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Yoshihiro Shigekami

Publications and source records attributed to Yoshihiro Shigekami.

At least 19 recordsLinked to original sources

Can a minimal radiative seesaw explain the LZ 248 keV event?

We interpret the recently reported 248~keV nuclear recoil event in the LUX-ZEPLIN (LZ) experiment via inelastic dark matter scattering within the minimal Scotogenic model. A sub-MeV mass splitting between neutral inert scalars suppresses low-energy scattering while permitting signals from the high-velocity halo tail. Crucially, co-annihilation with nearly degenerate right-handed fermions accommodates the thermal relic density for dark matter masses up to $\sim {\cal O} (1)$~TeV, extending the viable range significantly beyond the pure inert doublet model limit while evading direct detection bounds. However, to resolve the severe tension with IceCube neutrino limits on solar capture, we extend this minimal framework by introducing a hidden $U(1)_X$ gauge symmetry that naturally leads us to tiny $λ_5$ coupling at the one-loop level. This realizes an isospin-violating scenario that suppresses dark matter capture in the Sun while preserving the coherent scattering signal in the Xenon-based LZ detector. We numerically verify that this extended framework naturally generates neutrino masses and satisfies constraints from Big Bang Nucleosynthesis and indirect detection, providing a robust and testable solution to the LZ anomaly.

hep-ph

Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology

We present a dynamical solution to the dark matter-baryon coincidence problem based on the neutron portal operator connecting the visible and dark sector asymmetries. This framework is motivated by the possibility that a strongly supercooled dark confinement phase transition accounts for the nano-Hz stochastic gravitational wave signal observed by pulsar timing arrays, while also generating the dark matter and baryon asymmetry in the Universe. We show that the GeV-scale mass of asymmetric dark matter can be naturally correlated with the (multi-)TeV scale cut-off for the neutron portal through its ultraviolet completion. The dark sector is governed by an approximate fixed point and confines once the heavy portal states are integrated out, dynamically generating a scale of $\mathcal{O} ({\rm GeV})$. We analyze both tree and loop-level ultraviolet completions and demonstrate how the resulting confinement scale is linked to the effective neutron portal scale. We also discuss cosmological constraints and experimental prospects in beam dump searches and colliders for probing the neutron portal.

hep-ph

The Minimal Supersymmetric Standard Model with Non-Invertible Selection Rules

We investigate a framework of the Minimal Supersymmetric Standard Model (MSSM) in which the quark and lepton flavor structure and suppression of flavor-changing neutral currents (FCNCs) are governed by non-invertible selection rules. By implementing such non-group-like fusion rules for matter fields, arising from gauging the outer automorphism $\mathbb{Z}_2$ of a discrete $\mathbb{Z}_N$ symmetry, we obtain realistic Yukawa textures that reproduce the observed quark and lepton masses and mixings while ensuring diagonal soft supersymmetry (SUSY) breaking masses and hence suppressing dangerous FCNC processes. We analyze mass insertion parameters under random $\mathcal{O}(1)$ coefficients and find that all flavor-violating effects are consistent with experimental limits on processes such as $μ\to e γ$ and meson mixings. We show that the Yukawa textures and soft terms remain stable under renormalization group evolution. Our results demonstrate that non-invertible selection rules provide a compelling new mechanism to address both the flavor structure and FCNC problems in supersymmetric models.

hep-ph

Radiative lepton model in a non-invertible fusion rule

We propose a radiatively induced lepton mass model introducing a $Z_2$ gauging $Z_5$ fusion rule. In our framework, the charged-lepton mass matrix is generated at one-loop level via dynamical breaking of the fusion rule. On the other hand, the neutrino mass matrix is induced at the one-loop level without breaking the fusion rule. As a direct consequence of the loop induced charged lepton masses, we can also consider lepton flavor violations, electron and muon $g-2$, and charged-lepton electric dipole moments that come into our valid phenomenological discussion. Then, we perform numerical analysis and show some interesting tendency on the Dirac $CP$ phase, two Majorana phases, charged-lepton electric dipole moments and the neutrinoless double beta decay, all of which depends on their arguments where we fix the absolute values of our free parameters in order to satisfy experimental data of the lepton masses and mixing angles.

