arXiv subjects

Shinya Kanemura

Publications and source records attributed to Shinya Kanemura.

At least 37 records · Page 2Linked to original sources

arXiv2024

New renormalization scheme in the two Higgs doublet models

We propose a new renormalization scheme in the two Higgs doublet models with a softly-broken $Z_2$ symmetry and CP-conservation in the Higgs sector. In this scheme, counterterms for mixing angles of the Higgs bosons are determined by using the decay rates of the discovered Higgs boson $h$, $i.e.$, $h \to τ^+τ^-$ and $h \to ZZ^* \to Z\ell^+\ell^-$ at next leading order (NLO) instead of using the renormalized two-point functions which are adopted in the previous scheme. We require that the decay rates at NLO are determined to be the corresponding predictions at NLO in the Standard Model (SM) times square of the scaling factor which describes the deviation of $h$ couplings at tree level from the SM value. The mixing angles then maintain the meaning of the ``alignmentness", $i.e.$, how the properties of $h$ are close to the SM predictions, while they lose such meaning in the previous scheme. We compare the predictions of the decay rates at NLO given in the new scheme and those in the previous scheme.

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arXiv2024

Loop induced $H^\pm W^\mp Z$ vertices in the general two Higgs doublet model with CP violation

Understanding symmetry of extended Higgs models helps to construct the theory beyond the standard model and gives important insights for testing these models by experiments. The custodial symmetry plays an important role as a leading principle to construct non-minimal Higgs sectors. On the other hand, new sources of CP violation in the Higgs sector are essentially important to explain the baryon asymmetry of the Universe. In the two Higgs doublet model, it has been known that the CP violation and the custodial symmetry are not compatible, and thus the CP violating Higgs potential in general violates the custodial symmetry in the Higgs sector. In this paper, we discuss the loop-induced $H^\pm W^\mp Z$ vertices in the two Higgs doublet model, which are induced due to the custodial symmetry violation of the sector of the particles in the loop, and study how the vertices are affected by the CP violation. We calculate the $H^\pm W^\mp Z$ vertices in the most general two Higgs doublet model with the CP violation at the one-loop level and evaluate the decay modes $H^\pm \to W^\pm Z$ caused by these vertices. We obtain new contributions to the $H^\pm W^\mp Z$ vertices from the CP violating part of the model, in addition to the known contributions from the CP conserving part. Moreover, we find that an asymmetry between the decays $H^+ \to W^+ Z$ and $H^- \to W^- Z$ is caused by the CP violating phases in the model, some of which are important for the electroweak baryogenesis. We also briefly mention testability of these vertices at future collider experiments.

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arXiv2024

The electric dipole moment in a model for neutrino mass, dark matter and baryon asymmetry of the Universe

The electric dipole moment is examined in a three-loop neutrino mass model with dark matter originally proposed in Aoki et al. [Phys. Rev. Lett. 102, 051805 (2009)]. The model contains a $CP$-violating phase in the Higgs potential which plays an important role in electroweak baryogenesis and is thus expected to explain the baryon asymmetry of the Universe simultaneously. However, such a $CP$-violating phase is severely constrained by the measurements of the electron electric dipole moment (eEDM), and a suppression mechanism for the eEDM is necessary to explain the observed baryon asymmetry while avoiding the constraint. In this paper, we examine neutrino mass, lepton-flavor-violating processes, dark matter, and the eEDM in the model. We show that the eEDM can be suppressed by destructive interference between the $CP$-violating phases in the Higgs sector and the dark sector with large $CP$-violating phases. We propose some benchmark scenarios including $O(1)$ $CP$-violating phases where tiny neutrino mass and dark matter can be explained while avoiding all current experimental and theoretical constraints. These $CP$-violating phases are expected to be large enough to generate the observed baryon asymmetry in the electroweak baryogenesis scenario.

