arXiv subjects

Motoi Endo

Publications and source records attributed to Motoi Endo.

At least 19 recordsLinked to original sources

arXiv2026

Systematic derivation of the Boltzmann equation for electroweak baryogenesis

We derive the Boltzmann equation for electroweak baryogenesis within a field-theoretical framework. We first algebraically solve the Kadanoff--Baym equations by incorporating the effects of gradients of the CP-violating bubble-wall background order by order. We then apply approximations and assumptions appropriate for electroweak baryogenesis and identify the on-shell parts of the solutions, which describe quasiparticle states propagating in a plasma. Using these on-shell solutions, we derive the Boltzmann equations for flavor-diagonal quasiparticle modes, while neglecting the off-diagonal components describing quantum coherence between different flavor modes. Our derivation generalizes previous field-theoretical treatments by retaining self-energy corrections, and our results are consistent with the known Boltzmann equations when these corrections are neglected. This framework provides a systematic and transparent derivation applicable to multi-flavor bosonic and fermionic systems.

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arXiv2026

Dark Matter Interpretation of the Super-Kamiokande Antineutrino Excess in $\mathrm{U}(1)_{L_\mu-L_\tau}$ model

Recent Super-Kamiokande analyses of the diffuse supernova neutrino background, based on data across all SK phases, indicate a mild preference over the zero-DSNB hypothesis at the level of about $2.3\sigma$ with electron-like antineutrino events at $E_{\bar{\nu}_e} \simeq 20\,\mathrm{MeV}$. We investigate whether this excess can be explained by MeV-scale dark matter annihilation into neutrinos in a $\mathrm{U}(1)_{L_\mu - L_\tau}$ model. The dark matter is a Dirac fermion with $m_\chi \simeq 22\,\mathrm{MeV}$ that annihilates via a light $Z'$ mediator into $\nu_\mu \bar{\nu}_\mu$ and $\nu_\tau \bar{\nu}_\tau$, which are partly converted into $\bar{\nu}_e$ through flavor oscillations. We find that this scenario simultaneously accounts for the excess and the observed relic abundance via thermal freeze-out. We further discuss the relevant laboratory and cosmological constraints, including neutrino trident production, NA64-$\mu$, Borexino, and the contribution to $\Delta N_\mathrm{eff}$.

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arXiv2026

Semileptonic sum rules in heavy-to-light charm decays

We investigate semileptonic sum rules in heavy-to-light charm decays, motivated by analogous relations in $b \to c$ and $b \to u$ transitions. Focusing on the $c \to d\overline{\ell}\nu$ decays, $D \to \pi\overline{\ell}\nu$, $D \to \rho\overline{\ell}\nu$, and $\Lambda_c \to n\overline{\ell}\nu$, we examine a relation among their lepton-flavor universality ratios $R_H^{\mu e}$. Although the charm sum rule is less precise than the relations in bottom-hadron decays, current low-energy and high-$p_T$ constraints on new physics restrict the actual deviation from the relation to below the percent level. The relation can therefore provide a useful consistency check of charm semileptonic measurements. As an application, we derive a prediction for the yet-unmeasured ratio $R_n^{\mu e}$ in $\Lambda_c \to n\overline{\ell}\nu$.

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arXiv2026

Kaon Portal to Freeze-in Dark Matter

We investigate freeze-in production of light dark matter through the quark flavor-changing operator $(\bar{s}\gamma_\mu d)(\bar{\chi}\gamma^\mu\chi)$ in a low-reheating cosmology. For reheating temperatures below the QCD crossover, kaon decays and scatterings generate the dark matter abundance through $K\to\pi\chi\bar{\chi}$ and $K\pi\to\chi\bar{\chi}$. The same interaction induces the rare kaon decays $K^+\to\pi^+\chi\bar{\chi}$ and $K_L\to\pi^0\chi\bar{\chi}$. This links the freeze-in relic abundance to searches at NA62, KOTO, and KOTO II. We find that lower reheating temperatures require larger couplings to compensate for the Boltzmann-suppressed kaon abundance, making kaon-driven freeze-in dark matter testable at rare kaon decay experiments.