hep-ph

A Minimal Realization of Radiative Dirac Neutrino Masses via a Non-Invertible Fusion Rule

We propose a minimal one-loop radiative framework for Dirac neutrino mass matrix. As a consequence, the Yukawa hierarchies among the SM fermions are alleviated, and radiative type-I seesaw framework is realized. To regulate divergent loop contributions, we introduce an effective cutoff scale $Λ\sim 100 \, {\rm TeV}$. By introducing a scalar leptoquark and imposing appropriate assignments of ising fusion rule to the particle content, we successfully realize a minimal construction. Furthermore, the presence of the leptoquark leads to rich phenomenology, including semi-leptonic decays, neutral meson mixing, lepton flavor violations and lepton $g-2$, thereby rendering the model experimentally testable. After formulating each sector of our model, we perform a comprehensive numerical analysis, taking into account all relevant experimental constraints for both normal and inverted hierarchies of neutrino masses. Our analysis reveals characteristic tendencies within the viable parameter space.

hep-ph

Two-loop rainbow neutrino masses in a non-invertible symmetry

We propose two-loop rainbow type of the neutrino mass model via ${Z}_2$ gauging of ${Z}_6$ non-invertible symmetry in which we introduce three families of isospin doublet vector-like fermions, heavy right-handed neutrinos and isospin doublet and singlet bosons. All new fields, which have nonzero charges under the non-invertible symmetry, can be dark matter candidates, since the non-invertible symmetry possesses a remnant ${Z}_2$ symmetry that plays a role in assuring the stability of our dark matter candidate. Even though the non-invertible symmetry is dynamically broken at one-loop level, its violation does not affect our scenario. In this paper, we especially consider the lightest mode of the neutral components in the doublet vector-like fermions as our main dark matter candidate. The dark matter is potentially degenerated to the other two families of neutral fermions, since the mass difference is induced at one-loop level. Thus, we consider our dark matter candidate in rather simpler co-annihilation system among their particles. Considering all the constraints of neutrino oscillation data, lepton flavor violations, muon $g-2$ and the relic density of dark matter, we perform the numerical analysis and show some allowed regions for these phenomenology. Due to our dark matter nature, the sum of neutrino masses in case of normal hierarchy is larger than that in case of inverted hierarchy, which is opposite situation compared with typical active neutrino models.

hep-ph

Models for the Electric Dipole Moment and Anomalous Magnetic Moment of the Tau Lepton

The Belle II experiment and other ongoing and projected lepton facilities are expected to greatly enhance the sensitivity to the electric dipole moment (EDM) and anomalous magnetic moment ($g-2$) for the tau lepton, making it timely to explore models that predict these observables. We present a class of models that generate a sizable EDM and $g-2$ of the tau lepton via radiative tau mass generation. Two benchmark models with different hypercharge assignments are investigated. The first model contains neutral fermions and charged scalars. We find that the model can predict a large signal of the tau EDM, $d_τ = \mathcal{O}(10^{-19}) \, e \, {\rm cm}$, and $g-2$, $a_τ = \mathcal{O}(10^{-5})$, which are within the reach of future updates of their measurements. In contrast, the second model, containing a charged fermion and neutral scalars, yields a similar magnitude for the $g-2$ but predicts a comparatively smaller EDM signal. Our models serve as benchmarks for new physics generating sizable EDM and $g-2$ of the tau lepton.

