arXiv subjects

Masato Yamanaka

Publications and source records attributed to Masato Yamanaka.

At least 19 recordsLinked to original sources

arXiv2025

Cosmological Bounds on Scotogenic Model with Asymmetric Mediator

We study cosmological constraints on the asymmetric mediator scenario, a variant of the scotogenic model that addresses the origins of neutrino masses, dark matter (DM), and the baryon asymmetry. An SU(2)$_L$ doublet scalar $\eta$ mediates between the visible and dark sectors, while a singlet scalar $\sigma$ serves as the DM candidate. We evaluate the DM relic abundance by solving the Boltzmann equations including $\eta$ decay and scattering processes prior to the freeze-out of the $\eta$ asymmetry, and show Big Bang nucleosynthesis constraints from late-time $\eta$ decays. Combining the DM abundance and BBN bounds, we find the favored parameter space of this model, for instance, the mediator masses of $m_\eta \lesssim \mathcal{O}(10)$ TeV.

hep-ph↗

arXiv2025

Thermal Leptogenesis in $SO(10)\times U(1)_A$ SUSY GUT

We investigate thermal leptogenesis within a supersymmetric grand unified theory (SUSY GUT) based on the $SO(10) \times U(1)_A$ symmetry, where both the doublet--triplet splitting problem and the unrealistic Yukawa relations are resolved under the natural assumption that all symmetry-allowed interactions appear with $\mathcal{O}(1)$ coefficients. In this framework, the structures of the Dirac neutrino Yukawa couplings and right-handed neutrino masses are determined entirely by the symmetry. The baryon asymmetry of the Universe is computed taking into account the flavor effects, Higgs asymmetry contributions, and the impact of the second-lightest right-handed neutrino. While the predicted asymmetry is too small when all $\mathcal{O}(1)$ coefficients of the Dirac neutrino Yukawa couplings are set to unity, a moderate enhancement factor $r_1 \sim 5.4$ for the lightest right-handed neutrino mass reproduces the observed baryon asymmetry without spoiling low-energy neutrino data. This corresponds to a suppressed lightest neutrino mass, $m_{\nu_1} \sim (1/r_1) \times (\text{symmetry-determined value})$, typically $m_{\nu_1} \sim 4.5 \times 10^{-4}\,\text{eV}$. We further explore cases where the $\mathcal{O}(1)$ coefficients of the Dirac neutrino Yukawa couplings are $\pm 1$, and find that roughly half of them successfully generate the observed baryon asymmetry for $r_1\leq 11$. Moreover, the others yield results of the correct order of magnitude, although they do not generate the observed Baryon asymmetry. These findings demonstrate that thermal leptogenesis is realized in this $SO(10)\times U(1)_A$ SUSY GUT, establishing a link between the observed baryon asymmetry and predictions for the lightest neutrino mass.

hep-ph↗

arXiv2025

Probing Scalar-Mediator Quark Couplings via CLFV Lepton-Nucleon Scattering

We investigate charged lepton flavor violating (CLFV) deep-inelastic scattering, focusing on the gluon-initiated subprocess $\ell_i g \to \ell_j g$ via the gluon effective operator $\phi\, G_{\mu \nu}^a G_a^{\mu \nu}$, and demonstrate how to probe the nature of the CLFV mediator $\phi$, specifically its mass and interaction with quarks. We consider two benchmark scenarios for the mediator-quark coupling: (i) $h$-like scenario, in which the mediator couples to heavy quarks in proportion to their masses, and (i\hspace{-1pt}i) $b$-only scenario, where the coupling is restricted to bottom quark only. We demonstrate that these scenarios can be discriminated by examining the dependence of the differential cross section on the momentum transfer. Furthermore, we show that the peak position of the differential cross section exhibits a pronounced sensitivity to both the mass of the mediator and the coupling strengths with quarks.

hep-ph↗

arXiv2025

Lepton flavor violating decay of true muonium: $\boldsymbol{(\mu^+ \mu^-) \to \mu^\pm e^\mp}$

We propose a new channel for probing charged lepton flavor violation (CLFV): the decay of true muonium into a lepton pair of different flavor, $(\mu^+ \mu^-) \to \mu^\pm e^\mp$. This purely leptonic two-body decay provides a clean experimental signature in the form of energetic, oppositely charged leptons. It is sensitive not only to photonic dipole interactions but also to four-fermion contact interactions, and is free from hadronic uncertainties in theoretical predictions. We evaluate the branching ratios induced by scalar-, vector-, and dipole-type CLFV operators. Our results show that the branching ratio can reach up to $\mathcal{O}(10^{-20})$ within current experimental bounds. This decay mode may be discovered as an early-stage physics opportunity of the muon collider program by utilizing the large number of muons produced at its front end.

