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Shinya Kanemura

Publications and source records attributed to Shinya Kanemura.

At least 199 records · Page 11Linked to original sources
arXiv2006

Dimension-six top-Higgs interaction and its effect in collider phenomenology

Measurement of the Yukawa interaction between the top quark and the Higgs boson should be useful to clarify the mechanism of fermion mass generation. We discuss the impact of non-standard interactions characterized by dimension-six operators on the effective top Yukawa coupling. The cross section of the process $e^-e^+ \to W^-W^+ ν\bar ν\to t \bar t ν\bar ν$ is calculated including these operators, and possible deviation from the standard model prediction is evaluated under the constraint from perturbative unitarity and current experimental data. We find that if the new physics scale is in a TeV region, the cross section can be significantly enhanced due to the non-standard interactions. Such a large effect should be detectable at the International Linear Collider.

hep-ph↗
arXiv2006

Lepton flavor violation in Higgs boson decays under the rare tau decay results

We study lepton flavor violation (LFV) associated with tau leptons in the framework of the two Higgs doublet model, in which LFV couplings are introduced as a deviation from Model II Yukawa interaction. Parameters of the model are constrained from experimental results and also from requirements of theoretical consistencies such as vacuum stability and perturbative unitarity. Current data for rare tau decays provide substantial upper limits on the LFV Yukawa couplings in the large $\tanβ$ region ($\tanβ\gtrsim 30 $), which are comparable with predictions in fundamental theories. Here $\tanβ$ is the ratio of vacuum expectation values of the two Higgs doublets. We show that a search for the LFV decays $ϕ^{0} \to τ^\pm μ^\mp$ $(τ^\pm e^\mp)$ of neutral Higgs bosons ($ϕ^{0} =h,H$ and $A$) at future collider experiments can be useful to further constrain the LFV couplings especially in the relatively small $\tanβ$ region ($\tanβ\lesssim 30 $), where rare tau decay data cannot give any strong limit.

hep-ph↗
arXiv2005

Enhancement of lepton flavor violation in a model with bi-maximal mixing at the grand unification scale

We study phenomenological predictions in the scenario with the quasi-degenerate relation among neutrino Dirac masses, m_D1 simeq m_D2 < m_D3, assuming the bi-maximal mixing at the grand unification scale in supersymmetric standard models with right-handed neutrinos. A sufficient lepton number asymmetry can be produced for successful leptogenesis. The lepton flavor violating process mu to e gamma can be enhanced due to the Majorana phase, so that it can be detectable at forthcoming experiments. The processes tau to e gamma and tau to mu gamma are suppressed because of the structure of neutrino Dirac masses, and their branching ratios are smaller than that of mu to e gamma.

hep-ph↗
arXiv2005

Measurement of lepton flavor violating Yukawa couplings at ILC

We discuss lepton flavor violation (LFV) associated with tau leptons in the general framework of the two Higgs doublet model, in which LFV couplings are introduced as a deviation from Model II Yukawa interaction. Parameters of the model are constrained from experimental results and also from requirements of theoretical consistencies such as vacuum stability and perturbative unitarity. Current data for LFV rare tau decays provide substantial upper limits on the LFV Yukawa couplings in the large $\tanβ$ region, which are comparable with predictions in fundamental theories. Here $\tanβ$ is the ratio of vacuum expectation values of the two Higgs doublets. A search for the LFV decays $ϕ^{0} \to τ^\pm μ^\mp$ $(τ^\pm e^\mp)$ of neutral Higgs bosons ($ϕ^{0} =h,H$ and $A$) at future collider experiments can be useful to further constrain the LFV couplings, especially in the relatively small $\tanβ$ region ($\tanβ\lesssim 30 $), where rare tau decay data cannot give any strong limit.

hep-ph↗
arXiv2005

Electroweak baryogenesis and the triple Higgs boson coupling

We study collider signatures of electroweak baryogenesis in the two Higgs doublet model and the minimal supersymmetric standard model. It is found that the trilinear coupling of the lightest Higgs boson receives large quantum corrections if the electroweak phase transition is strongly first order for successful baryogenesis. In the two Higgs doublet model, the magnitude of the deviation from the standard model value is shown to be larger than 10 percent. Such a deviation can be detected at a future electron-positron linear collider.

hep-ph↗
arXiv2005

Phase effects from the general neutrino Yukawa matrix on lepton flavor violation

We examine contributions from Majorana phases to lepton flavor violating processes in the framework of the minimal supersymmetric standard model with heavy right-handed neutrinos. All phases in the complex neutrino Yukawa matrix are taken into account in our study. We find that in the scenario with universal soft-breaking terms sizable phase effects can appear on the lepton flavor violating processes such as $μ\to e γ$, $τ\to e γ$, and $τ\to μγ$. In particular, the branching ratio of $μ\to e γ$ can be considerably enhanced due to the Majorana phases, so that it can be much greater than that of $τ\to μγ$.

