ$ in models with Higgs extensions

The decays of a Higgs boson to the $γγ$ and $Zγ$ final states are purely quantum mechanical phenomena that are closely related to each other. We study the effects of an extended Higgs sector on the decay rates of the two modes. We propose that a simultaneous determination of them and the $ZZ$ mode is a useful way to see whether the Higgs boson recently observed by the LHC experiments is of the standard model type or could be a member of a larger Higgs sector.

hep-ph↗
arXiv2012

Radiative corrections to the Higgs boson couplings in the triplet model

We calculate a full set of one-loop corrections to the Higgs boson coupling constants as well as the electroweak parameters. We compute the decay rate of the standard model (SM)-like Higgs boson ($h$) into diphoton. Renormalized Higgs couplings with the weak gauge bosons $hVV$ ($V=W$ and $Z$) and the trilinear coupling $hhh$ are also calculated at the one-loop level in the on-shell scheme. Magnitudes of the deviations in these quantities are evaluated in the parameter regions where the unitarity and vacuum stability bounds are satisfied and the predicted W boson mass at the one-loop level is consistent with the data. We find that there are strong correlations among deviations in the Higgs boson couplings $hγγ$, $hVV$ and $hhh$. For example, if the event number of the $pp\to h\toγγ$ channel deviates by +30% (-40%) from the SM prediction, deviations in the one-loop corrected $hVV$ and $hhh$ vertices are predicted about -0.1% (-2%) and -10% $(+150%)$, respectively. The model can be discriminated from the other models by measuring these coupling constants accurately at future collider experiments.

hep-ph↗
arXiv2012

Testing the custodial symmetry in the Higgs sector of the Georgi-Machacek model

We study how the custodial symmetry in the Higgs sector of the Georgi-Machacek (GM) model can be tested at the LHC. As the minimal extension of the Higgs triplet model, in which tiny neutrino masses are generated via the Type-II Seesaw Mechanism, the GM model keeps the electroweak $ρ$ parameter at unity at tree level. In the GM model, there are 5-plet ($H_5$), 3-plet ($H_3$) and singlet ($H_1$) Higgs bosons under the classification of the custodial $SU(2)_V$ symmetry, in addition to the standard model-like Higgs boson ($h$). These new Higgs bosons have the following characteristic features at the tree level: (1) the masses of the Higgs bosons belonging to the same $SU(2)_V$ multiplet are degenerate; and (2) $H_5$ and $H_1$ couple to the electroweak gauge bosons but not SM quarks, whereas $H_3$ couples to the quarks but not the gauge bosons. We find that the $H_5$ production from the weak vector boson fusion process and the Drell-Yan process associated with $H_3$ are useful in testing the custodial symmetry of the Higgs sector at the LHC. In addition, these processes can also be used to discriminate from other models that contain singly-charged Higgs bosons and extra neutral Higgs bosons. We also investigate a possible enhancement in the $h\to γγ$ as well as $h\to Zγ$ decays.

hep-ph↗
arXiv2012

Renormalization of the Higgs sector in the triplet model

We study radiative corrections to the mass spectrum and the triple Higgs boson coupling in the model with an additional Y=1 triplet field. In this model, the vacuum expectation value for the triplet field is strongly constrained from the electroweak precision data, under which characteristic mass spectrum appear at the tree level; i.e., $m_{H^{++}}^2-m_{H^+}^2\simeq m_{H^+}^2-m_A^2$ and $m_A^2\simeq m_H^2$, where the CP-even ($H$), the CP-odd ($A$) and the doubly-charged ($H^{\pm\pm}$) as well as the singly-charged ($H^\pm$) Higgs bosons are the triplet-like. We evaluate how the tree-level formulae are modified at the one-loop level. The $hhh$ coupling for the standard model-like Higgs boson ($h$) is also calculated at the one-loop level. One-loop corrections to these quantities can be large enough for identification of the model by future precision data at the LHC or the International Linear Collider.