hep-ph↗

arXiv2025

Explosive production of Higgs particles and implications for heavy dark matter

It is widely believed that the parameter space for Higgs-portal dark matter that achieves the relic abundance through thermal freeze-out has already been tightly constrained, typically at masses on the order of ${\cal O}(10-100)$ GeV. We point out the possibility that the multiple Higgs production due to its self-interaction dramatically changes this picture. We show that the multiplicity can be as large as ${\cal O}(200)$ for the parameters of the Standard Model Higgs, independently of the kinematics of the particle production process. Consequently, heavy Higgs-portal dark matter of $m_\chi\gtrsim{\cal O}(1)$ TeV can achieve the required relic abundance in the same mechanism with that for canonical weakly interacting massive particle models.

hep-ph↗

arXiv2024

$\mu^- \to e^-\gamma$ in a muonic atom as a probe for effective lepton flavor violating operators involving photon fields

We propose the $\mu^-\to e^-\gamma$ process in a muonic atom as a novel means to investigate charged lepton flavor violation (CLFV). We demonstrate its sensitivity in probing effective CLFV operators associated with both single and double photon fields. In comparison to $\mu^+\to e^+\gamma$ using free positive muon decays at rest, the emitted electron and photon in $\mu^-\to e^-\gamma$ exhibit non-monochromatic spectra, presenting non-trivial case. We derive the decay rate formula and demonstrate that the potential rate of the process is significant enough to motivate exploration in future muon CLFV experiments.

hep-ph↗

arXiv2022

Asymmetric Mediator in Scotogenic Model

The scotogenic model is the Standard Model (SM) with Z_2 symmetry and the addition of Z_2 odd right-handed Majorana neutrinos and SU(2)_L doublet scalar fields. We have extended the original scotogenic model by an additional Z_2 odd singlet scalar field that plays a role in dark matter. In our model, the asymmetries of the lepton and Z_2 odd doublet scalar are simultaneously produced through CP-violating right-handed neutrino decays. While the former is converted into baryon asymmetry through the sphaleron process, the latter is relaid to the DM density through the decay of SU(2)_L doublet scalar that is named "asymmetric mediator". In this way, we provide an extended scotogenic model that predicts the energy densities of baryon and dark matter being in the same order of magnitude, and also explains the low-energy neutrino masses and mixing angles.

hep-ph↗

arXiv2022

Determination of coupling patterns by parallel searches for $μ^-\to e^+$ and $μ^-\to e^-$ in muonic atoms

We investigate a possibility that the $μ^-\to e^+$ conversion is discovered prior to the $μ^-\to e^-$ conversion, and its implications to the new physics search. We focus on the specific model including the mixing of the $SU(2)_L$ doublet- and singlet-type scalar leptoquarks, which induces not only the lepton flavor violation but also the lepton number violation. Such a structure is motivated by R-parity violating (RPV) supersymmetric models, where a sbottom mediates the conversion processes. We formulate the $μ^-\to e^+$ rate in analogy with the muon capture in a muonic atom, and numerically evaluate it using several target nuclei. The lepton flavor universality test of pion decay directly limits the $μ^-\to e^+$ rate, and the maximally allowed $μ^-\to e^+$ branching ratio is $\sim 10^{-18}$ under the various bounds on RPV parameters. We show that either $μ^-\to e^-$ or $μ^-\to e^+$ signals can be discovered in near future experiments. This indicates that parallel searches for these conversions will give us significant information on the pattern of coupling constants.

hep-ph↗

arXiv2021

Charged lepton flavor violation associated with heavy quark production in deep inelastic lepton-nucleon scattering via scalar exchange

We study charged lepton flavor violation (CLFV) associated with heavy quark pair production in lepton-nucleon deep-inelastic scattering $\ell_i N \to \ell_j q\bar{q} X$. Here $\ell_i$ and $\ell_j$ denote the initial and final leptons; $N$ and $X$ are respectively the initial nucleon and arbitrary final hadronic system. We employ a model Lagrangian in which a scalar and pseudoscalar mediator generates the CLFV. We derive heavy quark structure functions for scalar and pseudoscalar currents and compute momentum distributions of the final lepton for the process. Our focus is on the heavy quark mass effects in the final lepton momentum distribution. We clarify the necessity of inclusion of the heavy quark mass to obtain reliable theory predictions for the CLFV signal searches in the deep-inelastic scattering.

