arXiv subjects

Junichiro Kawamura

Publications and source records attributed to Junichiro Kawamura.

At least 19 recordsLinked to original sources

arXiv2026

Taming lepton portal dark matter by a non-invertible selection rule

We point out that a non-invertible selection rule can control the flavor violation inherent in lepton portal dark matter, a weakly interacting massive particle candidate that can evade severe constraints from direct detection experiments. We show that ordinary invertible symmetries cannot suppress flavor-violating portal couplings while accommodating the large mixing angles in the PMNS matrix. We then present a simple construction based on the $\mathbb{Z}_2$ gauging of a $\mathbb{Z}_5$ symmetry, which realizes one-flavor dominance of the portal couplings consistently with the PMNS mixing matrix. We also discuss possible flavor leakage in a type-I seesaw completion and study experimental probes of the model through lepton observables, especially lepton flavor non-universality in $Z$ boson decays.

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arXiv2026

Finite modular Coleman-Weinberg inflation

We propose a modular symmetric inflationary model based on a Coleman--Weinberg potential generated by integrating out heavy vector-like quarks that couple to the complex modulus field $τ$ through modular forms. In this framework, the imaginary part of modulus $τ$ plays the role of the inflaton, while the real part is identified with a heavy axion. We show that the model successfully explains the current cosmological observations. We further discuss reheating through modulus-dependent gauge kinetic functions and the cosmology of the axion. The axion oscillation dominates over the Universe after the reheating via inflaton decay, and then it decays before Big Bang Nucleosynthesis in the viable parameter region. The quantum fluctuation of the axion can be of order $\mathcal{O}(1)\% $ of that of the inflaton, which would induce isocurvature perturbations that may be detectable in future observations.

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arXiv2026

Chiral enhancement in the vector-like fourth family: Case of $b \to s γ$

We demonstrate that a vector-like fourth family of quarks induces a genuine chiral enhancement in $b\to sγ$, which is absent in the Standard Model (SM). The coexistence of doublet and singlet states allows the chirality flip to occur inside the loop, leading to contributions proportional to the heavy vector-like mass. The resulting amplitude is enhanced by a factor $\overlineλ_d v_H/m_b$, which can be as large as $\mathcal{O}(40)$ for moderate Yukawa couplings. This leads to sizable deviation from the SM prediction even for $\mathcal{O}(\mathrm{TeV})$ vector-like quark masses and small mixing angles. We find that $\mathrm{Br}(\overline{B}\to X_sγ)$ provides the most stringent constraint on this scenario among a wide range of precision observables.

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arXiv2024

Large and small hierarchies from finite modular symmetries

We study the moduli stabilization by the radiative corrections due to the moduli dependent vector-like masses invariant under the finite modular symmetry. The radiative stabilization mechanism can stabilize the modulus $τ$ of the finite modular symmetry $Γ_N$ ($N \in \mathbb{N}$) at $\mathrm{Im}\,τ\gg 1$, where the shift symmetry $τ\to τ+1$ remains unbroken approximately. The shift symmetry can be considered as the residual $\mathbb{Z}_N$ symmetry which realizes the Froggatt-Nielsen mechanism with the hierarchy parameter $e^{- 2π\mathrm{Im}\,τ/N} \ll 1$. In this work, we study the stabilization of multiple moduli fields, so that various hierarchical values of the modular forms coexist in a model. For example, one modulus stabilized at $\mathrm{Im}\,τ_1 \sim 3$ is responsible for the hierarchical structure of the quarks and leptons in the Standard Model, and another modulus stabilized at $\mathrm{Im}\,τ_2 \sim 15$ can account for the flatness of the $\mathrm{Re}\,τ_2$ direction which may be identified as the QCD axion.