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arXiv2026

$b \to c$ semileptonic sum rule: orbitally excited hadrons

We study semileptonic sum rules for $b \to c \tau \overline{\nu}$ transitions involving orbitally excited charm hadrons. Starting from the amplitude-level relation implied by the heavy quark symmetry, we construct sum rules relating these decays. We then examine deviations from the small-velocity limit. Our numerical analysis shows that the deviations generally increase once excited hadrons are involved, with tensor contributions often inducing sizable effects. At present, however, the relevant form factors are not yet sufficiently constrained. Further improvements in the hadronic inputs are essential for these sum rules to yield robust predictions for the corresponding lepton-universality ratios.

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arXiv2026

$b \to c$ semileptonic sum rule: exploring a sterile neutrino loophole

We investigate the $b\to c$ semileptonic sum rule in the presence of a massive sterile neutrino. Recent measurements of charged-current semitauonic $B$-meson decays exhibit a $\sim4\sigma$ deviation from the Standard Model predictions, whereas no such tension has been reported for the lowest-lying baryonic counterpart, the $\Lambda_b$ decay. Since these decay rates are related through the sum rule, accommodating such a mismatch beyond the level of uncertainties is nontrivial. We revisit this issue by considering dimension-six operators involving a massive sterile neutrino, and evaluate the resulting violation of the sum rule. We find that the induced effect remains negligible compared with the current experimental uncertainties, further strengthening the sum rule as a consistency check for the experimental data.

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arXiv2026

Feasibility Study of Lepton Number Violation in Rare $B$ and $K$ Meson Decays

We study lepton-number-violating interactions at dimension seven in the Standard Model effective field theory that contribute to the meson decays $B \to K \nu \nu$ and $K \to \pi \nu \nu$. Such interactions could washout the baryon asymmetry of the Universe and also contribute to the neutrinoless double beta decay, even though the interactions involve a change in down-type quark flavors. We clarify conditions under which excesses in meson decay rates over the Standard Model predictions can be successfully observed. We also show that, although these interactions contribute to neutrino masses at the two-loop level, the Weinberg operator can be introduced consistently without spoiling the scenario.

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arXiv2025

Constructing heavy-quark sum rule for $b \to c$ meson and baryon decays

We study heavy-hadron semileptonic decays proceeding via $b \to c$ transition, such as $B \to D^{(*)}\tau\bar{\nu}_\tau$ and $\Lambda_b \to \Lambda_c\tau\bar{\nu}_\tau$. In the heavy-quark limit, where the heavy-quark symmetry holds, we provide a fundamental framework for heavy-quark sum rules among these decays based on the spin decomposition picture. The relation holds directly for the squared amplitudes without requiring phase-space integration. We then apply this relation to reproduce the sum rule among $B \to D^{(*)}\tau\bar{\nu}_\tau$ and $\Lambda_b \to \Lambda_c\tau\bar{\nu}_\tau$. Furthermore, we derive new sum rules for $\Omega_b \to \Omega_c^{(*)}$ transitions and those involving excited states, such as $B \to \{D_0^*,D_1^*\}$ and $B \to \{D_1,D_2^*\}$.

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arXiv2025

$b \to c$ semileptonic sum rule: Current status and prospects

The $b \to c$ semileptonic sum rules provide relations between the decay rates of $B \to D^{(*)} \tau\bar\nu$ and $\Lambda_b \to \Lambda_c \tau\bar\nu$. Starting from the heavy quark and zero-recoil limits, we revisit the derivation of the sum rule for total decay rates. We then examine deviations from the limits and investigate corrections arising from realistic hadron masses and higher-order contributions to form factors, taking account of uncertainties. We show that these corrections are negligible compared to current experimental uncertainties, indicating that the sum rule is useful for cross-checking experimental consistency and testing the validity of the Standard Model predictions. In future, precise determinations of the form factors particularly for the tensor operator will be necessary to compare the sum rule predictions with $\Lambda_b \to \Lambda_c \tau\bar\nu$ data from the LHCb experiment and the Tera-Z projects.

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arXiv2025

$b \to c$ semileptonic sum rule: Extension to angular observables

Lepton flavor universality is a key prediction of the Standard Model of particle physics and any violation of it immediately indicates the existence of new physics. Given the recent more than $4\sigma$ discrepancy in charged current semileptonic B meson decays and the absence of evident signals at the large hadron collider, independent cross-checks become invaluable. In this context, $b \to c$ semileptonic sum rules based on heavy quark symmetry are interesting since they allow us to check the consistency of experimental results. In this paper, we report newly found sum rules among angular observables of mesonic and baryonic $b\to c l \overline{\nu}$ decays holding exactly in the large mass limit of heavy quarks. Moreover, we investigate corrections to the sum rule in realistic situations and discuss phenomenological implications.

