arXiv subjects

Chikako Idegawa

Publications and source records attributed to Chikako Idegawa.

12 recordsLinked to original sources

arXiv2026

Precise bubble wall velocity in a specific phase transition pattern in the CxSM and beyond

The bubble wall velocity is a key quantity in cosmological first-order phase transitions, with important implications for electroweak baryogenesis, gravitational wave signals, the dark matter relic density and primordial black holes formed during the phase transition, and so on. However, it is often treated as a free input in phenomenological studies, while a self-consistent determination remains challenging. In this work, taking the complex singlet extension of the Standard Model as an example, we investigate the bubble wall dynamics and velocity in a specific electroweak phase transition pattern where both the Higgs field and the coupled singlet scalar experience friction. The microscopic friction arising from particle interactions with the plasma is evaluated using Boltzmann transport equations, while the macroscopic plasma response is described through hydrodynamic analysis. By applying the steady state force balance condition, we numerically determine the bubble wall velocity for different model parameters. We show that the wall velocity is governed by the competition between the driving force from the effective potential and plasma friction, and that its variation can significantly affect the baryon asymmetry. Our study provides a quantitative investigation of bubble wall dynamics in this overlooked phase transition pattern and its implications for early Universe phenomenology.

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arXiv2026

Model Parameter Reconstruction of Electroweak Phase Transition with TianQin and LISA: Insights from the Dimension-Six Model

We investigate the capability of TianQin and LISA to reconstruct the model parameters in the Lagrangian of new physics scenarios that can generate an electroweak SFOPT. Taking the dimension-six Higgs operator extension of the Standard Model as a representative scenario for a broad class of new physics models, we establish the mapping between the model parameter $Λ$ and the observable spectral features of the stochastic gravitational wave background. We begin by generating simulated data incorporating Time Delay Interferometry channel noise, astrophysical foregrounds, and signals from the dimension-six model. The data are then compressed and optimized, followed by geometric parameter inference using both Fisher matrix analysis and Bayesian nested sampling with PolyChord, which efficiently handles high-dimensional, multimodal posterior distributions. Finally, machine-learning techniques are employed to achieve precise reconstruction of the model parameter $Λ$. For benchmark points producing strong signals, parameter reconstruction with both TianQin and LISA yields relative uncertainties of approximately $20$-$30\%$ in the signal amplitude and sub-percent precision in the model parameter $Λ$. The sub-percent precision reflects the statistical reconstruction capability of the detectors in an idealized setting: it incorporates the machine-learning inference uncertainty and is established at a fixed bubble wall velocity, while theoretical uncertainties in the effective potential calculation are not included.

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arXiv2026

Two Higgs doublet model with a complex singlet scalar and Multi-critical Point Principle

We study a two Higgs doublet model extended by a complex singlet scalar, in which the imaginary part of the singlet serves as a dark matter (DM) candidate. In this model, degenerate masses of the three neutral Higgs bosons are crucial for achieving consistency with current constraints from DM direct-detection experiments and Higgs searches. This is called the degenerate scalar scenario. To provide a theoretical motivation for such a degenerate Higgs spectrum, we impose the tree-level Multiple Point Principle (MPP), which requires the electroweak and singlet vacua to be degenerate, and analyze its implications for the scalar potential, DM phenomenology, and the electroweak phase transition. We show that the tree-level MPP favors large SU(2)$_L$ doublet-singlet mixing parameters, which compete with the degenerate scalar scenario. Nevertheless, we demonstrate that viable parameter regions still exist in which the observed DM constraints are satisfied. Furthermore, although the tree-level MPP forbids a tree-level-driven first-order electroweak phase transition, we show that thermal loop effects can induce a strong first-order transition compatible with electroweak baryogenesis.

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arXiv2026

Primordial Black Hole Formation and Multimessenger Signals in a Complex Singlet Extension of the Standard Model

We investigate the formation of primordial black holes (PBHs) induced by a first-order electroweak phase transition in a realistic renormalizable framework, the complex singlet extension of the Standard Model. We perform a quantitative analysis of the PBH abundance and identify parameter regions consistent with current microlensing constraints. Furthermore, we show that the same parameter space predicts observable stochastic gravitational waves within the sensitivities of future space-based detectors, as well as a sizable deviation in the Higgs triple coupling that can be probed at future lepton colliders. Our results highlight a comprehensive multimessenger framework in which PBH, gravitational wave, and collider observations can jointly test the dynamics of a strongly first-order electroweak phase transition in the early Universe.

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arXiv2025

One-loop analysis of dark matter constraints in a complex scalar extension of the Standard Model

We investigate the complex singlet extension of the Standard Model, which provides a scalar dark matter candidate. We impose the constraints from the observed relic abundance together with the most stringent limits from direct detection experiments on the model. The counterterms are determined so that the vacuum and mass conditions are consistently satisfied at one-loop order, and the one-loop corrections to scalar self-interactions are fully included in the amplitudes involving the dark matter particle. As a result, the allowed parameter region shows clear deviations from the tree-level analysis, demonstrating the impact of quantum corrections on the phenomenology of dark matter.

