arXiv subjects

Takashi Shimomura

Publications and source records attributed to Takashi Shimomura.

At least 19 recordsLinked to original sources

arXiv2026

Probing CP violation in dark scalar decays at the SHiP experiment

We investigate CP violation in the decays of a GeV-scale dark scalar boson into mesons, muons and photons. Both scalar and pseudoscalar couplings to the SM fermions are considered to study how the CP structure of the dark scalar boson can be probed through its decays. The scalar and pseudoscalar interactions predominantly induce decays into two-meson and three-meson final states, respectively. We propose the simultaneous observation of these two classes of decay modes as a probe of CP violation in the dark scalar sector. The expected signal rates at the SHiP and FASER2 experiments are evaluated based on the dark scalar production from heavy-meson decays. We find that the ratio of three-meson to two-meson decay events at SHiP has the potential to probe a wide range of the CP-violating parameter space for dark scalar masses around the GeV scale. We also show that the two-muon and di-photon channels can provide complementary information for the CP violation search.

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arXiv2026

Dark photon and U(1)$_{B-L}$ gauge boson from dark Higgs boson decays at FASER and SHiP

We study the sensitivity to dark photons and U(1)$_{B-L}$ gauge bosons produced via dark Higgs boson decays at the FASER and SHiP experiments. In addition to pair production of these vector bosons from both on-shell and off-shell dark Higgs boson decays, a new production process of single vector boson associated with the standard model particles is taken into account. Constraints on the parameter space of dark photon are derived with including the latest results from the FASER experiment. The expected sensitivity regions to the dark photon and U(1)$_{B-L}$ gauge boson of the future FASER2 and SHiP experiments are presented. The sensitivity to the U(1)$_{B-L}$ model with freeze-in sterile neutrino dark matter is also discussed.

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arXiv2025

New rare meson decay constraints on a light vector in $U(1)_{B-L}, U(1)_R$ and the dark photon

We evaluate constraints from flavor changing rare meson decays to a light vector boson $X$, followed by the decay of the on-shell $X$ into the SM fermions. The flavor changing meson decay emitting the light $X$ is induced by loop processes where the up-type quarks, the $W$ boson, or charged scalar bosons are running inside loops. We calculate all one-loop diagrams with neglecting all masses of light quarks except for the top quark in a general anomaly free extra $U(1)$ model. Our theoretical evaluation of the branching ratio of charged $B$ meson decay and charged kaon decay is compared to experimental results, and we derive new constraints for dark photon, $U(1)_{B-L}$ and $U(1)_R$ models.

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arXiv2025

Freeze-in sterile neutrino dark matter in a feebly gauged $B-L$ model

We consider the gauged $U(1)_{B-L}$ model and examine the situation where the sterile neutrino is a dark matter candidate produced by the freeze-in mechanism. In our model, the dark matter $N$ is mainly produced by the decay of a $U(1)_{B-L}$ breaking scalar boson $ϕ$. We point out that the on-shell production of $ϕ$ through annihilation of the $U(1)_{B-L}$ gauge boson $Z'$ plays an important role. We find that the single production of $Z'$ from the gluon bath in the early Universe can become the main production modes for $Z'$ in some parameter regions. To prevent $N$ from being overproduced, we show that the $U(1)_{B-L}$ gauge coupling constant $g_{B-L}$ must be as small as $10^{-16}$--$10^{-10}$. We also consider the case where the decay of $ϕ$ into $N$ is kinematically forbidden. In this case, $N$ is generated by the scattering of $Z'$ and the $g_{B-L}$ takes values of $10^{-10}$--$10^{-6}$, which can be explored in collider experiments like FASER and SHiP.

hep-ph↗

arXiv2025

Dark photon pair production via off-shell dark Higgs at FASER

We consider a dark photon model in which the dark U(1) gauge symmetry is spontaneously broken by a vacuum expectation value of a new scalar boson. We focus on the ForwArd Search ExpeRiment (FASER) and calculate its sensitivity to the dark photon produced from the off-shell decay of the new scalar boson. It is found that the off-shell production extends the sensitivity region beyond the kinematical threshold of the on-shell decay of the scalar boson, and that the sensitivity region can be spanned to unexplored region. We also show the parameter space in which perturbative calculation is valid for the unitarity of an S matrix.