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arXiv2024

Moduli stabilization and light axion by Siegel modular forms

We discuss the stabilization of multiple moduli by utilizing Siegel modular forms in the framework of $Sp(2g,\mathbb{Z})$ modular invariant theories. We derive the stationary conditions at CP-conserving fixed points for a generic modular- and CP-invariant scalar potential. The stabilization of multiple moduli is explicitly demonstrated in $Sp(4,\mathbb{Z})$ and $Sp(6,\mathbb{Z})$ modular invariant scalar potentials. Furthermore, it turns out that there exists a light axion when the moduli are stabilized nearby a fixed point.

hep-th↗

arXiv2024

Finite modular axion and radiative moduli stabilization

We propose a simple setup which can stabilize a modulus field of the finite modular symmetry by the Coleman-Weinberg potential. Our scenario leads to a large hierarchy suppressing instanton-like corrections $e^{2πiτ}$ and to a light axion identified as $\mathrm{Re} τ$, where $τ$ is the modulus field. This stabilization mechanism provides the axion solution to the strong CP problem. The potential has a minimum at a large $\mathrm{Im}τ$ which suppresses explicit $U(1)_{\mathrm{PQ}}$ violation terms proportional to $e^{-2π{\mathrm{Im}τ}}$, and hence the quality of the axion is ensured by the residual symmetry associated with the $T$-transformation, $τ\to τ+1$, around the fixed point $τ\sim i\infty$.

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arXiv2024

Finite modular majoron

We point out that the accidental $U(1)_{B-L}$ symmetry can arise from a finite modular symmetry $Γ_N$ in the type-I seesaw. The finite modular symmetry is spontaneously broken in such a way that the residual $\mathbb{Z}^T_N$ discrete symmetry, associated with the $T$-transformation which shifts the modulus $τ\to τ+ 1$, remains unbroken. This discrete $\mathbb{Z}^T_N$ symmetry mimics $U(1)_{B-L}$, and hence the majoron appears as a pseudo Nambu-Goldstone boson of $U(1)_{B-L}$. Without introducing additional interactions, the modulus $τ$ can be stabilized by the Coleman-Weinberg (CW) potential given by the Majorana mass terms of the right-handed neutrinos. We study cosmological implications of the majoron, with particular interests in the dark matter and dark radiation, where the latter may alleviate the Hubble tension. We also find that the CW potential can have a wide range of nearly exponential shape which prevents $τ$ from overshooting, and makes the amount of dark radiation not too large.

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arXiv2024

Semi-visible dark photon in a model with vector-like leptons for the $(g-2)_{e,μ}$ and $W$-boson mass anomalies

We propose a model realizes that a semi-visible dark photon which can contribute to the anomalous magnetic moment ($g-2$) of both electron and muon. In this model, the electron $g-2$ is deviated from the Standard Model (SM) prediction by the 1-loop diagrams involving the vector-like leptons, while that of muon is deviated due to a non-vanishing gauge kinetic mixing with photons. We also argue that the $W$-boson mass can be deviated from the SM prediction due to the vector-like lepton loops, so that the value obtained by the CDF II experiment can be explained. Thus, this model simultaneously explains the recent three anomalies in $g-2$ of electron and muon as well as the $W$-boson mass. The constraints on the $\mathcal{O}(1)~\mathrm{GeV}$ dark photon can be avoided because of the semi-invisible decay of the dark photon, $A^\prime \to 2 N \to 2ν\,2χ\to 2ν\,4e$, where $N$ is a SM singlet vector-like neutrino and $χ$ is a CP-even Higgs boson of the $U(1)^\prime$ gauge symmetry.

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arXiv2024

Taking aim at the wino-higgsino plane with the LHC

In this work we explore multiple search strategies for higgsinos and mixed higgsino-wino states in the MSSM and project the results onto the $(μ,M_2)$ plane. Assuming associated production of higgsino-like pairs with a $W/Z$ boson, we develop a search in a channel characterized by a hadronically tagged vector boson accompanied by missing energy. We use as our template an ATLAS search for dark matter produced in association with a hadronically decaying vector boson, but upgrade the search by implementing a joint likelihood analysis, binning the missing transverse energy distribution, which greatly improves the search sensitivity. For higgsino-like states (more than 96% admixture) we find sensitivity to masses up to 550 GeV. For well-mixed higgsino-wino states (70-30% higgsino) we still find sensitivities above 300 GeV. Using this newly proposed search, we draw a phenomenological map of the wino-higgsino parameter space, recasting several complementary searches for disappearing tracks, soft leptons, trileptons, and hadronic diboson events in order to predict LHC coverage of the $(μ,M_2)$ mass plane at integrated luminosities of up to $3\,\text{ab}^{-1}$. Altogether, the full run of the HL-LHC can exclude much of the "natural" ($μ,M_2 <$ 500 GeV) wino-higgsino parameter space.