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arXiv2024

Exploring loop-induced first-order electroweak phase transition in the Higgs effective field theory

The nearly aligned Higgs Effective Field Theory (naHEFT) is based on the general assumption: all deviations in the Higgs boson couplings are originated from quantum one-loop effects of new particles that are integrated out. If the new particles integrated out have the same non-decoupling property, physics of the electroweak symmetry breaking can be then described by several parameters in the naHEFT, so that there is a correlation among the Higgs boson couplings such as $h γγ$, $hWW$ and $hhh$ couplings. In this paper, we analyze the strongly first-order electroweak phase transition (EWPT) with the condition of sphaleron decoupling and the completion condition of the phase transition, and investigate the relation among the deviations in the Higgs boson couplings and the dynamics of the EWPTs. We also take into account the gravitational wave spectrum as well as the primordial black hole predicted at the EWPT. We show that if the new particles integrated out include charged scalar states future precision measurements of the $h γγ$ coupling can give a useful prediction on the $hhh$ coupling to realize the strongly first-order EWPT. We can explore the nature of EWPT and the new physics behind it by the combination of precision measurements of various Higgs boson couplings at future collider experiments, gravitational wave observations at future space-based interferometers and searches for primordial black holes.

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arXiv2024

Primordial black holes from slow phase transitions: a model-building perspective

We investigate the formation of primordial black holes (PBHs) through delayed vacuum decay during slow cosmic first-order phase transitions. Two specific models, the polynomial potential and the real singlet extension of the Standard Model, are used as illustrative examples. Our findings reveal that models with zero-temperature scalar potential barriers are conducive to the realization of this mechanism, as the phase transition duration is extended by the U-shaped Euclidean action. We find that the resulting PBH density is highly sensitive to the barrier height, with abundant PBH formation observed for sufficiently high barriers. Notably, the phase transition needs not to be ultra-supercooled (i.e. the parameter $α\gg1$), and the commonly used exponential nucleation approximation $Γ(t)\sim e^{βt}$ fails to capture the PBH formation dynamics in such models.

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arXiv2024

Gravitational Waves from Particle Decays during Reheating

Gravitational waves have become an irreplaceable tool for exploring the post-inflationary universe. Their cosmological and astrophysical origins have been attracting numerous attention. In this Letter, we point out a novel source of ultra-high frequency gravitational waves: the decay of particles produced during the reheating era. We highlight the decay of the Higgs boson as a representative case, showing how it yields a testable gravitational wave spectrum by future observations.

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arXiv2024

On the criterion of perturbativity with the mass-dependent beta function in extended Higgs models

In order to realize electroweak first order phase transition, a category of extended Higgs models with relatively large self-coupling constants is often considered. In such a scenario, the running coupling constants can blow up at an energy scale much below the Planck scale. To clarify the allowed parameter space of the model, it is important to evaluate the scale where perturbation calculation breaks down. In the renormalization group equation analysis using the mass-independent renormalization scheme, we need the matching condition to connect the low energy theory and the high energy theory. Consequently, the analysis depends on the detail of the matching condition. On the other hand, the analysis with the mass-dependent beta function is performed in a simpler way, because the threshold effect is automatically included in the beta function. Therefore, in this paper, using the mass-dependent beta function at the one-loop level, we discuss the application limit of perturbation calculation. First, in a toy model with two complex scalar fields, we explain the essence of our method and compare our results with those based on the mass-independent beta function. We then apply our method to the more realistic model; i.e., the inert doublet model. We find that in our method, in which the threshold effect is automatically included, the energy scale where the perturbation calculation breaks down can be higher than the one using the mass-independent beta function, especially when the blow up scale is relatively low.

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arXiv2024

Dark phase transition from WIMP: complementary tests from gravitational waves and colliders

A dark sector is an interesting place where a strong first-order phase transition, observable gravitational waves and/or a dark matter candidate could arise. However, the experimental tests for such a dark sector could be ambiguous due to the dark content, largely unconstrained parameter space and the connection to the visible world. We consider a minimal dark scalar-vector boson plasma to realize the three mentioned phenomena, with a unique connection to the Standard Model via the Higgs portal coupling. We discuss the important features of the Higgs portal in such a minimal dark sector, namely the dark thermalization, collider tests, and direct detection of dark matter. We perform numerical analyses of the dark phase transition associated with stochastic gravitational waves and dark matter, discussing the complementarity of collider detection, dark matter direct detection and space-based/terrestrial interferometers as a promising avenue to hear and see the minimal dark sector.