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arXiv2023

Refinement of Bratteli-Vershik models

In the zero-dimensional systems, the Bratteli-Vershik models can be built upon certain closed sets that are called `quasi-sections' in this article. There exists a bijective correspondence between the topological conjugacy classes of triples of zero-dimensional systems and quasi-sections and the topological conjugacy classes of Bratteli-Vershik models. Therefore, we can get refined Bratteli-Vershik models if we get certain refined quasi-sections. The basic sets are such refined quasi-sections that bring `closing property' on the corresponding Bratteli-Vershik models. We show a direct proof on the existence of basic sets. Thorough investigations on quasi-sections and basic sets are done. Furthermore, it would be convenient for the Bratteli-Vershik models to concern minimal sets. To this point, we show the existence of the Bratteli-Vershik models whose minimal sets are properly ordered. On the other hand, we can get certain refinements with respect to the Bratteli-Vershikizability condition or the decisiveness.

math.DS↗

arXiv2023

New Constraint on Dark Photon at T2K Off-Axis Near Detector

The T2K experiment is one of the most powerful long-baseline experiments to investigate neutrino oscillations. The off-axis near detector called ND280 is installed 280 m downstream from the neutrino production target to measure the neutrino energy spectrum. In this paper, we study the capability of the ND280 detector to search for the dark photon produced through the meson rare decay and proton bremsstrahlung processes at the proton beam dump. We find that the ten-year operation of T2K with the ND280 detector excludes the unexplored parameter region for the dark photon mass and kinetic mixing. We also show that a broader parameter region can be searched by the ND280 in the future T2K operation for dark photon as well as U(1)$_{B-L}$ gauge boson.

hep-ph↗

arXiv2023

Neutrinophilic DM annihilation in a model with $U(1)_{L_μ-L_τ} \times U(1)_{H}$ gauge symmetry

We propose a model with two different extra $U(1)$ gauge symmetries; muon minus tauon symmetry $U(1)_{{L_μ}-L_τ}$ and hidden symmetry $U(1)_H$. Then, we explain muon anomalous magnetic moment, semi-leptonic decays $b\to s\ell\bar\ell$, and dark matter. In particular, we find an intriguing dark matter candidate to be verified by Hyper-Kamiokande and JUNO in the future that request neutrinophilic DM with rather light dark matter mass$\sim{\cal O}(10)$ MeV.

hep-ph↗

arXiv2023

Search for Lepton Flavor Violating Decay at FASER

FASER is one of the promising experiments which search for long-lived particles beyond the Standard Model. In this paper, we consider charged lepton flavor violation (CLFV) via a light and weakly interacting boson and discuss the detectability by FASER. We focus on four types of CLFV interactions, i.e., the scalar-, pseudoscalar-, vector-, and dipole-type interaction, and calculate the sensitivity of FASER to each CLFV interaction. We show that, with the setup of FASER2, a wide region of the parameter space can be explored. Particularly, it is found that FASER2 has a sensitivity to very small coupling regions in which the rare muon decays, such as $μ\rightarrow eγ$, cannot place bounds, and that there is a possibility to detect CLFV decays of the new light bosons.

hep-ph↗

arXiv2023

Multi muon/anti-muon signals via productions of gauge and scalar bosons in a $U(1)_{L_μ-L_τ}$ model at muonic colliders

We discuss the discovery potential of a promising signals, $3 \times μ^+ μ^-$ at a $μ^+ μ^-$ collider and $μ^+ μ^+ μ^+ μ^+ μ^- μ^- $ at a $μ^+ μ^+$ collider, that are obtained via production of $Z'$ and a new scalar boson $ϕ$ in a spontaneously broken local $U(1)_{L_μ- L_τ}$ model. We consider the $Z'$ associated production from the process $μ^+ μ^- \to ϕZ'$ in addition to a muonphilic $Z'$ fusion process $μ^+ μ^\pm \to ϕμ^+ μ^\pm$. The scalar boson is associated with $U(1)_{L_μ- L_τ}$ symmetry breaking and dominantly decays into $Z' Z'$ mode. We carry out numerical simulation analysis for signal and background processes to estimate a discovery significance for different benchmark points. It is shown that our signal can be observed with integrated luminosity less than $\mathcal{O}(100)$ fb$^{-1}$ for both $μ^+ μ^-$ and $μ^+ μ^+$ colliders with more than 5-$σ$ significance.