hep-ph

Three-loop induced neutrino mass model in a non-invertible symmetry

We propose a new type of radiatively induced neutrino masses at three-loop level based on the Ma model, introducing a non-invertible symmetry in the class under a ${\mathbb Z_2}$ gauging of ${\mathbb Z_6}$ symmetry and adding three isospin doublet vector-like fermions $L'$ and singlet boson $S_0$. Under this symmetry, the Yukawa interactions directly related to the neutrino masses are not allowed at tree-level. However it is allowed at one-loop level due to $L'$ and $S_0$ as well as $η$, which is no longer invariant under this symmetry. Therefore, the symmetry is dynamically broken. Intriguingly, $η$ plays important roles in contributing to both the radiative matrices $y^η$ and $m_ν$. After constructing our model, we show some numerical analyses to satisfy the lepton flavor violations, muon anomalous magnetic dipole moment, and a boson dark matter candidate $S_0$ or $η_R$ for the cases of normal hierarchy and inverted hierarchy. Then, we demonstrate allowed space for our input parameters.

hep-ph

Proton Lifetime in Minimal Supersymmetric SU(5) with Gauge Mediation

In this paper, we discuss the predicted proton lifetimes in minimal supersymmetric (SUSY) $SU(5)$ grand unified theory (GUT) with gauge mediated supersymmetry breaking (GMSB). We focus on the case of $\mathbf{5} + \mathbf{\bar{5}}$ messengers and determine the low-scale mass spectrum of the scalar particles and gauginos using the renormalization group equations. With the obtained mass spectrum, we calculate the dominant proton decay mode for SUSY $SU(5)$ GUT, $p \to K^+ \barν$. In our setup, we assume the messenger scale to be $\mathcal{O}(10^3)$ TeV in order to obtain a proper Higgs mass in GMSB scenario. For this messenger scale, we find the proton lifetime is consistent with current experimental limits and can be tested by future proton decay experiments.

hep-ph

Probing CP Violation in Dark Sector through the Electron Electric Dipole Moment

The Two Higgs Doublet Model (2HDM) stands as a promising framework for exploring physics beyond the Standard Model (SM). Within this context, we explore the possibility that the two Higgs doublets may serve as a window into CP-violating dark sectors, neutral under the SM gauge groups. Specifically, our focus is on investigating the electric dipole moment (EDM) of the electron, generated solely by CP violation in the dark sector. We present a general formula for the electron EDM, without specifying the structure of the dark sectors, and discuss the current constraints on various dark sector models. It is noteworthy that even in the case of a CP-conserving 2HDM, the resulting electron EDM is capable of reaching the current experimental limit, with CP violation arising exclusively from the dark sectors. Furthermore, we introduce a heavy dark sector (HDS) approximation for the analytic calculation of the EDM, assuming that the dark sector particles are much heavier than the physical states in the 2HDM. This approximation yields simplified analytic results that are consistent with the full numerical calculations.

hep-ph

Light Dilaton in Rare Meson Decays and Extraction of its CP Property

The dilaton $ϕ$ is a pseudo-Nambu-Goldstone boson associated with the spontaneous breaking of scale invariance in a nearly conformal theory, and couples to the trace of the stress-energy tensor. We analyze experimental constraints on a light dilaton with mass in the MeV-GeV range from rare meson decays. New model-independent inclusive bounds for the $b \to s ϕ$ transition largely exclude the parameter space of a light dilaton that could explain the muon $g-2$ anomaly. Despite similarities between a dilaton and a Higgs-portal scalar, the dilaton-photon coupling is enhanced compared to the Higgs-portal scalar due to contributions from loops of the conformal sector. Consequently, the shortened lifetime of the dilaton relaxes bounds from $K \to π$ + invisible searches at the NA62 experiment and constraints from the Big Bang Nucleosynthesis. We utilize this fact to search for the dilaton signature at a lepton collider such as the ongoing Belle II experiment. Further, we demonstrate how to extract the CP property of the dilaton using the variation of the differential cross-section of $e^+ e^- \to e^+ e^- ϕ$ with the azimuthal angle between the outgoing leptons.