hep-ph↗
arXiv2012

Radiative corrections to electroweak parameters in the Higgs triplet model and implication with the recent Higgs boson searches

We study radiative corrections to the electroweak parameters in the Higgs model with the Y=1 triplet field, which is introduced in the scenario of generating neutrino masses based on the so-called type II seesaw mechanism. In this model, the rho parameter deviates from unity at the tree level. Consequently, the electroweak sector of the model is described by the four input parameters such as $α_{\text{em}}$, $G_F$, $m_Z$ and $\sin^2θ_W$. We calculate the one loop contribution to the W boson mass as well as to the rho parameter in order to clarify the possible mass spectrum of the extra Higgs bosons under the constraint from the electroweak precision data. We find that the hierarchical mass spectrum among $H^{\pm\pm}$, $H^{\pm}$ and $A$ (or $H$) is favored by the precision data especially for the case of $m_A$ $(\simeq m_H)>m_{H^+}>m_{H^{++}}$, where $H^{\pm\pm}$, $H^{\pm}$, $A$ and $H$ are the doubly-charged, singly-charged, CP-odd and CP-even Higgs bosons mainly originated from the triplet field. We also discuss phenomenological consequences of such a mass spectrum with relatively large mass splitting. The decay rate of the Higgs boson decay into two photons is evaluated under the constraint from the electroweak precision data, regarding the recent Higgs boson searches at the CERN LHC.

hep-ph↗
arXiv2012

Studies on Extended Higgs Sectors as a Probe of New Physics Beyond the Standard Model

In this thesis, we discuss theoretical properties of various Higgs sectors, and we analyze constraints from current experimental data, and then we study collider signatures in each Higgs sector. In Part I, we focus on the phenomenology of various extended Higgs sectors such as the two Higgs doublet model, the Higgs triplet model and Higgs sectors in supersymmetric standard models. In Part II, we discuss new physics models at the TeV scale, in which the Higgs sector is extended, and neutrino masses, dark matter and/or baryon asymmetry of the Universe can be explained.

hep-ph↗
arXiv2012

Testing the Higgs triplet model with the mass difference at the LHC

In general, there can be mass differences among scalar bosons of the Higgs triplet field with the hypercharge of Y=1. In the Higgs triplet model, when the vacuum expectation value $v_Δ$ of the triplet field is much smaller than that $v$ ($\simeq 246$ GeV) of the Higgs doublet field as required by the electroweak precision data, a characteristic mass spectrum $ m_{H^{++}}^2-m_{H^+}^2\simeq m_{H^+}^2-m_{ϕ^0}^2(\equiv ξ)$ appears, where $m_{H^{++}}$, $m_{H^+}$, $m_{ϕ^0}$ are the masses of the doubly-charged ($H^{++}$), the singly-charged ($H^+$) and the neutral ($ϕ^0=H^0$ or $A^0$) scalar bosons, respectively. It should be emphasized that phenomenology with $ξ\neq 0$ is drastically different from that in the case with $ξ=0$ where the doubly-charged scalar boson decays into the same sign dilepton $\ell^+\ell^+$ or the diboson $W^+W^+$ depending on the size of $v_Δ$. We find that, in the case of $ξ> 0$, where $H^{++}$ is the heaviest, $H^{++}$ can be identified via the cascade decays such as $H^{++}\to H^+W^{+(*)}\to ϕ^0 W^{+(*)}W^{+(*)}\to b\bar{b}\ell^+ν\ell^+ν$. We outline how the Higgs triplet model can be explored in such a case at the LHC. By the determination of the mass spectrum, the model can be tested and further can be distinguished from the other models with doubly-charged scalar bosons.