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arXiv2022

Pseudo-Nambu-Goldstone Dark Matter from Non-Abelian Gauge Symmetry

We propose a pseudo-Nambu-Goldstone boson (pNGB) dark matter (DM) model based on an additional non-Abelian gauge symmetry $SU(2)_D$. The gauge symmetry $SU(2)_D$ is spontaneously broken to a global custodial symmetry $U(1)_V$ via the nonvanishing vacuum expectation values of $SU(2)_D$ doublet and triplet scalar fields. Due to the exact global symmetry $U(1)_V$, the lightest $U(1)_V$ charged particle becomes stable. We assume that the lightest charged particle in the model is the charged complex pNGB, which we regard as DM. It avoids the strong constraints from current DM direct detection experiments due to the property of NGB. We find that the measured energy density of DM can be reproduced when the DM mass is larger than the half of the Higgs mass, where the lower limit generally comes from the constraint of DM invisible decay and the upper limit from DM direct detection experiments depends on the model parameters.

hep-ph↗

arXiv2022

Dark Photon from Light Scalar Boson Decays at FASER

FASER is one of the promising experiments which search for long-lived particles beyond the Standard Model. In this paper, we focus on dark photon associating with an additional U(1) gauge symmetry, and also a scalar boson breaking this U(1) gauge symmetry. We study the sensitivity to the dark photon originated from U(1)-breaking scalar decays. We find that a sizable number of dark photon signatures can be expected in wider parameter space than previous studies.

hep-ph↗

arXiv2022

Revisiting sterile neutrino dark matter in gauged $U(1)_{B-L}$ model

We reexamine sterile neutrino dark matter in gauged $U(1)_{B-L}$ model. Improvements have been made by tracing and careful evaluation of the evolution of the number densities of sterile neutrinos $N$ and extra neutral gauge bosons $Z'$. As a result, the cosmologically-interesting gauge coupling of $U(1)_{B-L}$ for freeze-in sterile neutrinos turns out to be smaller than the values reported in the literature. This avoids the overproduction of $Z'$ so that it is consistent with the big bang nucleosynthesis and the cosmic microwave background constraints on the effective number of neutrino species. Similarly, the free-streaming length constraints exclude a large parameter space derived in previous studies. In addition to known freeze-in pair production of $N$ from the standard model fermion pairs, we find the case that $N$ is dominantly produced from a pair of $Z'$ at the temperature characterized by the $B-L$ breaking scalar mass. Thus, the naive truncation of the $U(1)_{B-L}$ scalar contribution made in the literature is not valid.

hep-ph↗

arXiv2022

On some properties of basic sets

In the theory of zero-dimensional systems and their relation to $C^*$-algebras, Poon (1990) introduced a class of closed sets. We call the closed sets quasi-sections. Medynets (2006) introduced basic sets that are part of quasi-sections in his study of aperiodic zero-dimensional systems and their relation to Bratteli--Vershik models and $C^*$-algebras. Downarowicz and Karpel (2019) introduced the notion of decisiveness in the theory of Bratteli--Vershik models. We previously clarified that particular quasi-sections can be the "bases" of the decisive Bratteli--Vershik models for zero-dimensional systems with dense aperiodic orbits. We call them continuously decisive quasi-sections. However, even the basic topological properties of quasi-sections and the basic sets have not been studied systematically. This paper presents such a systematic study. Some properties are defined, stated, and proved in the general settings of compact Hausdorff topological dynamics. For example, if a topological dynamical system has dense aperiodic orbits and no wandering points, then every basic set is continuously decisive. If a zero-dimensional system has dense aperiodic orbits, then there exists a minimal continuously decisive basic set such that for every minimal set, there exists a unique common point.