hep-ph

A Large Muon EDM from Dark Matter

We explore a model of dark matter (DM) that can explain the reported discrepancy in the muon anomalous magnetic moment and predict a large electric dipole moment (EDM) of the muon. The model contains a DM fermion and new scalars whose exclusive interactions with the muon radiatively generate the observed muon mass. Constraints from DM direct and indirect detection experiments as well as collider searches are safely evaded. The model parameter space that gives the observed DM abundance and explains the muon $g-2$ anomaly leads to the muon EDM of $d_μ \simeq (4$-$5) \times 10^{-22} \, e \, {\rm cm}$ that can be probed by the projected PSI muEDM experiment. Another viable parameter space even achieves $d_μ = \mathcal{O}(10^{-21}) \, e \, {\rm cm}$ reachable by the ongoing Fermilab Muon $g-2$ experiment and the future J-PARC Muon $g-2$/EDM experiment.

hep-ph

Muon Electric Dipole Moment as a Probe of Flavor-Diagonal CP Violation

Electric dipole moments (EDMs) of elementary particles are powerful probes of new physics with flavor-diagonal CP violation. The reported discrepancy in the muon anomalous magnetic moment motivates us to explore to what extent new physics with flavor-diagonal CP violation to address the discrepancy is probed by searches for the muon EDM. As a benchmark, we focus on a CP-violating two-Higgs-doublet model to explain the muon $g-2$ anomaly where the muon exclusively couples to one Higgs doublet. Since contributions to flavor violating processes as well as the electron EDM are suppressed, the muon EDM becomes an essential probe of the model. Our result shows that some viable parameter space leads to the muon EDM of around $d_μ \simeq 6 \times 10^{-23} \, e \, \rm cm$ probed by the PSI experiment and most of the parameter space is covered by the proposed J-PARC experiment.

hep-ph

Probing an intrinsically flavorful ALP via tau-lepton flavor physics

Any axionlike particle (ALP) intrinsically possesses flavorful couplings to the standard model (SM) fermions arising as a consequence of the right-handed flavor rotation within the SM. In this paper we discuss this intrinsically flavored ALP, and explore the correlation of a minimal set of the couplings in a view of coherence in flavor physics observables. We focus particularly on the tau-lepton flavor violation (LFV). The ALP is assumed to be tau-philic on a current-eigenstate basis, a la Pecci-Quinn. The ALP has the intrinsic flavorful coupling structure for fermions, which allows coupling also to muon and electron only in a right-handed specific manner. Several LFV processes are generated including radiative tau decays and also anomalous magnetic moments of electron and muon. We first pay attention to two separated limits: electron scenario with the ALP coupled to tau which mixes only with right-handed electron, and muon scenario as the muonic counterpart of the electron scenario. It turns out that those scenarios are highly constrained by experimental limits, to require a mu or electron - tau flipped feature in the mass eigenbasis when coupled to the ALP. We then examine a hybrid scenario, and find a fully viable parameter space on the ALP mass-photon coupling plane, which limits the ALP mass to be (1.7 - 10) GeV and the ALP decay constant $f_a$ to be (12.8 - 67.9) GeV. We find that the same-sign multilepton signal at Belle II is a smoking-gun to probe the present ALP signal, and the polarization asymmetry in LFV radiative $τ$ decay is a punchline, which definitely predicts preference of the right-handed polarization, in sharp contrast to the SM plus massive Dirac neutrinos having the highly left-handed preference, and also other light-new physics candidates. Possible model-building to underlie the present third-generation specific ALP is also briefly addressed.

