arXiv subjects

Shinya Kanemura

Publications and source records attributed to Shinya Kanemura.

At least 91 records · Page 5Linked to original sources

arXiv2019

Single Higgs production in association with a photon at electron-positron colliders in extended Higgs models

We study associated Higgs production with a photon at electron-positron colliders, $e^+e^-\to hγ$, in various extended Higgs models, such as the inert doublet model (IDM), the inert triplet model (ITM) and the two Higgs doublet model (THDM). The cross section in the standard model (SM) is maximal around $\sqrt{s}=$250 GeV, and we present how and how much the new physics can enhance or reduce the production rate. We also discuss the correlation with the $h\toγγ$ and $h\to Zγ$ decay rates. We find that, with a sizable coupling to a SM-like Higgs boson, charged scalars can give considerable contributions to both the production and the decay if their masses are around 100 GeV. Under the theoretical constraints from vacuum stability and perturbative unitarity as well as the current constraints from the Higgs measurements at the LHC, the production rate can be enhanced from the SM prediction at most by a factor of two in the IDM. In the ITM, in addition, we find a particular parameter region where the $hγ$ production significantly increases by a factor of about six to eight, but the $h\toγγ$ decay still remains as in the SM. In the THDM, possible deviations from the SM prediction are minor in the viable parameter space.

hep-ph↗

arXiv2018

Gravitational Waves from Phase Transitions in Models with Charged Singlets

We investigate the effect of extra singlets on the electroweak phase transition (EWPT) strength and the spectrum of the corresponding gravitational waves (GWs). We consider here the standard model (SM) extended with a singlet scalar with multiplicity N coupled to the SM Higgs doublet. After imposing all the theoretical and experimental constraints and defining the region where the EWPT is strongly first order, we obtain the region in which the GWs spectrum can be reached by different future experiments such as LISA and DECIGO.

hep-ph↗

arXiv2018

Fingerprinting models of first-order phase transitions by the synergy between collider and gravitational-wave experiments

We investigate the sensitivity of future space-based interferometers such as LISA and DECIGO to the parameters of new particle physics models which drive a first-order phase transition in the early Universe. We first perform a Fisher matrix analysis on the quantities characterizing the gravitational wave spectrum resulting from the phase transition, such as the peak frequency and amplitude. We next perform a Fisher analysis for the quantities which determine the properties of the phase transition, such as the latent heat and the time dependence of the bubble nucleation rate. Since these quantities are determined by the model parameters of the new physics, we can estimate the expected sensitivities to such parameters. We illustrate this point by taking three new physics models for example: (1) models with additional isospin singlet scalars (2) a model with an extra real Higgs singlet, and (3) a classically conformal $B-L$ model. We find that future gravitational wave observations play complementary roles to future collider experiments in pinning down the parameters of new physics models driving a first-order phase transition.

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arXiv2018

Probing CP violating Higgs sectors via the precision measurement of coupling constants

We study how effects of the CP violation can be observed indirectly by precision measurements of Higgs boson couplings at a future Higgs factory such as the international linear collider. We consider two Higgs doublet models with the softly broken discrete symmetry. We find that by measuring the Higgs boson couplings very precisely we are able to distinguish the two Higgs doublet model with CP violation from the CP conserving one.

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arXiv2018

Physics Case for the 250 GeV Stage of the International Linear Collider

The International Linear Collider is now proposed with a staged machine design, with the first stage at 250 GeV with a luminosity goal of 2 ab-1. In this paper, we review the physics expectations for this machine. These include precision measurements of Higgs boson couplings, searches for exotic Higgs decays, other searches for particles that decay with zero or small visible energy, and measurements of e+e- annihilation to W+W- and 2-fermion states with improved sensitivity. A summary table gives projections for the achievable levels of precision based on the latest full simulation studies.

hep-ex↗

arXiv2018

Loop effects on the Higgs decay widths in extended Higgs models

In order to identify the Higgs sector using future precision data, we calculate the partial decay widths of the discovered Higgs boson with the mass of 125 GeV into fermion pairs and gauge-boson pairs with one-loop electroweak and one-loop QCD corrections in various extended Higgs models, such as the Higgs singlet model and four types of two Higgs doublet models. In the tree-level analysis, the patterns of deviations from the standard model predictions in the partial decay widths for various decay modes are distinctive for each model, due to the mixing of the Higgs boson with other neutral scalars. Our present analysis shows that even with a full set of radiative corrections we can discriminate these extended Higgs models via the partial decay widths as long as any of the deviations is detected at future precision measurements. Furthermore, we quantitatively show that in each model the magnitude of the deviations can provide important information on the mass scale of extra Higgs bosons under the theoretical constraints from perturbative unitary and vacuum stability, which can be obtained without discovery of the additional Higgs bosons.

hep-ph↗

arXiv2018

Gravitational waves from first order electroweak phase transition in models with the $U(1)_X^{}$ gauge symmetry

