arXiv subjects

Kazuki Enomoto

Publications and source records attributed to Kazuki Enomoto.

16 recordsLinked to original sources

arXiv2026

Improved analyses of the electroweak phase transition and its phenomenology in the Georgi-Machacek model

The Georgi-Machacek model is a promising new physics scenario with isospin-triplet scalar fields, whose effects can qualitatively change the nature of the electroweak phase transition compared with the Standard Model. We examine the electroweak phase transition in this model by using the thermally resummed one-loop effective potential. We also incorporate effects of custodial symmetry breaking due to radiative corrections by evolving the couplings and classical fields according to the renormalization group equations. We identify viable parameter regions by Bayesian analysis using theoretical requirements and the latest experimental bounds. We then investigate the electroweak phase transition in the resulting parameter regions and discuss its phenomenological implications, such as Higgs boson decays into neutral gauge bosons, di-Higgs production at high-energy colliders, and stochastic gravitational waves associated with a first-order electroweak phase transition. We find that parameter points for the strong first-order electroweak phase transition can be verified by these observables at next-generation experiments.

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arXiv2026

Oblique corrections in general dark $U(1)$ models

We investigate the impact of dark Abelian gauge bosons on the electroweak precision measurements at the one-loop level. The dark gauge boson couples to the standard model fermions generally via two kinds of mixing with the electroweak gauge bosons: the kinetic mixing and the mass mixing. We solve the Schwinger-Dyson equation for the gauge boson propagators and derive a renormalization scheme-independent representation of the scattering amplitudes for four-fermion processes, including the full oblique corrections. We define the running parameters at the one-loop level and show that the leading new physics effects, including the mixing, in the electroweak precision observables can be described by the oblique parameters $S$, $T$, and $U$ as in the standard electroweak gauge theory when the new physics scale is sufficiently high and the dark gauge boson mass lies away from the $Z$ pole. We consider the dark doublet scalar boson as an example and numerically show that a novel one-loop effect can drastically change the parameter region allowed by the electroweak precision tests.

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arXiv2026

Doubly charged Higgs boson at same-sign lepton colliders

We investigate the search for doubly charged Higgs bosons in the Georgi-Machacek (GM) model at same-sign lepton colliders. The dominant production mode is vector boson fusion, through which the particle is singly produced and decays into a pair of same-sign $W$ bosons if the triplet vacuum expectation value is sufficiently large, as allowed in the GM model. Considering the leptonic decays of the $W$ bosons, we discuss the discovery reach of the signal and a method to extract the mass of the doubly charged Higgs boson. It is impossible to construct the transverse mass of the decay product to obtain the mass because of the neutrinos radiated from the initial-state leptons. Alternatively, we propose to make use of the invariant mass of the same-sign leptons. We perform $χ^2$ fitting using the analytical formula for the invariant mass distribution and demonstrate that this approach is effective in extracting the information on the mass. We also compare the process with pair production processes at the opposite-sign lepton colliders.

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arXiv2025

Refined approaches in second leptogenesis for the baryon-lepton asymmetry discrepancy

The temperature-dependent mass of the heavy neutrino can lead to the second leptogenesis occurring below the electroweak scale, potentially explaining the large discrepancy between baryon and lepton asymmetries. We investigate this scenario further, exploring the intricate interplay of the weak interaction processes within this framework. It includes notable shifts in the dominant decay channels of heavy neutrinos around the electroweak symmetry breaking, along with the resonance behavior of the scattering processes near the $W/Z$ mass. The $CP$ asymmetry can also vary over cosmic history due to the temperature-dependent mass, allowing the $B-L$ asymmetry generation to be amplified in the late epoch. These findings elucidate how such alterations in the dynamics of second leptogenesis contribute to addressing the observed discrepancies in baryon-lepton asymmetry.

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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

Second leptogenesis: Unraveling the baryon-lepton asymmetry discrepancy

We propose a novel scenario to explain the matter-antimatter asymmetry by twofold leptogenesis, wherein heavy Majorana neutrinos exhibit temperature-dependent masses and engage in $CP$-violating decays. This scenario envisages two distinct phases of leptogenesis: one occurring above the electroweak scale and the other below it. The sphaleron process converts the first lepton asymmetry to baryon asymmetry, but not the second one due to its decoupling. This mechanism potentially explains the significant discrepancy between baryon and lepton asymmetries, as suggested by recent observations of Helium-4. Furthermore, our model implies that the present masses of Majorana neutrinos are lighter than the electroweak scale, offering a tangible avenue for experimental verification in various terrestrial settings.

