arXiv subjects

Shinya Kanemura

Publications and source records attributed to Shinya Kanemura.

At least 19 recordsLinked to original sources

arXiv2026

Entanglement entropy minimization and global symmetry violation in scatterings

The concept of quantum information has provided new ways to investigate the theoretical structure of particle interactions. In particular, it has been shown that extrema of quantum entanglement in scattering and decay processes can be related to symmetries and characteristic properties of particles. We focus on a more specific question: whether the minimization of entanglement entropy generated in scattering processes is systematically related to the suppression of interactions that violate global symmetries or selection rules. We conjecture that the minimization of the entanglement entropy can select the symmetry-preserving point when a new interaction opens a symmetry-violating scattering channel. We investigate the plausibility of this conjecture by considering interactions beyond the Standard Model (BSM) that violate global symmetries or the corresponding selection rules, including lepton number violation, baryon number violation, lepton flavor violation, and flavor-changing neutral currents. We show that, when symmetry-violating interactions open final-state sectors that are orthogonal to the symmetry-preserving sector, the symmetry-preserving point becomes a local minimum of the entanglement entropy. Our results support the possibility that the minimization of entanglement provides a common information-theoretic principle underlying the suppression of BSM phenomena relevant to symmetry-violating interactions.

hep-ph↗

arXiv2026

Radiative corrections to decays of the 125 GeV Higgs boson in the complex Higgs triplet model

The extension of the Higgs sector with an additional complex triplet field is often considered for generating the neutrino mass by the Type-II seesaw mechanism. Such an extension generally predicts $ρ\neq1$, where $ρ$ is the electroweak rho parameter at the tree level, so that the renormalization of the electroweak parameters is different from models like the standard model (SM) and two Higgs doublet models. In this paper, we present a full set of radiative corrections to decays of the 125 GeV Higgs boson ($h$) in this model. One-loop contributions of the extra Higgs bosons as well as SM fermions and gauge bosons to the decay rates of $h$ are calculated in the on-shell scheme. Gauge dependence appearing in the counter terms of mixing angles is eliminated by the pinch technique. Higher-order QCD corrections are also implemented. We find that the decay rates can significantly deviate from the predictions in the SM and other extensions such as the two Higgs doublet models and the singlet model. For example, the decay rates of $h\to WW^\ast$ and $h\to ZZ^\ast$ can be a few percent larger than the SM value under current experimental and theoretical constraints. In this case, deviations in $h\to γγ$ and Higgs self-coupling can reach about $-20\%$ and $100\%$, respectively. The pattern of the deviations is different from the other extended Higgs models. These characteristic predictions are expected to be detected at the High-Luminosity LHC or future Higgs factories.

hep-ph↗

arXiv2026

Gravitational waves from supermassive right-handed neutrinos produced at preheating

The post-inflationary production of supermassive particles can have profound implications for the thermal history of the universe and may leave observable imprints in the gravitational wave (GW) background. In scenarios where the inflaton couples predominantly to heavy fields, say right-handed neutrino (RHN), non-perturbative mechanisms such as parametric resonance can lead to their efficient production, even when their masses exceed the inflaton mass. Once produced, the RHNs emit gravitons through bremsstrahlung as they decay into the Standard Model (SM) particles via $N\rightarrow \ell + H$, enabled by the unavoidable minimal coupling to gravity, sourcing a stochastic GW background. We study this mechanism within the framework of $α-$attractor inflationary models, highlighting how the resulting GW spectrum carries indirect imprints of the heavy sector and the post-inflationary dynamics. This offers an observational window into otherwise inaccessible supermassive particles and provides a powerful probe of high-scale physics beyond the SM.

hep-ph↗

arXiv2026

Asteroid-mass soliton as the dark matter-baryon coincidence solution

Nontopological solitons formed during first-order phase transitions can serve as macroscopic dark matter candidates, with their stability ensured by a charge asymmetry traditionally assumed to originate from baryogenesis. Following this generic pattern, we demonstrate that solitogenesis after baryogenesis makes the solitons a coincident dark matter candidate, providing new explanations for the coincidence problem between baryon and dark matter energy densities. We derive a novel and robust conclusion: asteroid-mass coincident soliton dark matter is always accompanied by detectable gravitational waves observable by LISA, $μ$Ares, and Theia, providing a new candidate beyond primordial black holes in this mass window. Additionally, we propose a simple neutrino-ball scenario that addresses baryon asymmetry, dark matter, and neutrino masses, featuring new particles below the electroweak scale and correlated observable signals, including lensing, gravitational waves, and soliton evaporation or collisions.