hep-ph↗

arXiv2021

Probing $μe γγ$ contact interactions with $μ\to e$ conversion

Contact interactions of a muon, an electron and two photons can contribute to the decay $μ\to e γγ$, but also to the conversion of a muon into an electron in the electric field of a nucleus. We calculate the $μ\to e$ conversion rate, and show that for the coefficients of operators involving the combination $FF \propto |\vec{E}|^2$ (as opposed to $F\tilde{F} \propto \vec{E} \cdot \vec{B}$), the current bound on $μ\to e$ conversion is more sensitive than the bound on $μ\to e γγ$.

hep-ph↗

arXiv2019

Momentum distribution of the electron pair from the charged lepton flavor violating process $μ^-e^-\to e^-e^-$ in muonic atoms with a polarized muon

The $μ^-e^-\to e^-e^-$ process in a muonic atom is one of the promising probes to study the charged lepton flavor violation (CLFV). We have investigated the angular distribution of electrons from the polarized muon of the atomic bound state. The parity violating asymmetric distribution of electrons is analyzed by using lepton wave functions under the Coulomb interaction of a finite nuclear charge distribution. It is found that the asymmetry parameters of electrons are very sensitive to the chiral structure of the CLFV interaction and the contact/photonic interaction. Therefore, together with the atomic number dependence of the decay rate studied in our previous work, the angular distribution of electrons from a polarized muon should be a very useful tool to constrain the model beyond the standard model.

hep-ph↗

arXiv2019

Selecting mu -> e Conversion Targets to distinguish Lepton Flavour-Changing Operators

The experimental sensitivity to $μ\to e$ conversion on nuclei is set to improve by four orders of magnitude in coming years. However, various operator coefficients add coherently in the amplitude for $μ\to e$ conversion, weighted by nucleus-dependent functions, and therefore in the event of a detection, identifying the relevant new physics scenarios could be difficult. Using a representation of the nuclear targets as vectors in coefficient space, whose components are the weighting functions, we quantify the expectation that different nuclear targets could give different constraints.We show that all but two combinations of the 10 Spin-Independent (SI) coefficients could be constrained by future measurements, but discriminating among the axial, tensor and pseudoscalar operators that contribute to the Spin-Dependent (SD) process would require dedicated nuclear calculations. We anticipate that $μ\to e$ conversion could constrain 10 to 14 combinations of coefficients; if $μ\to e γ$ and $μ\to 3e$ constrain eight more, that leaves 60 to 64 "flat directions" in the basis of QED$\times$QCD-invariant operators which describe $μ\to e$ flavour change below $m_W$.

hep-ph↗

arXiv2018

Big-bang nucleosynthesis and Leptogenesis in CMSSM

We have investigated the constrained minimal supersymmetric standard model with three right-handed Majorana neutrinos whether there still is a parameter region which is consistent with all existing experimental data/limits such as Leptogenesis and the dark matter abundance and we also can solve the Lithium problem. Using Casas-Ibarra parameterization, we have found that a very narrow parameter space of the complex orthogonal matrix elements where the lightest slepton can have a long lifetime, that is necessary for solving the Lithium problem. Further, under this condition, there is a parameter region that can give an explanation for the experimental observations. We have studied three cases of the right-handed neutrino mass ratio \mbox{\em (i)} $M_{2}=2 \times M_{1}$, \mbox{\em (ii)} $M_{2}=4 \times M_{1}$, \mbox{\em (iii)} $M_{2}=10 \times M_{1}$ while $M_{3}=40 \times M_{1}$ is fixed. We have obtained the mass range of the lightest right-handed neutrino mass that lies between $10^9$ GeV and $10^{11}$ GeV. The important result is that its upper limit is derived by solving the Lithium problem and the lower limit comes from Leptogenesis. Calculated low-energy observables of these parameter sets such as BR($μ\to e γ$) is not yet restricted by experiments and will be verified in the near future.

hep-ph↗

arXiv2017

Improved analysis for $μ^-e^-\to e^-e^-$ in muonic atoms by photonic interaction

Studies of the charged lepton flavor violating process of $μ^-e^-\to e^-e^-$ in muonic atoms by the four Fermi interaction [Y. Uesaka \textit{et al}., Phys. Rev. D {\bf 93}, 076006 (2016)] are extended to include the photonic interaction. The wave functions of a muon and electrons are obtained by solving the Dirac equation with the Coulomb interaction of a finite nuclear charge distribution. We find suppression of the $μ^-e^-\to e^-e^-$ rate over the initial estimation for the photonic interaction, in contrast to enhancement for the four Fermi interaction. It is due to the Coulomb interaction of scattering states and relativistic lepton wave functions. This finding suggests that the atomic number dependence of the $μ^-e^-\to e^-e^-$ rate could be used to distinguish between the photonic and the four Fermi interactions.