hep-ph↗
arXiv2005

The $H^\pm W^\mp Z^0$ vertex and single charged Higgs boson production via $WZ$ fusion at the Large Hadron Collider

There is a variety of new physics scenarios which deduce extended Higgs sectors in the low energy effective theory. The coupling of a singly-charged Higgs boson ($H^\pm$) with weak gauge bosons, $H^\pm W^\mp Z^0$, directly depends on the global symmetry structure of the model, so that its experimental determination can be useful to test each scenario. We discuss predictions on this coupling in several models, such as the model with additional real and complex triplets, the Littlest Higgs model, the two Higgs doublet model and the minimal supersymmetric standard model. In order to measure the $H^\pm W^\mp Z^0$ coupling we consider single $H^\pm$ production via the $WZ$ fusion mechanism at the CERN Large Hadron Collider. The production rate is hierarchically different among these models, so that this process can be useful to explore new physics scenarios.

hep-ph↗
arXiv2005

Enhancement of W+/- H-/+ Production at Hadron Colliders in the Two Higgs Doublet Model

We discuss the associated W+/- H-/+ production at the CERN Large Hadron Collider. The dependence of the hadronic cross section on the Higgs sector parameters is investigated in detail in the framework of the general Two Higgs Doublet Model (THDM). We study the possible enhancement of the THDM prediction for the cross section compared to the prediction of the Minimal Supersymmetric Standard Model (MSSM). We find regions in the THDM parameter space where the THDM prediction can exceed the one of the MSSM by two orders of magnitude. These regions of large cross section are in agreement with theoretical bounds on the model, derived from the requirement of vacuum stability and perturbative unitarity, and are not excluded by experimental constraints.

hep-ph↗
arXiv2004

Electroweak baryogenesis and quantum corrections to the triple Higgs boson coupling

A phenomenological consequence of electroweak baryogenesis is studied in connection with the Higgs physics. In a two Higgs double model, the first order phase transition can be strong enough to allow the electroweak baryogenesis due to the effect of extra Higgs bosons. We investigate the quantum correction to the triple coupling of the lightest Higgs boson in such a scenario, and find that the condition of the strong first order phase transition necessarily leads to the deviation of at least 10 percent from the standard model prediction. Such magnitude of the deviation can be identified in future $e^+e^-$ linear collider experiments. We also discuss the minimal supersymmetric standard model with the light stop scenario, and point out that a sizable correction appears in the triple coupling for successful electroweak baryogenesis.

hep-ph↗
arXiv2004

New physics from Higgs self-coupling measurement

Radiative correction to the triple Higgs boson coupling is studied in the two Higgs doublet model in connection with the electroweak baryogenesis. It is shown that the one loop correction is large enough to be identified in future linear collider experiments, if the baryon number of the Universe is generated at the first order electroweak phase transition.

hep-ph↗
arXiv2004

A Study of Lepton Flavor Violating $μN (e N) \to τX$ Reactions in Supersymmetric Models

We study a lepton flavor violating $μN \to τX$ reaction in deep inelastic scattering region in supersymmetric models. The contribution from the Higgs boson mediation could be important for this reaction. For that case, the cross section is constrained by the experimental limit of the pseudo-scalar coupling from $τ\to μη$ decays. We find that at a muon energy ($E_μ$) higher than 50 GeV, the predicted cross section increases significantly due to the contribution from sea $b$-quarks. As a result, with $10^{20}$ muons per year, at most a number of $\mathcal{O}(10^4)$ is expected for $μN \to τX$ events at $E_μ$= 300 GeV, whereas $\mathcal{O}(10^2)$ events are given at $E_μ= 50$ GeV. Furthermore, the $μN \to τX$ phenomenology, in particular that for the signal and backgrounds, is briefly discussed. Another promising possibility to search for the $e N \to τX$ reaction at an electron-positron linear collider is also discussed. Searches for these reactions would be competitive to studies of rare tau decays and have potential to improve sensitivities to lepton flavor violation significantly.

hep-ph↗
arXiv2004

Higgs coupling constants as a probe of new physics

We study new physics effects on the couplings of weak gauge bosons with the lightest CP-even Higgs boson ($h$), $hZZ$, and the tri-linear coupling of the lightest Higgs boson, $hhh$, at the one loop order, as predicted by the two Higgs doublet model. Those renormalized coupling constants can deviate from the Standard Model (SM) predictions due to two distinct origins; the tree level mixing effect of Higgs bosons and the quantum effect of additional particles in loop diagrams. The latter can be enhanced in the renormalized $hhh$ coupling constant when the additional particles show the non-decoupling property. Therefore, even in the case where the $hZZ$ coupling is close to the SM value, deviation in the $hhh$ coupling from the SM value can become as large as plus 100 percent, while that in the $hZZ$ coupling is at most minus 1 percent level. Such large quantum effect on the Higgs tri-linear coupling is distinguishable from the tree level mixing effect, and is expected to be detectable at a future linear collider.