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arXiv2024

Dark matter as the trigger of flavor changing neutral current decays of the top quark

We suggest a simplified model that simultaneously addresses the dark-matter problem and give rise to top quark flavor changing neutral current (FCNC) interactions at the one-loop order. The model consists of two extra $SU(2)_L$ gauge singlets: a colored mediator of spin zero ($S$) and a right-handed fermion ($χ$) both are odd under an ad-hoc $Z_2$ symmetry. The right-handed fermion plays the role of the dark-matter candidate. In this model, the presence of the two dark sector particles generates one-loop induced FCNC decays of the top quark into light quarks and bosons such as the gluon, the photon, the $Z$-boson or the Higgs boson. As a case study, we analyze the top quark FCNC decays into light quarks ($u$ or $c$) and a $Z$ or Higgs bosons. We then study the reliable solutions to the dark-matter problem by estimating the regions in the parameter space that are consistent with the \textsc{Planck} measurement of the dark-matter relic density. We also revisit the bounds from the searches of dark matter in events with at least one high-$p_T$ jet and large missing transverse energy at the Large Hadron Collider (LHC). We then define four benchmark points that are consistent with the existing constraints from collider experiments and cosmology. We finally estimate, for these benchmark scenarios, the rates of a broad range of channels that can be used to probe the connection between the top FCNC transitions and dark matter both at the HL-LHC and a future $100$ TeV collider.

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arXiv2024

Gravitational Waves from Phase Transitions in Scale Invariant Models

We investigate the properties of the gravitational waves (GWs) generated during a strongly first order electroweak phase transition (EWPT) in models with the classical scale invariance (CSI). Here, we distinguish two parameter space regions that correspond to the cases of (1) light dilaton and (2) purely radiative Higgs mass (PRHM). In the CSI models, the dilaton mass, or the Higgs mass in the PRHM case, in addition to some triple scalar couplings are fully triggered by the radiative corrections (RCs). In order to probe the RC effects on the EWPT strength and on the GW spectrum, we extend the standard model by a real singlet to assist the electroweak symmetry breaking and an additional scalar field $Q$ with multiplicity $N_Q$ and mass $m_Q$. After imposing all theoretical and experimental constraints, we show that a strongly first order EWPT with detectable GW spectra can be realized for the two cases of light dilaton and PRHM. We also show the corresponding values of the relative enhancement of the cross section for the di-Higgs production process, which is related to the triple Higgs boson coupling. We obtain the region in which the GW spectrum can be observed by different future experiments such as LISA and DECIGO. We also show that the scenarios (1) and (2) can be discriminated by future GW observations and measurements of the di-Higgs productions at future colliders.

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arXiv2023

H-COUP Version 3: A program for one-loop corrected decays of any Higgs bosons in non-minimal Higgs models

The H-COUP program is provided as a package of Fortran codes, which can compute observables related to Higgs bosons including radiative corrections in various extended Higgs sectors. We give a manual for the latest version of H-COUP (H-COUP_3.0), in which decay rates and branching ratios of all the Higgs bosons can be calculated at one-loop level in EW and Higgs interactions with QCD corrections in the Higgs singlet model, four types of the two Higgs doublet model with a softly-broken $Z_2$ symmetry, and the inert doublet model. The previous version (H-COUP_2.0) can evaluate those only for the standard model like Higgs boson with the mass of 125 GeV ($h$). In H-COUP_3.0, renormalized quantities are computed based on the gauge independent on-shell renormalization scheme. The source code of H-COUP_3.0 can be downloaded via the following link: \url{http://www-het.phys.sci.osaka-u.ac.jp/~hcoup}. By using H-COUP_3.0, we can compare the precise measurements of the properties of $h$ and direct searches for additional Higgs bosons with their predictions at one-loop level, by which we can reconstruct the structure of the Higgs sector.