We consider a standard model extension equipped with a dark sector where the $U(1)_X^{}$ Abelian gauge symmetry is spontaneously broken by the dark Higgs mechanism. In this framework, we investigate patterns of the electroweak phase transition as well as those of the dark phase transition, and examine detectability of gravitational waves (GWs) generated by such strongly first order phase transition. It is pointed out that the collider bounds on the properties of the discovered Higgs boson exclude a part of parameter space that could otherwise generate detectable GWs. After imposing various constraints on this model, it is shown that GWs produced by multi-step phase transitions are detectable at future space-based interferometers, such as LISA and DECIGO, if the dark photon is heavier than 25 GeV. Furthermore, we discuss the complementarity of dark photon searches or dark matter searches with the GW observations in these models with the dark gauge symmetry.

hep-ph↗

arXiv2018

The role of positron polarization for the inital $250$ GeV stage of the International Linear Collider

The International Linear Collider is now proposed with a staged machine design, with the first stage at $\sqrt{s}=$~250 GeV and an integrated luminosity goal of 2~ab$^{-1}$. One of the questions for the machine design is the importance of positron polarization. In this report, we review the impact of positron polarization on the physics goals of the $250$ GeV stage of the ILC and demonstrate that positron polarization has distinct advantages.

hep-ph↗

arXiv2017

Report by the Committee on the Scientific Case of the ILC Operating at 250 GeV as a Higgs Factory

This document is the final report by the Committee on the Scientific Case of the ILC Operating at 250 GeV as a Higgs Factory. This committee was commissioned by the Japan Association of High Energy Physicists. The purpose of this committee is to investigate and compare the capability to determine the energy scale of new phenomena and new principles and the capability to uncover the origin of matter-antimatter asymmetry.

hep-ex↗

arXiv2017

H-COUP: a program for one-loop corrected Higgs boson couplings in non-minimal Higgs sectors

We describe a numerical calculation tool H-COUP written in Fortran, which provides one-loop electroweak corrected vertices for the discovered Higgs boson $h(125)$ in various Higgs sectors. The renormalization is based on the improved on-shell scheme without gauge dependence. In the first version H-COUP_1.0, the following models are included, namely, the Higgs singlet model, four types (Type-I, Type-II, Type-X, Type-Y) of two Higgs doublet models with a softly-broken $Z_2$ symmetry and the inert doublet model. We first briefly introduce these models and then explain how to install and run this tool in an individual machine. A sample of numerical outputs is provided for user information.

hep-ph↗

arXiv2017

Gauge invariant one-loop corrections to Higgs boson couplings in non-minimal Higgs models

We comprehensively evaluate renormalized Higgs boson couplings at one-loop level in non-minimal Higgs models such as the Higgs Singlet Model (HSM) and the four types of Two Higgs Doublet Models (THDMs) with a softly-broken $Z_2$ symmetry. The renormalization calculation is performed in the on-shell scheme improved by using the pinch technique to eliminate the gauge dependence in the renormalized couplings. We first review the pinch technique for scalar boson two-point functions in the Standard Model (SM), the HSM and the THDMs. We then discuss the difference in the results of the renormalized Higgs boson couplings between the improved on-shell scheme and the ordinal one with a gauge dependence appearing in mixing parameters of scalar bosons. Finally, we widely investigate how we can identify the HSM and the THDMs focusing on the pattern of deviations in the renormalized Higgs boson couplings from predictions in the SM.

hep-ph↗

arXiv2017

Neutrino Mass, Dark Matter and Baryon Asymmetry without Lepton Number Violation

We propose a model to explain tiny masses of neutrinos with the lepton number conservation, where neither too heavy particles beyond the TeV-scale nor tiny coupling constants are required. Assignments of conserving lepton numbers to new fields result in an unbroken $Z_2$ symmetry that stabilizes the dark matter candidate (the lightest $Z_2$-odd particle). In this model, $Z_2$-odd particles play an important role to generate the mass of neutrinos. The scalar dark matter in our model can satisfy constraints on the dark matter abundance and those from direct searches. It is also shown that the strong first-order phase transition, which is required for the electroweak baryogenesis, can be realized in our model. In addition, the scalar potential can in principle contain CP-violating phases, which can also be utilized for the baryogenesis. Therefore, three problems in the standard model, namely absence of neutrino masses, the dark matter candidate, and the mechanism to generate baryon asymmetry of the Universe, may be simultaneously resolved at the TeV-scale. Phenomenology of this model is also discussed briefly.

hep-ph↗

arXiv2017

One-loop corrections to the Higgs self-couplings in the singlet extension

We investigate predictions on the triple Higgs boson couplings with radiative corrections in the model with an additional real singlet scalar field. In this model, the second physical scalar state ($H$) appears in addition to the Higgs boson ($h$) with the mass 125 GeV. The $hhh$ vertex is calculated at the one-loop level, and its possible deviation from the predictions in the standard model is evaluated under various theoretical constraints. The decay rate of $H \to hh$ is also computed at the one-loop level. We also take into account the bound from the precise measurement of the $W$ boson mass, which gives the upper limit on the mixing angle $α$ between two physical Higgs bosons for a given value of the mass of $H$ ($m_H^{}$). We find that the deviation in the $hhh$ coupling from the prediction in the standard model can maximally be about 250\%, 150\% and 75\% for $m_H^{}=300$, 500 and 1000 GeV, respectively, under the requirement that the cutoff scale of the model is higher than 3 TeV. We also discuss deviations from the standard model prediction in double Higgs boson production from the gluon fusion at the LHC using the one-loop corrected Higgs boson vertices.