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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.

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arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

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arXiv2025

Heavy quark symmetry behind $b \to c$ semileptonic sum rule

Lepton flavor universality violations in semileptonic $b \to c$ transitions have garnered attention over a decade. For $R_{H_c}={\rm{BR}}(H_b\to H_c \tau\bar\nu_\tau)/{\rm{BR}}(H_b\to H_c \ell\bar\nu_\ell)$ with $\ell$ being $e,\, \mu$, a sum rule among $R_{D}$, $R_{D^*}$ and $R_{\Lambda_c}$ was proposed to check consistency in the experimental results independently of new physics models. We revisit this relation from the perspective of the heavy quark symmetry. We derive a sum rule holding exactly in the heavy quark limit and clarify how model-dependent corrections are introduced in a realistic situation.

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arXiv2024

Heavy photophobic ALP at the LHC

We study the photophobic ALP model in high-mass regions under LHC Run-II. Since the ALP is predominantly coupled with electroweak gauge bosons such as $ZZ$, $WW$, and $Zγ$, and less with di-photon, the model may be probed via multi-boson final-state processes. We find that on-shell ALP productions with $Zγ$ final states currently provide the best sensitivities for $m_{a} > 40~{\rm GeV}$.

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arXiv2023

Higgs Probes of axion-like particles

We study axion-like particle contributions to the Higgs boson decays. The particle is assumed to couple with the standard model electroweak gauge bosons. Although direct productions of axion-like particles have often been discussed, we investigate indirect contributions to the Higgs boson decays into fermions, photons, $W$, and $Z$ bosons at the one-loop level. It is found that the corrections to the fermions are suppressed, whereas precise measurements of the di-photon channel of the Higgs boson decay can provide a significant probe of the model especially when the axion-like particle is heavy and its coupling to di-photon is suppressed.

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arXiv2023

Electroweak precision test of axion-like particles

We study the contributions of an axion-like particle to the electroweak precision observables. The particle is assumed to couple with the standard model electroweak gauge bosons. We provide the formulae of the contributions valid for any mass of the axion-like particle. It is found that the effects arise not only via the oblique $S$ and $U$ parameters but also via radiative corrections to the gauge couplings. Besides, the decay of $Z\to aγ$ affects the total width of the $Z$ boson. All of those contributions are considered simultaneously in the global fit analysis of the electroweak precision observables. Also, we discuss the recent CDF result of the $W$-boson mass measurement. Since the model is tightly constrained by flavor and collider constraints, it is found that the discrepancy from the standard model prediction is solved only when the axion-like particle is heavier than 500 GeV and its coupling to di-photon is suppressed.

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arXiv2022

New physics interpretation of $W$-boson mass anomaly

The CDF collaboration has recently reported an updated result on the $W$-boson mass measurement, showing a $7σ$ deviation from the standard model prediction. The discrepancy may indicate new contributions to the Fermi coupling constant. We study simple extensions of the standard model by introducing an extra scalar, fermion or vector field. It is found that the tension implies the new physics existing in multi-TeV scales if the new coupling to the electron and/or muon is of order unity.

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arXiv2022

Stau study at the ILC and its implication for the muon g-2 anomaly

Once all the sleptons as well as the Bino are observed at the ILC, the Bino contribution to the muon anomalous magnetic dipole moment (muon $g-2$) in supersymmetric (SUSY) models can be reconstructed. Motivated by the recently confirmed muon $g-2$ anomaly, we examine the reconstruction accuracy at the ILC with $\sqrt{s}$ = 500 GeV. For this purpose, measurements of stau parameters are important. We quantitatively study the determination of the mass and mixing parameters of the staus at the ILC. Furthermore, we discuss the implication of the stau study to the reconstruction of the SUSY contribution to the muon $g-2$. At the benchmark point of our choice, we find that the SUSY contribution to the muon $g-2$ can be determined with a precision of $\sim 1\%$ at the ILC.

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