hep-ph↗
arXiv2011

Testing the Higgs model with triplet fields at the ILC

Higgs triplet fields are introduced various new physics models such as the type-II seesaw model, the left-right symmetric model and so on. The vertex of a charged Higgs boson and weak gauge bosons, $H^\pm W^\mp Z$, appears at the tree level in these models. The magnitude of this vertex is proportional to the vacuum expectation value (VEV) of Higgs triplet fields. We discuss the possibility of measuring this vertex at the ILC and study how precisely determine the VEV of the triplet field.

hep-ph↗
arXiv2011

Doubly-charged scalar bosons from the doublet

We consider the extended Higgs models, in which one of the isospin doublet scalar fields carries the hypercharge Y=3/2. Such a doublet field $Φ_{3/2}$ is composed of a doubly charged scalar boson as well as a singly charged one. We first discuss a simple model with $Φ_{3/2}$ (Model I), and study its collider phenomenology at the LHC. We then consider a new model for radiatively generating neutrino masses with a dark matter candidate (Model II), in which $Φ_{3/2}$ and an extra Y=1/2 doublet as well as vector-like singlet fermions carry the odd quantum number for an unbroken discrete $Z_2$ symmetry. We also discuss the neutrino mass model (Model III), in which the exact $Z_2$ parity in Model II is softly broken. It is found that the doubly charged scalar bosons in these models show different phenomenological aspects from those which appear in models with a Y=2 isospin singlet field or a Y=1 triplet one. They could be clearly distinguished at the LHC.

hep-ph↗
arXiv2011

Decoupling property of the supersymmetric Higgs sector with four doublets

In supersymmetric standard models with multi Higgs doublet fields, selfcoupling constants in the Higgs potential come only from the D-terms at the tree level. We investigate the decoupling property of additional two heavier Higgs doublet fields in the supersymmetric standard model with four Higgs doublets. In particular, we study how they can modify the predictions on the quantities well predicted in the minimal supersymmetric standard model (MSSM), when the extra doublet fields are rather heavy to be measured at collider experiments. The B-term mixing between these extra heavy Higgs bosons and the relatively light MSSM-like Higgs bosons can significantly change the predictions in the MSSM such as on the masses of MSSM-like Higgs bosons as well as the mixing angle for the two light CP-even scalar states. We first give formulae for deviations in the observables of the MSSM in the decoupling region for the extra two doublet fields. We then examine possible deviations in the Higgs sector numerically, and discuss their phenomenological implications.

hep-ph↗
arXiv2011

Light Charged Higgs bosons at the LHC in 2HDMs

We present a discussion of light charged Higgs boson searches at the Large Hadron Collider (LHC) in CP-conserving 2-Higgs Doublet Models (2HDMs). Taking into account all available experimental and theoretical constraints we review all possible processes that would allow for a detection of such a particle with a mass below the top quark mass. Two different types of processes are analysed: one that depends only on tan beta and on the charged Higgs boson mass because it involves only the charged Higgs boson Yukawa couplings; the other that depends on almost all model parameters, mainly due to the presence of Higgs self-couplings. We discuss the regions of parameter space of 2HDMs that can be covered be each type of process and define some guidelines for experimental searches at the LHC.

hep-ph↗
arXiv2011

Non-decoupling effects in supersymmetric Higgs sectors

A wide class of Higgs sectors is investigated in supersymmetric standard models. When the lightest Higgs boson (h) looks the standard model one, the mass (m_h) and the triple Higgs boson coupling (the hhh coupling) are evaluated at the one-loop level in each model. While m_h is at most 120-130 GeV in the minimal supersymmetric standard model (MSSM), that in models with an additional neutral singlet or triplet fields can be much larger. The hhh coupling can also be sensitive to the models: while in the MSSM the deviation from the standard model prediction is not significant, that can be 30-60 % in some models such as the MSSM with the additional singlet or with extra doublets and charged singlets. These models are motivated by specific physics problems like the mu-problem, the neutrino mass, the scalar dark matter and so on. Therefore, when h is found at the CERN Large Hadron Collider, we can classify supersymmetric models by measuring m_h and the hhh coupling accurately at future collider experiments.