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arXiv2023

Multi-photon signatures as a probe of CP-violation in extended Higgs sectors

We propose a novel signature with four-photon final states to probe CP-violating (CPV) extended Higgs sectors via $f \bar{f} \to Z^* \to H_1H_2 \to 4 γ$ processes with $H_{1,2}$ being additional neutral Higgs bosons. We focus on the nearly Higgs alignment scenario, in which the discovered Higgs boson almost corresponds to a neutral scalar state belonging to the isospin doublet field with the vacuum expectation value $v \simeq 246$ GeV. We show that the branching ratios of $H_{1,2} \to γγ$ can simultaneously be sizable when CPV phases in the Higgs potential are of order one due to the enhancement of charged-Higgs boson loops. Such branching ratios can be especially significant when the fermiophobic scenario is taken into account. As a simple example, we consider the general two Higgs doublet model, and demonstrate that the cross section for the four-photon process can be 0.1 fb at LHC with the masses of $H_{1,2}$ to be a few 100 GeV in the Higgs alignment limit under the constraints from electric dipole moments (EDMs) and LHC Run-II data. We also illustrate that the searches for EDMs and di-photon resonances at high-luminosity LHC play complementary roles to explore CPV extended Higgs sectors.

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arXiv2023

Electroweak baryogenesis in the three-loop neutrino mass model with dark matter

Baryon asymmetry of the Universe is evaluated in the model originally proposed in Phys. Rev. Lett. 102 (2009) 051805, where Majorana masses of neutrinos are generated via three-loop diagrams composed of additional scalar bosons including the dark matter candidate which is odd under an unbroken $Z_2$ symmetry. In order for the model to include multiple CP-violating phases, we do not impose the softly broken $Z_2$ symmetry imposed in the original model to avoid the flavor-changing neutral current at tree level. Instead, for simplicity, we assume the flavor alignment structure in the Yukawa interactions. We also simply assume the alignment structure in the Higgs potential so that the Higgs couplings coincide with those in the SM at tree level. Under these phenomenological simplifications, the model still contains multiple CP-violating phases. By using destructive interferences among them, it is compatible with the stringent constraint from the electric dipole moment measurements to generate the observed baryon asymmetry along with the scenario of electroweak baryogenesis. We show a benchmark scenario which can explain neutrino mass, dark matter and baryon asymmetry of the universe simultaneously and can satisfy all the other available experimental data. Some phenomenological predictions of the model are also discussed.

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arXiv2023

Charged Higgs induced 5 and 6 lepton signatures from heavy neutrinos at the LHC

We propose an anomaly free gauged U$(1)$ extension of the SM where three right handed heavy neutrinos, being charged under the general U$(1)$ gauge group, are introduced to explain the origin of the tiny neutrino mass through the seesaw mechanism after the general U$(1)$ symmetry is broken. Due to the breaking of the general U$(1)$ symmetry a neutral beyond the standard model gauge boson $Z^\prime$ acquires mass. There are two Higgs doublets in this model where one interacts with the SM fermions and the other one interacts with the right handed heavy neutrinos and charged leptons. The charged multiplet of the second Higgs can completely decay into the heavy neutrinos and charged lepton in the neutrinophilic limit of the model parameters. The charged Higgs pair production can be influenced due to presence of the $Z^\prime$ boson at the High Luminosity LHC (HL-LHC) in addition to the neutral SM gauge bosons. The pair produced charged Higgs bosons decay into SM charged leptons and heavy neutrinos. Following the leading decay modes of the heavy neutrinos into charged leptons and $W$ boson we study the 5 and 6 lepton final states after the leptonic and hadronic decay of the $W$ bosons considering solely muons and electrons in the final state. Combining the electron and muon final states we estimate the significance of the 5 and 6 charged lepton processes in the $m_{H^\pm}-m_N$ plane for different benchmark points of $m_{Z^\prime}$. It is found that the 5 (6) charged lepton processes could be probed at the High Luminosity LHC (HL-LHC) with at least 5 (3) $σ$ significance and there are parameter regions where the significance could be larger.