hep-ph↗
arXiv2004

Lepton Flavor Violation in the Higgs Boson Decay at a Linear Collider

We study possibility of observing the process $h^{0} \to τ^{\pm} μ^{\mp}$ at a linear collider. The branching ratio is constrained to be of the order of $10^{-4}$ by the $τ^{-} \to μ^{-} η$ result. Supersymmetric standard models can reproduce such amount of the branching ratio by taking a specific parameter set. The Higgsstrahlung process $e^{+}e^{-} \to Zh^{0}$ is preferable because of its simple kinematic structure, then, the signal process is $e^{+} e^{-} \to Z h^{0} \to Z τ^{\pm} μ^{\mp}$. The most serious background comes from the process, $e^{+} e^{-} \to Zh^{0} \to Zτ^{\pm} τ^{\mp} \to τ^{\pm} μ^{\mp} ν\barν$. We estimate the significance of the signal, taking into account the background reduction.

hep-ph↗
arXiv2004

Search for Lepton Flavor Violation in the Higgs Boson Decay at a Linear Collider

We discuss possibility of direct search for lepton flavor violation (LFV) in Yukawa interaction by measuring the branching ratio for the decay of the lightest Higgs boson ($h^0$) into a $τ$-$μ$ pair at a linear collider. We study the significance of the signal process, $e^+e^- \to Z^\ast \to Z h^0 \to Z τ^\pm μ^\mp$, against the backgrounds such as $e^{+}e^{-} \to Z τ^{+}τ^{-} \to Z τ^{\pm}μ^{\mp}+$ missings. After taking appropriate kinematic cuts, the number of the background event is considerably reduced, so that the signal can be visible when the branching ratio of $h^0 \to τ^\pm μ^\mp$ is larger than about $10^{-4}$. In a Minimal Supersymmetric Standard Model scenario, the effective coupling of $h^0 τ^\pm μ^\mp$ can be generated at loop level due to the slepton mixing. When supersymmetric mass parameters are larger than TeV scales, the branching ratio can be as large as several times $10^{-4}$. Therefore, the signal can be marginally visible at a LC. In the general two Higgs doublet model, the possible maximal value for the branching ratio of $h^0 \to τ^\pm μ^\mp$ can reach to a few times $10^{-3}$ within the available experimental bound, so that we can obtain larger significance.

hep-ph↗
arXiv2004

SUSY Loop Contributions to the $W$-Pair Production in $e^+e^-$ Collisions

We discuss one-loop contributions of supersymmetry (SUSY) particles to the process $e^+e^- \to W^+W^-$ in the minimal supersymmetric standard model. The calculation is tested by using the sum rule among the form factors, which is based on the BRS invariance. The overall normalization factor of the amplitudes is tested by using the decoupling property of the SUSY particles in the large mass limit. The correction due to the one-loop effect of the sfermions can be of a few times $\pm 0.1$ % level, whereas that of the charginos and neutralinos can be order 1%. We also study the CP-violating effects in the chargino and neutralino sector.

hep-ph↗
arXiv2004

New mechanism for the top-bottom mass hierarchy

We propose a mechanism to generate hierarchy between masses of the top and bottom quarks without fine tuning of the Yukawa coupling constants in the context of the two Higgs doublet model (THDM). In the THDM with a discrete symmetry, there exists the vacuum where only the top quark receives the mass of the order of the electroweak symmetry breaking scale $v(\simeq 246 GeV)$, while the bottom quark remains massless. By introducing a small soft-breaking parameter $m_3^2$ of the discrete symmetry, the bottom quark perturbatively acquires a nonzero mass. We show a model in which the small $m_3^2 [\sim v^2/(4π)^2]$ is generated by the dynamics above the cutoff scale of the THDM. The ratio $\tanβ$ of the two vacuum expectation values is necessarily very large; i.e., $\tan β\sim m_t/m_b$. We also find a salient relation, $1/\tanβ\simeq m_3^2/m_H^2$, where $m_H$ is the mass of the extra CP-even Higgs boson. Our scenario yields some specific features that can be tested in future collider experiments.

hep-ph↗
arXiv2004

Testing Supersymmetry in $A H^\pm$ Associated Production

In the Minimal Supersymmetric Standard Model, the masses of the charged Higgs boson ($H^\pm$) and the CP-odd scalar ($A$) are related by $M_{H^+}^2=M_A^2+m_W^2$ at the Born level. Because the coupling of $W^-$-$A$-$H^+$ is fixed by gauge interaction, the Born level production rate of $q \bar q' \to W^{\pm \ast} \to A H^\pm$ depends only on one supersymmetry parameter -- the mass ($M_A^{}$) of $A$. We illustrate how to test the mass relation between $A$ and $H^+$ in case that the signal is found at the LHC. If the signal is not found, the product of the decay branching ratios of $A$ and $H^\pm$ predicted by the MSSM is bounded from above as a function of $M_A$.

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