math.DS↗

arXiv2021

Inelastic Dark Matter from Dark Higgs Boson Decays at FASER

We consider inelastic dark matter scenarios with dark photon mediator and a dark Higgs boson. The dark Higgs boson spontaneously breaks the gauge symmetry associated with the dark photon, and gives the mass to the dark photon and the mass difference to dark particles. Such a dark Higgs boson can decay into the dark particles and hence can be another source of the dark particles at collider experiments. We analyze the sensitivity to decays of the excited state into the dark matter and charged particles at the FASER 2 experiment in fermion and scalar inelastic dark matter scenarios. We consider two mass spectra as illustrating examples in which the excited state can be produced only through the decay of dark Higgs boson. We show that unprobed parameter region can be explored in fermion dark matter scenario for the illustrating mass spectra.

hep-ph↗

arXiv2021

Resolving the Hubble tension in a U(1)$_{L_μ-L_τ}$ model with Majoron

In this paper, we explore possibilities of resolving the Hubble tension and $(g-2)_μ$ anomaly simultaneously in a U(1)$_{L_μ- L_τ}$ model with Majoron. We only focus on a case where the Majoron $ϕ$ does not exist at the beginning of the universe and is created by neutrino inverse decay $νν\to ϕ$ after electron-positron annihilation. In this case, contributions of the new gauge boson $Z'$ and Majoron $ϕ$ to the effective number of neutrino species $N_{\rm eff}$ can be calculated in separate periods. These contribution are labelled $N'_{\rm eff}$ for the U(1)$_{L_μ- L_τ}$ gauge boson and $ΔN_{\rm eff}^\prime$ for the Majoron. The effective number $N_{\rm eff} = N'_{\rm eff} + ΔN_{\rm eff}^\prime$ is evaluated by the evolution equations of the temperatures and the chemical potentials of light particles in each period. As a result, we found that the heavier $Z'$ mass $m_{Z^\prime}$ results in the smaller $N_{\mathrm{eff}}^\prime$ and requires the larger $ΔN_{\mathrm{eff}}^\prime$ to resolve the Hubble tension. Therefore, compared to previous studies, the parameter region where the Hubble tension can be resolved is slightly shifted toward the larger value of $m_{Z^\prime}$.

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arXiv2021

Electron Beam Dump Constraints on Light Bosons with Lepton Flavor Violating Couplings

We study constraints on light and feebly interacting bosons with lepton flavor violation from electron beam dump experiments. Scalar, vector, and dipole interactions of the bosons are analyzed, respectively, and excluded regions from the searches for decays into electron-positron pairs are derived. It is found that parameter regions unconstrained by flavor violating decays of muon can be excluded using the results of the E137 experiment. We also discuss the impact of the search for flavor violating decays of the light bosons in electron beam dump experiments.

hep-ph↗

arXiv2021

Soft supersymmetry breaking terms and lepton flavor violations in modular flavor models

We study the soft supersymmetry (SUSY) breaking terms due to the modulus F-term in the modular flavor models of leptons. It is found that the soft SUSY breaking terms are constrained by the modular forms, and specific patterns are derived. Those phenomenological implications are discussed in such as the lepton flavor violation, $μ\rightarrow e + γ$ decay. In order to examine numerically, two modular flavor $A_4$ models are taken. The SUSY breaking scale is significantly constrained by inputting the observed upper bound of the $μ\rightarrow e + γ$ decay. The SUSY mass scale is larger than around $8$ TeV and $5$ TeV for the two $A_4$ models, respectively. Therefore, the current experimental upper bound for the $μ\to e + γ$ decay corresponds to the new physics of the SUSY particle at the $5$ -- $10$ TeV scale in the modular flavor models. The predicted branching ratio depends on a modulus $τ$ significantly. It decreases of one order at the large ${\rm Im}\,τ$. We also calculate the branching ratios of tauon decays to $e + γ$ and $μ+ γ$. Predicted ones are at most ${\cal O}(10^{-15})$, which are much below the current experimental bounds.

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