hep-ph

Higgs flavor phenomenology in a supersymmetric left-right model with parity

In this paper, we focus on the supersymmetric model with left-right (LR) symmetry, that is especially proposed in our previous work [1]. In this model, there are four Higgs doublets in order to realize the Standard Model (SM) fermion masses and the Cabibbo-Kobayashi-Maskawa matrix. The heavy Higgs doublets unavoidably have flavor changing couplings to the SM fermions and induce flavor-changing neutral currents at tree level. We study broader parameter space than the previous work with including the renormalization group corrections to the Yukawa couplings between the LR breaking scale, $\mathcal{O}(10^{13})$ GeV, and the supersymmetry breaking scales, $\mathcal{O}(100)$ TeV. The CP violating observable in $K$-$\overline{K}$ mixing, $ε_K$, strongly constrains the model, so that heavy Higgs mass should be heavier than $\mathcal{O}(100)$ TeV. We study the lepton flavor violating (LFV) processes setting heavy Higgs masses to be 170 TeV. The branching ratios of $μ\to 3 e$ and the $μ$-$e$ conversion can be larger than $10^{-16}$ that could be covered by the future experiments. We also study the degree of fine-tuning in the parameter region that predicts testable LFV processes.

hep-ph

New perspective in searching for axion-like particles from flavor physics

We propose new perspective in searching for axion-like particles (ALPs) from quark and lepton flavor physics: measurements of the time-dependent CP asymmetry in $B^0 \to K_S^0 π^0 γ$ and the branching ratio of $B_s \to e^\pm μ^\mp$ decay possess, along with the anomalous magnetic moment of muon. In the mass range of sub-GeV, accessible by the flavorful ALPs search, the experimental sensitivity for these flavor observables reaches the maximum at around the pion mass scale (called the {\it sweetest} spots), where a couple of loopholes (unexplored regions) for the ALP parameter space have heretofore been present, because of an unavoidable contamination with pion background events. The proposed complementary probes can precisely determine the ALP coupling to photon at these {\it sweetest} spots/loopholes, and will significantly help cover whole parameter spaces in the ALP search including the present loopholes in the future.

hep-ph

$(g-2)_μ$ Versus $K \to π+E_{miss}$ Induced by the $(B-L)_{23}$ Boson

To address the long-standing $(g-2)_μ$ anomaly via a light boson, in Ref. [1] we proposed to extend the standard model (SM) by the local $(B-L)_{23}$, under which only the second and third generations of fermions are charged. It predicts an invisible $Z'$ with mass ${\cal O}(100)$ MeV, and moreover it has flavor-changing neutral current (FCNC) couplings to the up-type quarks at tree level. Such a $Z'$, via $K_L \to π^0 + Z'(\to ν\barν)$ at loop level, may be a natural candidate to account for the recent KOTO anomaly. In this article, we investigate this possibility, to find that $Z'$ can readily do this job if it is no longer responsible for the $(g-2)_μ$ anomaly. We further find that both anomalies can be explained with moderate tuning of the CP violation, but may contradict the $B$ meson decays.

hep-ph

Signatures of a Flavor Changing $Z'$ Boson in $B_q \to γZ'$

Rare $B$ meson decays offer an opportunity to probe a light hidden $Z'$ boson. In this work we explore a new channel $B_q \to γZ'$ ($q = d, s$) followed by a cascade decay of $Z'$ into an invisible (neutrino or dark matter) or charged lepton pair $\ell^+ \ell^-$ ($\ell=e ,μ)$. The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with $Z'$. For the first time, we calculate ${\rm BR}(B_q \to γZ')$ at the leading power of $1/m_b$ and $1/E_γ$. Confronting with the strong constraints from semi-invisible decays of $B$ meson, we find that the branching ratio for $B_d \to {\rm invisible} + γ$ can be larger than its Standard Model prediction, leaving a large room for new physics, in particular for light dark matter. Additionally, the branching ratio for $B_d \to e^+ e^- γ$ can also be sizable when the corresponding flavor violating $Z'$ coupling to quarks is of the axial-vector type. On the other hand, the predicted branching ratios of $B_d \to μ^+ μ^- γ$ and $B_s \to \ell^+ \ell^- γ$ are severely constrained by the experimental measurements.

hep-ph