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arXiv2025

Degenerate scalar scenario of two Higgs doublet model with a complex singlet scalar

We study the two Higgs doublet model with a complex singlet scalar whose imaginary part acts as dark matter (DM). The scattering of DM and quarks, mediated by three CP-even scalars in this model, is suppressed when masses of CP-even scalars are degenerate; that is called the ``degenerate scalar scenario''. Based on this scenario, we show that the strong first-order electroweak phase transition (EWPT) can be achieved without conflicting with constraints from the DM relic density and the direct detection experiments. We also discuss a shift of scalar trilinear coupling from the Standard Model prediction, which could be a test of this model in collider experiments.

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arXiv2024

A complex singlet extension of the Standard Model with a singlet fermion dark matter

We examine a complex singlet scalar extension of the Standard Model (CxSM) with an extra singlet fermion. Both the singlet scalar and fermion are dark matter (DM) candidates. It is known that although the scalar potential in the CxSM can realize strong first-order electroweak phase transition, the scalar DM included in the model gives only a tiny amount of the relic density compared to the observed one. Therefore, a fermion DM is introduced to compensate for the lack of relic density. We find that the scattering of the fermion DM and nucleons is sufficiently suppressed when the masses of scalar mediators are degenerate, as well as in the case of the scalar DM. We show the range of a combination of the mass and the Yukawa coupling of the fermion DM, which satisfies both the observed relic density and conditions of strong first-order electroweak phase transition.

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arXiv2023

Electron electric dipole moment and electroweak baryogenesis in a complex singlet extension of the Standard Model with degenerate scalars

We study the possibility of electroweak baryogenesis in the standard model with a complex scalar field, focusing mainly on a degenerate scalar scenario. In our setup, CP violation is provided by dimensional-5 Yukawa interactions involving the complex scalar field. In contrast to previous studies in the literature, we exemplify a case in which a complex phase in the singlet scalar potential is transmitted to the fermion sector via the higher-dimensional operators and drives BAU. We point out that an electric dipole moment of the electron can be suppressed due to the Higgs mass degeneracy and the presence of a new electron Yukawa coupling. Thus, viable parameter space for electroweak baryogenesis is still wide open for the latest experimental bound set by the JILA Collaboration.

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arXiv2023

Analyzing cancellation mechanism of the dark matter-quark scattering in a complex singlet extension of the Standard Model

We investigate a suppression mechanism of dark matter and quark scattering amplitudes in a complex singlet extension of the Standard Model. It has been pointed out that, in a some variant of the model, the scattering amplitudes cancel each other in the limit in which two mediator scalars degenerate in their masses. We study the origin of such the cancellation mechanism and show that the operators describing the Higgs-singlet scalar mixing play essential role. We derive sum rules for couplings in the general scalar potential of the model, which guarantee the cancellation of the scattering amplitudes in the tree and the 1-loop level.

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arXiv2022

A complex singlet extension of the Standard Model and Multi-critical Point Principle

We study the Multi-critical Point Principle (MPP) in a complex singlet scalar extension of the Standard Model (CxSM). The MPP discussed in this study selects model parameters so that two low-energy vacua realized by scalar fields are degenerate. We further note that the MPP may inhibit the electroweak phase transition (EWPT) in a certain class of models where the tree-level potential plays an essential role in its realization. Despite that, we show that strong first-order EWPT still occurs even after imposing the MPP to the scalar potential of the CxSM due to the 1-loop corrections by the new scalar boson. We study the allowed parameter space where a mass of the additional scalar is degenerate with that of the Standard Model Higgs boson, which provides a built-in mechanism to circumvent constraints from dark matter direct detection experiments. The parameter space for the non-degenerate scalar scenario is also studied for comparison.

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arXiv2022

CP-violating effects on gravitational waves in a complex singlet extension of the Standard Model with degenerate scalars

We examine CP-violating effects on electroweak phase transition (EWPT) in the standard model with a complex singlet scalar focusing particularly on a scenario where additional scalars have masses close to 125 GeV. Such a high mass degeneracy makes collider signatures in the scenario standard model like, and current experimental data cannot distinguish them from the standard model predictions. We utilize a simplified scalar potential to understand impacts of CP violation on EWPT qualitatively. Then, one-loop effective potential with a thermal resummation is employed for full numerical evaluations. As a phenomenological consequence, gravitational waves from the first-order EWPT are also evaluated. We find that the strength of the first-order EWPT would get weaker as the CP-violating effect becomes larger. As a result, gravitational wave amplitudes are diminished by the size of the CP violation. Future gravitational wave experiments may shed light on CP violation in the singlet scalar sector as well as the experimental blind spot due to the high mass degeneracy.

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arXiv2021

Electroweak phase transition in a complex singlet extension of the Standard Model with degenerate scalars

We study the feasibility of strong first-order electroweak phase transition (EWPT) in a degenerate-scalar scenario of a complex singlet extension of the Standard Model, in which a mass of an additional scalar is nearly degenerate with that of the Higgs boson, 125 GeV. This scenario is known to provide an exquisite solution for circumventing constraints from dark matter direct detection experiments due to cancellations between two scattering amplitudes mediated by two scalars. In the analysis of EWPT, we employ two gauge-invariant calculation schemes on the scalar potential and two familiar resummation methods in evaluating one-loop (gauge dependent) effective potential. We point out that one of the conditions for the strong first-order EWPT is incompatible with the known suppression mechanism of a dark matter cross-section scattering off the nucleons. Nevertheless, we find that strong first-order EWPT is still possible in the degenerate-scalar scenario by dodging dark matter constraints differently.

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