hep-ph↗

arXiv2026

Classification of Higgs sectors from group theoretical properties of UV gauge theories

Extended Higgs sectors are often introduced to explain phenomena beyond the standard model (BSM). The existence of multiple scalar fields may cause the Landau pole below the Planck scale. In this case, the low-energy theory may be replaced by an asymptotic-free gauge theory. In this paper, we consider an $\mathrm{SU}(2)$ gauge theory with confinement as such an ultraviolet theory of the extended Higgs sectors. We investigate the relation between scalar particle contents at the low energy and group theoretical properties of fundamental fermions of the gauge theory. We find that particle contents of various extended Higgs sectors previously proposed to explain the BSM problems are deduced by each charge assignment of flavor symmetry of the fundamental fermions of the $\mathrm{SU}(2)$ gauge symmetry. Our findings may provide a new picture for the ultraviolet completion of the extended Higgs sectors.

hep-ph↗

arXiv2025

The mass-coupling effect in leptogenesis

Particle decay in leptogenesis provides a simple avenue to explain the baryon asymmetric universe, where the decaying particle can provide the out-of-equilibrium condition to create a net lepton asymmetry. It is widely anticipated that the lepton asymmetry would be changed significantly by varying couplings and the decaying particle mass, especially in the weak washout regime. Contrary to this naive expectation, we demonstrate a general phenomenon in a class of leptogenesis scenarios from heavy particle decay, where varying the mass and couplings would not modify the lepton asymmetry in a noticeable way, as these mass and coupling effects are largely canceled out from the evolution of the decaying particle. It points out that a much broader parameter space in the mass and couplings will open automatically once leptogenesis is realized in a benchmark point; however, tuning the mass and couplings to boost leptogenesis will be challenging.

hep-ph↗

arXiv2025

Bounds and detection of MeV-scale dark matter annihilation to neutrinos

Current and most upcoming neutrino detectors can only reach a dark matter annihilation cross section to neutrinos larger than the standard freeze-out value, but they open intriguing detection avenues for non-standard dark matter paradigms. An important corollary of these non-standard scenarios is relic dark matter annihilation after neutrino decoupling, which was previously overlooked in constraining MeV-scale dark matter. However, by combining the contributions from entropy injection during neutrino decoupling and from nonthermal neutrino energy release after decoupling, we derive significant constraints on the annihilation cross section to neutrinos, which in some mass regimes become stronger than the current bounds. Furthermore, we find that the lower bounds on dark matter masses become inconclusive under the recent data releases from the DESI, SPT-3G, and ACT collaborations. These bounds determine the extent to which upcoming neutrino detectors will probe dark matter annihilation into neutrinos.

hep-ph↗

arXiv2025

Forbidden neutrinogenesis

The origin of neutrino masses can be simply attributed to a new scalar beyond the Standard Model. We demonstrate that leptogenesis can explain the baryon asymmetry of the universe already in such a minimal framework, where the electroweak scalar is favored to enhance the baryon asymmetry. Different from traditional leptogenesis, the realization here exploits the thermal behavior of leptons at finite temperatures, which is otherwise kinetically forbidden in vacuum. We present detailed calculations of the CP asymmetry in the Schwinger-Keldysh Closed-Time-Path formalism, and compute the asymmetry evolution via the Kadanoff-Baym equation. Such minimal forbidden neutrinogenesis establishes a direct link between the baryon asymmetry and the CP-violating phase from neutrino mixing, making the scenario a compelling target in neutrino oscillation experiments. Complementary probes from cosmology, flavor physics and colliders are also briefly discussed.