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arXiv2023

Searching for new physics effects in future $W$ mass and $\sin^2θ_W (Q^2)$ determinations

We investigate the phenomenology of the dark $Z$ boson, $Z_d$, which is associated with a new Abelian gauge symmetry and couples to the standard model particles via kinetic mixing $\varepsilon$ and mass mixing $\varepsilon_Z^{}$. We examine two cases: (i) $Z_d$ is lighter than the $Z$ boson, and (ii) $Z_d$ is heavier than that. In the first case, it is known that $Z_d$ causes a deviation in the weak mixing angle at low energies from the standard model prediction. We study the prediction in the model and compare it with the latest experimental data. In the second case, the $Z$-$Z_d$ mixing enhances the $W$ boson mass. We investigate the effect of $Z_d$ on various electroweak observables including the $W$ boson mass using the $S$, $T$, and $U$ parameters. We point out an interesting feature: in the limit $\varepsilon \to 0$, the equation $S = - U$ holds independently of the mass of $Z_d$ and the size of $\varepsilon_Z^{}$, while $|S|\gg |U|$ in many new physics models. We find that the dark $Z$ boson with a mass of $O(100)~\mathrm{GeV}$ with a relatively large mass mixing can reproduce the CDF result within $2σ$ while avoiding all other experimental constraints. Such dark $Z$ bosons are expected to be tested at future high-energy colliders.

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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

Revisiting Affleck-Dine Leptogenesis with light sleptons

We revisit the Affleck-Dine leptogenesis via the $L H_u$ flat direction with a light slepton field. Although the light slepton field is favored in low-energy SUSY phenomenologies, such as the muon $g-2$ anomaly and bino-slepton coannihilation, it may cause a problem in the Affleck-Dine leptogenesis: it may create an unwanted charge-breaking vacuum in the Affleck-Dine field potential so that the Affleck-Dine field is trapped during the course of leptogenesis. We investigate the conditions under which such an unwanted vacuum exists and clarify that both thermal and quantum corrections are important for the (temporal) disappearance of the charge-breaking minimum. We also confirm that if the charge-breaking vacuum disappears due to the thermal or quantum correction, the correct baryon asymmetry can be produced while avoiding the cosmological gravitino problem.

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arXiv2022

New benchmark scenarios of electroweak baryogenesis in aligned two Higgs double models

We discuss electroweak baryogenesis in aligned two Higgs doublet models. It is known that in this model the severe constraint from the experimental results for the electron electric dipole moment can be avoided by destructive interference among CP-violating effects in the Higgs sector. In our previous work, we showed that the observed baryon number in the Universe can be explained without contradicting current available data in a specific scenario in the same model. We here first discuss details of the evaluation of baryon number based on the WKB method taking into account all order of the wall velocity. We then investigate parameter spaces which are allowed under the current available data from collider, flavor and electric dipole moment experiments simultaneously. We find several benchmark scenarios which can explain baryon asymmetry of the Universe. We also discuss how we can test these benchmark scenarios at future collider experiments, various flavor experiments and gravitational wave observations.

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arXiv2021

Electroweak baryogenesis in aligned two Higgs doublet models

We evaluate the baryon number abundance based on the charge transport scenario of top quarks in the CP-violating two Higgs doublet model, in which Yukawa interactions are aligned to avoid dangerous flavor changing neutral currents, and coupling constants of the lightest Higgs boson with the mass $125~\mathrm{GeV}$ coincide with those in the standard model at tree level to satisfy the current LHC data. In this model, the severe constraint from the electric dipole moment of electrons, which are normally difficult to be satisfied, can be avoided by destructive interferences between CP-violating phases in Yukawa interactions and scalar couplings in the Higgs potential. Viable benchmark scenarios are proposed under the current available data and basic theoretical bounds. We find that the observed baryon number can be reproduced in this model, where masses of additional Higgs bosons are typically $300$--$400~\mathrm{GeV}$. Furthermore, it is found that the triple Higgs boson coupling is predicted to be $35$--$55~\%$ larger than the standard model value.