hep-ph↗

arXiv2017

Higgs mediated CLFV processes $μN(eN)\rightarrowτX$ via gluon operators

We revisit charged lepton flavor violating (CLFV) scattering processes $\ell_{i} N \to τX \, (\ell_{i} \ni e, μ)$ mediated by Higgs. We point out that a new subprocess $\ell_{i} g \to τg$ via the effective interactions of Higgs and gluon gives the dominant contribution to $\ell_{i} N \to τX$ for an incident beam energy of $E_{\ell} \lesssim 1\,\text{TeV}$ in fixed target experiments. Furthermore, in the light of quark number conservation, we consider quark pair-production processes $\ell_{i} g \to τq \bar{q}$ ($q$ denotes quarks) instead of $\ell_{i} q \to τq$. This corrects the threshold energy of each subprocess contributing to $σ(\ell_{i} N \to τX)$. Reevaluation of $σ(\ell_{i} N \to τX)$ including all of relevant subprocesses shows that the search for $\ell_{i} N \to τX$ could serve a complementary opportunity with other relevant processes to shed light on the Higgs CLFV.

hep-ph↗

arXiv2016

Relic Abundance in a Secluded Dark Matter Scenario with a Massive Mediator

The relic abundance of the dark matter (DM) particle $d$ is studied in a secluded DM scenario, in which the $d$ number decreasing process dominantly occurs not through the pair annihilation of $d$ into the standard model particles, but via the $dd \to mm$ scattering process with a subsequently decaying mediator particle $m$. It is pointed out that the cosmologically observed relic abundance of DM can be accomplished even with a massive mediator having a mass $m_m$ non-negligibly heavy compared with the DM particle mass $m_d$. In the degenerated $d$-$m$ case ($m_d=m_m$), the DM relic abundance is realized by adjusting the $dd \to mm$ scattering amplitude large enough and by choosing an appropriate mediator particle life-time. The DM evolution in the early universe exhibits characteristic "terrace" behavior, or two-step number density decreasing behavior, having a "fake" freeze-out at the first step. Based on these observations, a novel possibility of the DM model buildings is introduced in which the mediator particle $m$ is unified with the DM particle $d$ in an approximate dark symmetry multiplet. A pionic DM model is proposed to illustrate this idea in a renormalizable field theory framework.

hep-ph↗

arXiv2016

Muon-Electron Conversion in a Family Gauge Boson Model

We study the $μ$-$e$ conversion in muonic atoms via an exchange of family gauge boson (FGB) $A_{2}^{\ 1}$ in a $U(3)$ FGB model. Within the class of FGB model, we consider three types of family-number assignments for quarks. We evaluate the $μ$-$e$ conversion rate for various target nuclei, and find that next generation $μ$-$e$ conversion search experiments can cover entire energy scale of the model for all of types of the quark family-number assignments. We show that the conversion rate in the model is so sensitive to up- and down-quark mixing matrices, $U^{u}$ and $U^{d}$, where the CKM matrix is given by $V_\text{CKM} = U^{u\dagger} U^d$. Precise measurements of conversion rates for various target nuclei can identify not only the types of quark family-number assignments, but also each quark mixing matrix individually.

hep-ph↗

arXiv2016

A Solution to Lithium Problem by Long-Lived Stau

We review a non-standard Big-Bang nucleosynthesis (BBN) scenario within the minimal supersymmetric standard model, and propose an idea to solve both ${}^{7}$Li and ${}^{6}$Li problems. Each problem is a discrepancy between the predicted abundance in the standard BBN and observed one. We focus on the stau, a supersymmetric partner of tau lepton, which is a long-lived charged particle when it is the next lightest supersymmetric particle and is degenerate in mass with the lightest supersymmetric particle. The long-lived stau forms a bound state with a nucleus, and provide non-standard nuclear reactions. One of those, the internal conversion process, accelerates the destruction of ${}^{7}$Be and ${}^{7}$Li, and leads to a solution to the ${}^{7}$Li problem. On the other hand, the bound state of the stau and ${}^{4}$He enhances productions of n, d, t, and ${}^{6}$Li. The over-production of ${}^{6}$Li could solve the ${}^{6}$Li problem. While, the over-productions of d and t could conflict with observations, and the relevant parameter space of the stau is strictly constrained. We therefore need to carefully investigate the stau-${}^{4}$He bound state to find a condition of solving the ${}^{6}$Li problem. The scenario of the long-lived stau simultaneously and successfully fit the abundances of light elements (d, t, ${}^{3}$He, ${}^{4}$He, ${}^{6}$Li, and ${}^{7}$Li) and the neutralino dark matter to the observed ones. Consequently parameter space both of the stau and the neutralino is determined with excellent accuracy.

hep-ph↗