hep-ph↗

arXiv2017

The Potential of the ILC for Discovering New Particles

This paper addresses the question of whether the International Linear Collider has the capability of discovering new particles that have not already been discovered at the CERN Large Hadron Collider. We summarize the various paths to discovery offered by the ILC, and discuss them in the context of three different scenarios: 1. LHC does not discover any new particles, 2. LHC discovers some new low mass states and 3. LHC discovers new heavy particles. We will show that in each case, ILC plays a critical role in discovery of new phenomena and in pushing forward the frontiers of high-energy physics as well as our understanding of the universe in a manner which is highly complementary to that of LHC. For the busy reader, a two-page executive summary is provided at the beginning of the document.

hep-ph↗

arXiv2016

Testing neutrino mass generation mechanisms from the lepton flavor violating decay of the Higgs boson

We investigate how observations of the lepton flavor violating decay of the Higgs boson ($h \to \ell\ell^\prime$) can narrow down models of neutrino mass generation mechanisms, which were systematically studied in Refs. [1,2] by focusing on the combination of new Yukawa coupling matrices with leptons. We find that a wide class of models for neutrino masses can be excluded if evidence for $h \to \ell\ell^\prime$ is really obtained in the current or future collider experiments. In particular, simple models of Majorana neutrino masses cannot be compatible with the observation of $h \to \ell\ell^\prime$. It is also found that some of the simple models to generate masses of Dirac neutrinos radiatively can be compatible with a significant rate of the $h \to \ell\ell^\prime$ process.

hep-ph↗

arXiv2016

LHC 750 GeV Diphoton excess in a radiative seesaw model

We investigate a possibility for explaining the recently announced 750\,GeV diphoton excess by the ATLAS and the CMS experiments at the CERN LHC in a model with multiple doubly charged particles, which was originally suggested for explaining tiny neutrino masses through a three-loop effect in a natural way. The enhanced radiatively generated effective coupling of a new singlet scalar $S$ with diphoton with multiple charged particles in the loop enlarges the production rate of $S$ in $pp\to S+X$ via photon fusion process and also the decay width $Γ(S\to γγ)$ even without assuming a tree level production mechanism. We provide detailed analysis on the cases with or without allowing the mixing between $S$ and the standard model Higgs doublet.

hep-ph↗

arXiv2016

Gravitational waves and Higgs boson couplings for exploring first order phase transition in the model with a singlet scalar field

We calculate the spectrum of gravitational waves originated from strongly first order electroweak phase transition in the extended Higgs model with a real singlet field. In order to calculate the bubble nucleation rate, we perform a two-field analysis to evaluate bounce solutions connecting the true and the false vacua using the one-loop effective potential at finite temperatures. Imposing the Sakharov condition of the departure from thermal equilibrium for baryogenesis, we survey allowed regions of parameters of the model. We then investigate the gravitational waves produced at electroweak bubble collisions in the early Universe, such as the sound wave, the bubble wall collision and the plasma turbulence. We find that the strength at the peak frequency can be large enough to be detected at future space-based gravitational interferometers such as eLISA, DECIGO and BBO. Predicted deviations in the various Higgs boson couplings are also evaluated at the zero temperature, and are shown to be large enough too. Therefore, in this model strongly first order electroweak phase transition can be tested by the combination of the precision study of various Higgs boson couplings at LHC, the measurement of the triple Higgs boson coupling at future lepton colliders and the shape of the spectrum of gravitational wave detectable at future gravitational interferometers.

hep-ph↗

arXiv2016

Implications of the 750 GeV gamma-gamma Resonance as a Case Study for the International Linear Collider

If the gamma-gamma resonance at 750 GeV suggested by 2015 LHC data turns out to be a real effect, what are the implications for the physics case and upgrade path of the International Linear Collider? Whether or not the resonance is confirmed, this question provides an interesting case study testing the robustness of the ILC physics case. In this note, we address this question with two points: (1) Almost all models proposed for the new 750 GeV particle require additional new particles with electroweak couplings. The key elements of the 500 GeV ILC physics program---precision measurements of the Higgs boson, the top quark, and 4-fermion interactions---will powerfully discriminate among these models. This information will be important in conjunction with new LHC data, or alone, if the new particles accompanying the 750 GeV resonance are beyond the mass reach of the LHC. (2) Over a longer term, the energy upgrade of the ILC to 1 TeV already discussed in the ILC TDR will enable experiments in gamma-gamma and e+e- collisions to directly produce and study the 750 GeV particle from these unique initial states.

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