hep-ph↗

arXiv2025

Resonant Forbidden CP Asymmetry from Soft Leptons

To explain the baryon asymmetry in the early universe via leptogenesis, quantum corrections to new particles are commonly invoked to generate the necessary CP asymmetry. We demonstrate, however, that a large CP asymmetry can already arise from Standard Model leptons. The mechanism relies on resummation of soft leptons at finite temperatures. The CP asymmetry, which is kinematically forbidden in vacuum, can be resonantly enhanced from thermally resummed leptons by seven orders of magnitude. Contrary to the resonance from exotic particles, we show that the resonant enhancement from soft leptons is protected by controlled widths under finite-temperature perturbation theory. We quantify such CP asymmetries in leptogenesis with secluded flavor effects and comment on the significance and application. The mechanism exploits the maximal role of leptons themselves, featuring low-scale leptogenesis, minimal model buildings and dark matter cogenesis.

hep-ph↗

arXiv2025

Leading two-loop corrections to the Higgs di-photon decay in the Inert Doublet Model

Leading two-loop contributions to the di-photon decay of the Higgs boson are evaluated for the first time in the Inert Doublet Model (IDM). We employ for this calculation the Higgs low-energy theorem, meaning that we obtain corrections to the Higgs decay process by taking Higgs-field derivatives of the leading two-loop contributions to the photon self-energy. Specifically, we have included corrections involving inert BSM Higgs bosons and gauge bosons, as well as external-leg contributions involving inert scalars, gauge bosons and fermions. Our calculation has been performed with a full on-shell renormalization, and in the gauge-less limit. Moreover, we performed two independent calculations, using the background-field method and the pinch technique, in order to apply the Higgs low-energy theorem consistently, and found full agreement between the two calculations. We investigate our results numerically in two scenarios of the IDM: one with a light dark matter (DM) candidate (Higgs resonance scenario), and another with all additional scalars heavy (heavy Higgs scenario). In both cases, we find that the inclusion of two-loop corrections qualitatively modifies the behavior of the decay width, compared with the one-loop ($i.e.$ leading) order, and that they increase the deviation from the Standard Model. Furthermore, we demonstrate that the inclusion of the newly-computed two-loop corrections is essential to reliably interpret the observation or non-observation of a deviation in the Higgs di-photon decay width at current and future colliders.

hep-ph↗

arXiv2025

Electroweak baryogenesis in 2HDM without EDM cancellation

We study two Higgs doublet models with successful electroweak baryogenesis but without cancellations of electric dipole moments (EDMs). For the baryogenesis, additional scalar bosons are favored to couple mainly with the top quark with CP violations. However, if they also couple to light fermions of the Standard Model, the model is limited severely by EDMs, and additional CP phases irrelevant to the baryogenesis are often introduced to cancel the contributions to the EDMs. Alternatively, we consider a scenario where the light-fermion couplings are suppressed to avoid the constraints. In our scenario, it is found that the leading contributions arise in the top-quark EDMs at the two-loop level. They induce the electron, neutron, and proton EDMs via radiative corrections. Since there is no additional CP-violating phase, they are correlated with the baryon asymmetry. We show that our scenario is compatible with the current experimental bounds and is within the scope of future EDM experiments.

hep-ph↗

arXiv2025

Super-critical primordial black hole formation via delayed first-order electroweak phase transition

The delay of the first-order electroweak phase transitions (EWPT) may lead to the emergence of baby universes inside wormhole structures due to the large vacuum energy density in false vacuum domains. Observers outside the false vacuum domains observe them as primordial black holes (PBHs), categorized as super-critical PBHs. We specifically investigate the dynamics of PBH formation due to delayed first-order EWPTs by solving the equations of bubble wall dynamics. We numerically confirm that such super-critical PBHs can be formed by the delayed first-order EWPT assuming spherically symmetric false vacuum domains with the thin-wall approximation for its boundary. Our numerical results show that a PBH formation criterion utilizing characteristic timescales is more appropriate than the conventional criterion based on density fluctuations. Employing our numerical results, we update the parameter regions of new physics models which can be explored by current and future constraints on the PBH abundance.

hep-ph↗

arXiv2024

Testing the gauged $\mathrm{U(1)}_{B-L}$ model for loop induced neutrino mass with dark matter

We present a new viable benchmark scenario under the current experimental data for the model which can explain tiny mass of active neutrinos and dark matter, as a summary of our results. Majorana masses of right-handed neutrinos are given by the spontaneous breaking of the $\mathrm{U(1)}_{B-L}$ gauge symmetry above the electroweak scale, and tiny neutrino masses are radiatively induced by quantum effects of particles of the dark sector including dark matter candidates. We first show benchmark points which satisfy current experimental data, and then give comments on how this model can be tested at collider experiments.