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arXiv2023

Electroweak baryogenesis via top-charm mixing

We investigate a scenario of electroweak baryogenesis in the two Higgs doublet model with quark flavor mixing. In general, off-diagonal components of quark Yukawa interactions with additional Higgs bosons are strongly constrained by the data for flavor changing neutral currents. However, top-charm quark mixing is not the case, so that a large off-diagonal element can be taken, which can contribute to generating baryon asymmetry of the universe. It turns out that CP violating phases of the off-diagonal element in the source term in the Boltzmann equation are eliminated by the rephasing. This result is somewhat different from previous works on electroweak baryogenesis by flavor off-diagonal Yukawa couplings. Instead, we find that the absolute value of the top-charm off-diagonal element enhances CP violating phases in the Higgs potential, by which sufficient amount of the baryon number can be generated to explain the observed baryon asymmetry of the universe. We find that such a scenario is viable under the current experimental data. The model can be tested by the current and future measurements of various flavor experiments like Kaon rare decays, in addition to high energy collider experiments as well as gravitational wave observations. Characteristic predictions of our model would be deviations in Kaon rare decays. Branching ratios of $K^+ \to π^+ ν\overlineν$ and $K_L \to π^0 ν\overlineν$ can deviate by the order of 10% and 1%, respectively, which may be tested at future Kaon experiments such as NA62 and KOTO step-2.

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arXiv2023

Probing chirality structure in lepton-flavour-violating Higgs decay $h\toτμ$ at the LHC

A phenomenological study for determining the chirality structure in lepton-flavor-violating Higgs (hLFV) decays $h \to τμ$ at the LHC is presented. We estimate the effects of the $τ$ polarization in the analysis andthe importance of determining the relative visible momentum ratio $x$, and show the analysis with a collinear mass $m_{col1}$ by assuming one missing particle is appropriate. We find that the sensitivity would be generically affected up to $\pm$ $4-6$~\% in terms of the BR$(h\to τμ)$ upper bound, and show the altered bounds on the $(|y_{μτ}|, |y_{τμ}|)$ plane. We further study the benchmark scenarios, and demonstrate the sensitivity study for the chirality structure using the relative visible momentum ratio. We find that the two fully polarized cases, the $τ_R$ and $τ_L$ scenarios consistent with the recently reported excess, are distinguishable at 2$σ$ level for 1000~fb$^{-1}$. We also show that a further improved study potentially provides a similar sensitivity already for 139~fb$^{-1}$.

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arXiv2023

Exploring Wrong Sign Scenarios in the Yukawa-Aligned 2HDM

We discuss scenarios with wrong-sign (WS) Yukawa couplings for the discovered Higgs boson in the Yukawa-aligned two Higgs doublet model. In the WS scenario, Yukawa couplings for down-type quarks and/or charged leptons have an opposite sign as compared to those of the Higgs boson in the standard model, which can be consistent with current flavour data and the Higgs signal strengths. The phenomenology of additional Higgs bosons in such a scenario can be significantly different from that with right-sign Yukawa couplings, mainly due to a larger Higgs boson mixing to be required in the wrong-sign case. We show the parameter space which is excluded or explored by direct searches for the additional Higgs bosons at the current and high-luminosity LHC under the constraints from perturbative unitarity and vacuum stability. In particular, we find that most of the parameter space is explored in the WS scenario with the Type-X (lepton specific) Yukawa interaction which is a special case of the Yukawa alignment realized by imposing a softly-broken $\mathbb{Z}_2$ symmetry. We propose that multi-Higgs events from pair productions of the additional Higgs bosons can be the smoking gun signature to probe the WS scenario, and give the expected number of events at the high-luminosity LHC.

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arXiv2022

Radiative corrections to decay branching ratios of the CP-odd Higgs boson in two Higgs doublet models

We calculate radiative corrections to decay rates of CP-odd Higgs boson $A$ for various decay modes in the four types of two Higgs doublet models with the softly broken discrete $Z_{2}$ symmetry. The decay branching ratios are evaluated at the next-to-leading order for electroweak corrections and the next-to-next-to-leading order for QCD corrections. We comprehensively study the impact of the electroweak corrections on the decay rates and the branching ratios. We find that the radiative corrections can sizably modify the branching ratios, especially for the $A\to Zh$ decay mode in the nearly alignment scenario, where coupling constants of the SM-like Higgs boson $h$ are close to those in the standard model. We also show correlations between the branching ratios of $A$ and the scaling factor of the SM-like Higgs boson coupling including higher-order corrections. In addition, we show characteristic predictions on the decay pattern depending on the types of Yukawa interaction, by which we can discriminate the types of Yukawa interaction in future collider experiments.

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