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arXiv2021

Probing doubly charged scalar bosons from the doublet at future high-energy colliders

The isospin doublet scalar field with hypercharge 3/2 is introduced in some new physics models such as tiny neutrino masses. Detecting the doubly charged scalar bosons from the doublet field can be a good probe of such models. However, their collider phenomenology has not been examined sufficiently. We investigate collider signatures of the doubly and singly charged scalar bosons at the LHC for the high-luminosity upgraded option (HL-LHC) by looking at transverse mass distributions etc. With the appropriate kinematical cuts we demonstrate the background reduction in the minimal model in the following two cases depending on the mass of the scalar bosons. (1) The main decay mode of the singly charged scalar bosons is the tau lepton and missing (as well as charm and strange quarks). (2) That is into a top bottom pair. In the both cases, we assume that the doubly charged scalar boson is heavier than the singly charged ones. We conclude that the scalar doublet field with $Y = 3/2$ is expected to be detectable at the HL-LHC unless the mass is too large.

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arXiv2020

Alternative dark matter phenomenology in a general $U(1)_X$ extension of the Standard Model

The existence of the neutrino mass and flavor mixing have been experimentally verified. These phenomena strongly motivate to extend the Standard Model (SM). Amongst many possibilities, a simple and interesting extension of the SM can be investigated using a general U$(1)_X$ extension of the SM gauge group. Demanding the cancellation of the gauge and mixed gauge gravity anomalies, three right handed neutrinos are introduced in this model where the U$(1)_X$ charge assignment becomes a linear combination of U$(1)_{\rm{B-L}}$ and U$(1)_Y$ hyper-charges. After the U$(1)_X$ breaking, an additional neutral gauge boson, $Z^\prime$ is evolved and the neutrino mass is generated by the seesaw mechanism. In such a model we investigate the properties of a Dark Matter (DM) candidate which is a massive weakly interacting particle and Dirac type in nature. The stability of the DM is protected by its U$(1)_X$ charge. Using the current bounds on the search results of $Z^\prime$ at the Large Hadron Collider (LHC) and the dark matter relic abundance we find a phenomenologically viable parameter space of our scenario.

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arXiv2020

Probing charged lepton number violation via $\ell^\pm \ell^{\prime \pm} W^\mp W^\mp$

We study impacts of dimension-five lepton-number violating operators associated with two same-sign weak bosons, $\ell^\pm \ell^{\prime \pm} W^\mp W^\mp$, on current and future experiments for neutrino oscillation, lepton-number violating rare processes and high-energy collider experiments. These operators can contain important information on the origin of tiny neutrino masses, which is independent of that from the so-called Weinberg operator. We examine constraints on the coefficients of the operators by the neutrino oscillation data. Upper bounds on the coefficients are also investigated by using the data for processes of lepton number violation such as neutrinoless double beta decays and $μ^-$-$e^+$ conversion. These operators can also be directly tested by searching for lepton-number violating dilepton production via the same-sign W boson fusion process at high-energy hadron colliders like the Large Hadron Collider. We find that these operators can be considerably probed by these current and future experiments.

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arXiv2020

Radiative generation of neutrino masses in a 3-3-1 type model

A new model for tiny neutrino masses is proposed in the gauge theory of $SU(3)_C \otimes SU(3)_L \otimes U(1)_X$, where neutrino masses are generated via the quantum effect of new particles. In this model, the fermion content is taken to be minimal to realize the gauge anomaly cancellation, while the scalar sector is extended from the minimal 3-3-1 model to have an additional $SU(3)_L$ triplet field. After $SU(3)_L\otimes U(1)_X$ is broken into $SU(2)_L\otimes U(1)_Y$, the "Zee model" like diagrams are naturally induced, which contain sufficient lepton flavor violating interactions to reproduce current neutrino oscillation data. Furthermore, the remnant $Z_2$ symmetry appears after the electroweak symmetry breaking, which guarantees the stability of dark matter. It is confirmed that this model can satisfy current dark matter data. As an important prediction to test this model, productions and decays of doubly-charged scalar bosons at collider experiments are discussed in successful benchmark scenarios.

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arXiv2019

New model for radiatively generated Dirac neutrino masses and lepton flavor violating decays of the Higgs boson

We propose a new mechanism to explain neutrino masses with lepton number conservation, in which the Dirac neutrino masses are generated at the two-loop level involving a dark matter candidate. In this model, branching ratios of lepton flavor violating decays of the Higgs boson can be much larger than those of lepton flavor violating decays of charged leptons. If lepton flavor violating decays of the Higgs boson are observed at future collider experiments without detecting lepton flavor violating decays of charged leptons, most of the models previously proposed for tiny neutrino masses are excluded while our model can still survive. We show that the model can be viable under constraints from current data for neutrino experiments, searches for lepton flavor violating decays of charged leptons and dark matter experiments.

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