hep-ph↗

arXiv2023

Current status on pair-produced muon-philic vectorlike leptons in multilepton channels at the LHC

In this work, we obtain the current limits on the pair production of vectorlike leptons decaying to a Standard Model gauge boson and a lepton in the second generation using the Run-2 data at the LHC. Since there is no dedicated search, we recast the ATLAS analyses searching for the type-III seesaw heavy leptons in the multi-lepton channels. There is no limit for the $SU(2)_L$ singlet vectorlike lepton beyond about 100 GeV, while the limit is about 780 GeV for the doublet one. Thus, dedicated searches for the vectorlike leptons are necessary, especially for the singlet one. We also study the general cases of the vectorlike lepton decays and future sensitivities at the HL-LHC.

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arXiv2023

Fermion Hierarchies in $SU(5)$ Grand Unification from $Γ_6^\prime$ Modular Flavor Symmetry

We construct a model in which the hierarchies of the quark and lepton masses and mixing are explained by the $Γ_6^\prime$ modular flavor symmetry. The hierarchies are realized by the Froggatt-Nielsen-like mechanism due to the residual $Z^T_6$ symmetry, approximately unbroken at $τ\sim i\infty.$ We argue that the $Γ_6^{(\prime)}$ symmetry is the minimal possibility to realize the up-type quark mass hierarchies, since the Yukawa matrix is symmetric. We find a combination of the representations and modular weights and then show numerical values of $\mathcal{O}(1)$ coefficients for the realistic fermion hierarchies.

hep-ph↗

arXiv2023

Neutrinos in Global SU(5) F-theory Model

In this talk, given at Corfu 2022 Workshop on the Standard Model and Beyond, I present work in collaboration with Junichiro Kawamura,Ref.~[arXiv:2212.00840]. The talk is also based on a number of papers on a Global $SU(5)$ F-theory GUT in collaboration with Herb Clemens. In the model $SU(5)$ is broken to the MSSM via a Wilson line. This is accomplished (without problems with vector-like exotics) by simultaneously describing the F-theory model and its Heterotic dual. The model has a twin MSSM sector and it's the neutrino sector of the field I consider in the talk.

hep-ph↗

arXiv2023

Quark and lepton hierarchies from $S_4^\prime$ modular flavor symmetry

We propose models in which the hierarchical structures of the masses and mixing in both quark and lepton sectors are explained by the $S_4^\prime$ modular flavor symmetry near the fixed point $τ\sim i\infty$. The model provides the first explicit example which explains hierarchies of both quarks and leptons. The hierarchies are realized by powers of $ε= e^{2πi τ/4} = \mathcal{O}(0.01)$ and $2\,\mathrm{Im}\,τ\sim 5$, where $τ$ being the modulus. The small parameter $ε$ plays a role of flavon in the Froggatt-Nielsen mechanism under the residual $Z_4^T$ symmetry, and powers of $2\,\mathrm{Im}\,τ$ in the Yukawa couplings are controlled by modular weights via the canonical normalization. The doublet quarks are identified to a $S_4^\prime$ triplet to explain the hierarchical structure of the quark mixing angles, while the doublet leptons are composed of three singlets for the large mixing angles in the lepton sector. We show that the $S_4^\prime$ modular symmetry alone can explain the hierarchies in both quark and lepton sectors by $\mathcal{O}(1)$ coefficients.

hep-ph↗

arXiv2023

Quark masses and CKM hierarchies from $S_4^\prime$ modular flavor symmetry

We propose models to explain the hierarchies of the quark masses and mixing by utilizing the $S_4^\prime$ modular flavor symmetry. The hierarchy is realized by the modulus $τ$ stabilized at $\mathrm{Im}\,τ\gg 1$, where the residual $Z_4^T$ symmetry is approximately unbroken and the Froggatt-Nielsen mechanism works. It is found that the quark hierarchies are realized only in a few cases of quark representations. We study two models with assigning the modular weights, so that the observed quark hierarchies are explained in the cases of both small and large ratios of the top to bottom Yukawa couplings. We also argue that $\mathcal{O}({0.1})$ hierarchies of the $\mathcal{O}({1})$ coefficients can be explained by imposing another $S_3$ modular symmetry.