hep-ph↗
arXiv2011

Testing Higgs models via the $H^\pm W^\mp Z$ vertex by a recoil method at the International Linear Collider

In general, charged Higgs bosons $H^\pm$ appear in non-minimal Higgs models. The $H^\pm W^\mp Z$ vertex is known to be related to the violation of the global symmetry (custodial symmetry) in the Higgs sector. Its magnitude strongly depends on the structure of the exotic Higgs models which contain higher isospin $SU(2)_L$ representations such as triplet Higgs bosons. We study the possibility of measuring the $H^\pm W^\mp Z$ vertex via single charged Higgs boson production associated with the $W^\pm$ boson at the International Linear Collider (ILC) by using the recoil method. The feasibility of the signal $e^+e^-\to H^\pm W^\mp \to \ell νjj$ is analyzed assuming the polarized electron and positron beams and the expected detector performance for the resolution of the two-jet system at the ILC. The background events can be reduced to a considerable extent by imposing the kinematic cuts even if we take into account the initial state radiation. For a relatively light charged Higgs boson whose mass $m_{H^\pm}$ is in the region of 120-130 GeV $< m_{H^\pm} < m_W+m_Z$, the $H^\pm W^\mp Z$ vertex would be precisely testable especially when the decay of $H^\pm$ is lepton specific. The exoticness of the extended Higgs sector can be explored by using combined information for this vertex and the rho parameter.

hep-ph↗
arXiv2011

Triviality and vacuum stability bounds in the three-loop neutrino mass model

We study theoretical constraints on the parameter space under the conditions from vacuum stability and triviality in the three-loop radiative seesaw model with TeV-scale right-handed neutrinos which are odd under the $Z_2$ parity. In this model, some of the neutrino Yukawa coupling constants can be of the order of one. Requirement of strongly first order phase transition for successful electroweak baryogenes is also prefers order-one coupling constants in the scalar sector. Hence, it is important to clarify whether this model satisfies those theoretical conditions up to a given cutoff scale. It is found that the model can be consistent up to the scale above 10 TeV in the parameter region where the neutrino data, the lepton flavor violation data, the thermal relic abundance of dark matter as well as the requirement from the strongly first order phase transition are satisfied.

hep-ph↗
arXiv2010

An R-parity conserving radiative neutrino mass model without right-handed neutrinos

The model proposed by A. Zee (1986) and K.S. Babu (1988) is a simple radiative seesaw model, in which tiny neutrino masses are generated at the two-loop level. We investigate a supersymmetric extension of the Zee-Babu model under R-parity conservation. The lightest superpartner particle can then be a dark matter candidate. We find that the neutrino data can be reproduced with satisfying current data from lepton flavour violation even in the scenario where not all the superpartner particles are heavy. Phenomenology at the Large Hadron Collider is also discussed.

hep-ph↗
arXiv2009

Models of Yukawa interaction in the two Higgs doublet model, and their collider phenomenology

Possible models of Yukawa interaction are discussed in the two Higgs doublet model (THDM) under the discrete symmetry imposed to avoid the flavor changing neutral current at the leading order. It is known that there are four types of such models corresponding to the possible different assignment of charges for the discrete symmetry on quarks and leptons. We first examine decay properties of Higgs bosons in each type of the models, and summarize constraints on the models from current experimental data. We then shed light on the differences among these models in collider phenomenology. In particular, we mainly discuss so-called the Type-II THDM and the Type-X THDM. The Type-II THDM corresponds to the model with the same Yukawa interaction as the minimal supersymmetric standard model (MSSM). On the other hand, in the Type-X THDM, additional Higgs bosons can predominantly decay into leptons. This scenario may be interesting because of the motivation for a light charged Higgs boson scenario such as in the TeV scale model of neutrino, dark matter and baryogenesis. We study how we can distinguish the Type-X THDM from the MSSM at the Large Hadron Collider and the International Linear Collider.

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