hep-ph↗

arXiv2024

New renormalization scheme in extended Higgs sectors for Higgs precision measurements

We discuss a new renormalization scheme for mixing angles in extended Higgs sectors for the coming era of the Higgs precise measurements at future lepton colliders. We focus on the two Higgs doublet models (2HDMs) with a softly-broken $Z_2$ symmetry as a simple and important example, in which two mixing angles $α$ and $β$ appear in the Higgs sector. In this new scheme, the counterterms for two mixing angles $δα$ and $δβ$ are determined by requiring that deviations in the decay rates of $h\to ZZ^* \to Z\ell^+\ell^-$ and $h \to ττ$ from the corresponding predictions in the standard model at NLO are given by the square of the scaling factor at tree level. We show how this scheme works in the 2HDMs, and demonstrate how the other decay rates (e.g., $h \to WW^*$, $h \to b\bar{b}$, etc.) are predicted at NLO.

hep-ph↗

arXiv2024

CP violation of the loop induced $H^\pm \to W^\pm Z$ decays in general two Higgs doublet model

New sources of CP violation are necessary to solve the problem of the baryon asymmetry of the Universe. Extending Higgs sector is one way to introduce such new CP violating phases, and studying observables resulting from the CP violation is important to test the model in future experiments. In these proceedings, we discuss the loop induced $ H^\pm W^\mp Z$ vertices in the CP violating general two Higgs doublet model, summarizing our results. We evaluate impacts of the CP violation on the decays $H^\pm \to W^\pm Z$ through these vertices, and find that the difference between the decays $H^+ \to W^+ Z$ and $H^- \to W^- Z$ is sensitive to the CP violating phases in the model.

hep-ph↗

arXiv2024

Revisiting the model for radiative neutrino masses with dark matter in the $\mathrm{U(1)}_{B-L}$ gauge theory

The radiative seesaw model with gauged $\mathrm{U(1)}_{B-L}\times\mathbb{Z}_2$ extension is a well-motivated scenario which gives consistent predictions of active neutrino masses and the abundance of dark matter. Majorana masses of right-handed neutrinos, the lightest of which can be identified as dark matter, are given by the spontaneous breaking of the $\mathrm{U(1)}_{B-L}$ gauge symmetry. We revisit this model with the latest constraints from dark matter searches, neutrino oscillations, flavor experiments and collider experiments. We explore the feasible parameter space of this model, and find that there are still allowed regions under the latest experimental constraints. We present new viable benchmark scenarios for this model, i.e., the right-handed neutrino dark matter scenario and the scalar dark matter scenario. We also mention the testability of these benchmark scenarios at future experiments.

hep-ph↗

arXiv2024

New collider implications on a strongly first order EWPT

In order to understand the early history of the universe, and to test baryogenesis models, determining the nature of the electroweak phase transition is imperative. The order and strength of this transition is strongly correlated to relatively large deviations in the $hhh$ coupling. In models where a considerable part of the $hhh$ coupling deviation is caused by charged particle loops, the $hγγ$ coupling is also expected to deviate considerably. In this talk, by using a model-independent approach, I explain how to obtain conditions that are sufficient for a strongly first order phase transition. After the $hγγ$ coupling is determined with precision at the HL-LHC, these conditions can be tested at Future Linear Colliders by measurements of the $hhh$ coupling, to conclusively determine the nature of the electroweak phase transition and the viability of electroweak baryogenesis on models with new charged scalars.

hep-ph↗

arXiv2024

Top quark flavor changing neutral currents at Future Linear Colliders

We discuss the production and the decay of top quark through flavor-changing neutral current (FCNC) interaction at future linear colliders. We first discuss the theoretical predictions of top quark FCNC decays into $qH$ and $qZ$ within a class of $t$-channel simplified dark matter models. For the existing bounds on the top quark FCNC interactions at the Large Hadron Collider, we estimate the production rates of top quark through FCNC interactions at future linear colliders for energies from $250$ GeV to $3$ TeV.

hep-ph↗