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arXiv2022

A Right-handed Neutrino Portal to the Hidden sector : Active Neutrinos and their Twins in an F-theory model

We analyze the neutrino phenomenology in an $SU(5)$ F-theory model with both a visible sector and a twin hidden sector. At low energies, the strong and weak scales of the two sectors may differ but the spectrum of states is described by the MSSM (MSSM$^\prime$) in the visible (twin) sectors. What is special about the model is that there are right-handed neutrinos which couple to both sectors via Yukawa couplings. As a result, assuming 3 right-handed neutrinos with a large mass much greater than the weak scale, at tree-level the seesaw mechanism results in 3 massive Majorana neutrinos and 3 massless ones. The massless neutrinos acquire mass via radiative corrections. In our analysis, the massless neutrinos are predominantly active neutrinos, while the massive neutrinos are predominantly sterile neutrinos. We fit the active neutrino masses and mixing angles and discuss the phenomenology of the lightest sterile neutrino. Finally we consider some possible scenarios for cosmology.

hep-ph↗

arXiv2022

$W$ mass in a model with vector-like leptons and $U(1)^\prime$

We study the effects of vector-like leptons on the $W$ boson mass in a model with a vector-like $U(1)^\prime$ gauge symmetry. This model provides simultaneous explanations for the recent anomalies in the muon anomalous magnetic moment and the semi-leptonic decays of $B$ mesons. We found that the recent result of the $W$ boson mass precise measurement at CDF can be explained if the charged (neutral) vector-like lepton is lighter than 250 (80) GeV. The light vector-like leptons may not be excluded by collider experiments if these decay to a physical mode of the $U(1)^\prime$ breaking scalar field.

hep-ph↗

arXiv2022

Importance of vector leptoquark-scalar box diagrams in Pati-Salam unification with vector-like families

We study lepton flavor violating meson decays induced by box diagrams involving a vector leptoquark (LQ) and scalar fields in Pati-Salam (PS) unification with vector-like families. The vector LQ corresponds to the massive gauge boson associated with the PS gauge symmetry breaking and the scalar fields are the physical degrees of freedom of the PS breaking field. The LQ generally causes the rapid flavor violating decays, such as $K_L \to μe$, at the tree-level unless its mass scale is higher than PeV scale. The vector-like families are introduced to suppress the tree-level contributions mediated by the LQ and explain the realistic fermion mass matrices. In this paper, we point out that there are inevitable one-loop contributions to those meson decays from the box diagrams mediated by both one LQ and one scalar field, even if the tree-level contributions are suppressed. We consider a concrete model for demonstration, and find that the vector-like fermion masses have an upper bound for a given LQ mass when the one-loop induced meson decays are consistent with the experimental limits. The vector-like fermion mass should be lighter than 3 TeV for 20 TeV LQ, if a combination of the couplings does not suppress $K_L \to μe$ decay. Our findings would illustrate importance of the box diagrams involving both LQ and physical modes of symmetry breaking scalars in generic models.

hep-ph↗

arXiv2022

Leptonic cascade decays of a heavy Higgs boson through vectorlike leptons at the LHC

We demonstrate the potential of fully leptonic cascade decays of a heavy neutral Higgs boson through vectorlike leptons as a simultaneous probe for extended Higgs sectors and extra matter particles at the LHC. The processes we explore are unique in that their event topologies lead to di-boson-like leptonic final states with a lepton pair which does not reconstruct the mass of a gauge boson. By recasting existing $2\ell + E_T^{\rm miss}$ and $3/4\ell$ searches channels using run2 data from the LHC we obtain $\textit{model independent}$ bounds on the masses of heavy scalars and vectorlike leptons and use these results to explore future prospects at the HL-LHC. Our results can be directly applied to any kind of new physics scenarios sharing the final states and the event topology. For concreteness, we apply our results to a benchmark scenario: a two Higgs doublet model type-II augmented with vectorlike leptons. Remarkably, even with current data the sensitivity of our analysis shows a reach for masses of a heavy neutral Higgs and vectorlike leptons up to 2 TeV and 1.5 TeV, respectively. Even for low $\tanβ\gtrsim 1,$ the analysis retains sensitivity to heavy Higgs masses slightly above 1 TeV. The future sensitivities at the HL-LHC extend the reach for heavy Higgses and new leptons to 2.7 TeV and 2 TeV, respectively.

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