arXiv subjects

Yukihiro Mimura

Publications and source records attributed to Yukihiro Mimura.

At least 19 recordsLinked to original sources

arXiv2023

Insights from the magnetic field dependence of the muonium-to-antimuonium transition

The muonium-to-antimuonium transition experiment is about to be updated. Notably, the experiment at J-PARC in Japan can explore the magnetic field dependence of the transition probability. In this paper, we investigate the information that we can extract from the transition probabilities across different magnetic field strengths, while also taking into account a planned transition experiment at CSNS in China. There are two model-independent parameters in the transition amplitude, and we ascertain the feasibility of determining these parameters, including their relative physical phase, from experimental measurements. This physical phase can be related to the electron electric dipole moment, which is severely constrained by experiments. The underlying mediator responsible for the transition can be either doubly charged particles or neutral particles. In the former case, typical magnetic fields yield specific probability ratios, while the latter presents a range of the probability ratio. We investigate several models with neutral mediators, and elucidate that the probability ratio is linked to the sign of new physics contribution to the electron $g-2$. The pivotal role of the J-PARC transition experiment in shedding light on these insights is emphasized.

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arXiv2023

Neutrinoless double beta decay and the muonium-to-antimuonium transition in models with a doubly charged scalar

The lepton number and flavor violations are important possible ingredients of the lepton physics. The neutrinoless double beta decay and the transition of the muonium into antimuonuim are related to those violations. The former can give us an essential part of fundamental physics, and there are plenty of experimental attempts to observe the process. The latter has also been one of the attractive phenomena, and the experimental bound will be updated in planned experiments at new high-intensity muon beamlines. In models with a doubly charged scalar, not only can those two processes be induced, but also the active neutrino masses can be induced radiatively. The flavor violating decays of the charged leptons constrain the flavor parameters of the models. We study how the muonium-to-antimuonium transition rate can be as large as the current experimental bound, and we insist that the updated bound of the transition rate will be useful to distinguish the models to generate the neutrinoless double beta decay via the doubly charged scalar. We also study a possible extension of the model to the left-right model which can induce the neutrinoless double beta decay.

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arXiv2022

Transverse positron polarization in the polarized $μ^+$ decay related with the muonium-to-antimuonium transition

The constructions of the new high-intensity muon beamlines are progressing in facilities around the world, and new physics searches related to the muons are expected. The facilities can observe the transverse positron polarization of the polarized $μ^+$ decay to test the standard model. The transition of muonium into antimuonium (Mu-to-$\bar{\text{Mu}}$ transition), which is one of the interesting possibilities in the models beyond the standard model, can be also tested. The near-future observation of the transition gives us a great impact since it indicates that there is an approximate discrete symmetry in the lepton sector. If the Mu-to-$\bar{\text{Mu}}$ transition operator is generated, a new muon decay operator can exist and it may interfere with the standard model muon decay operator to induce the corrections to the transverse positron polarization in the $μ^+$ decay. In this paper, we examine the possibility that the Mu-to-$\bar{\text{Mu}}$ transition and the correction to the transverse positron polarization are related, and we show that those two are related in the model of a neutral flavor gauge boson. We also investigate the models to generate the Mu-to-$\bar{\text{Mu}}$ transition, such as an inert $SU(2)_L$ doublet, a $SU(2)_L$ triplet for the type-II seesaw model, a dilepton gauge boson, and a left-right model. The non-zero value of the transverse polarization for one of the two directions, $P_{\rm T_2}$, violates the time-reversal invariance, and the experimental constraint of the electron electric dipole moment can provide a severe constraint on $P_{\rm T_2}$ depending on the model.

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arXiv2021

Models of the Muonium to Antimuonium Transition

Muonium is a bound state composed of an antimuon and an electron, and it constitutes a hydrogen-like atom. Because of the absence of the hadronic matter in the bound state, the muonium is a useful probe to explore new physics being free from the hadronic uncertainties. The process of the muonium-to-antimuonium transition is considered to be effective to identify fundamental interactions which relate to the lepton flavor and lepton number violation. New experiments are being planned at J-PARC in Japan and CSNS in China, and it is expected to attract more attention in the near future. In this paper, we will study what kind of model can be verified in the next generation of the muonium-to-antimuonium transition search experiments while escaping the limitations from other experiments. Though the transition probability is strongly suppressed by the lepton flavor conservation in the standard model, it can be much larger by the exchanges of neutral and doubly charged bosons, and by box loop diagrams in new physics beyond the standard model. We study the neutrino models with heavy Majorana neutrinos at TeV scale, a type-II seesaw model, left--right models, and models for radiative neutrino masses such as the Zee-Babu model in particular, in addition to other possible models to induce the sizable transition probability, which can be tested in the forthcoming experiments.

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arXiv2020

Renormalizable $SO(10)$ GUT with Suppressed Dimension-5 Proton Decays

We study a renormalizable SUSY $SO(10)$ GUT model where the Yukawa couplings of single ${\bf 10}$, single ${\bf \overline{126}}$ and single ${\bf 120}$ fields, $Y_{10},Y_{126},Y_{120}$, account for the quark and lepton Yukawa couplings and the neutrino mass. We pursue the possibility that $Y_{10},Y_{126},Y_{120}$ reproduce the correct quark and lepton masses, CKM and PMNS matrices and neutrino mass differences, and at the same time suppress dimension-5 proton decays (proton decays via colored Higgsino exchange) through their texture, so that the soft SUSY breaking scale can be reduced as much as possible without conflicting the current experimental bound on proton decays. We perform a numerical search for such a texture, and investigate implications of that texture on unknown neutrino parameters, the Dirac CP phase of PMNS matrix, the lightest neutrino mass and the $(1,1)$-component of the neutrino mass matrix in the charged lepton basis. Here we concentrate on the case when the active neutrino mass is generated mostly by the Type-2 seesaw mechanism, in which case the correlation between the suppression of dimension-5 proton decays and the neutrino parameters is expected to be most direct.

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arXiv2020

$μ$-$τ$ symmetry breaking and CP violation in the neutrino mass matrix

The $μ$-$τ$ exchange symmetry in the neutrino mass matrix and its breaking as a perturbation are discussed. The exact $μ$-$τ$ symmetry restricts the 2-3 and 1-3 neutrino mixing angles as $θ_{23} = π/4$ and $θ_{13} = 0$ at a zeroth order level. We claim that the $μ$-$τ$ symmetry breaking prefers a large CP violation to realize the observed value of $θ_{13}$ and to keep $θ_{23}$ nearly maximal, though an artificial choice of the $μ$-$τ$ breaking can tune $θ_{23}$, irrespective of the CP phase. We exhibit several relations among the deviation of $θ_{23}$ from $π/4$, $θ_{13}$ and Dirac CP phase $δ$, which are useful to test the $μ$-$τ$ breaking models in the near future experiments. We also propose a concrete model to break the $μ$-$τ$ exchange symmetry spontaneously and its breaking is mediated by the gauge interactions radiatively in the framework of the extended gauge model with $B-L$ and $L_μ- L_τ$ symmetries. As a result of the gauge mediated $μ$-$τ$ breaking in the neutrino mass matrix, the artificial choice is unlikely, and a large Dirac CP phase is preferable.

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arXiv2020

Enhanced $Γ(p\to K^0μ^+)/Γ(p\to K^+\barν_μ)$ as a Signature of Minimal Renormalizable SUSY $SO(10)$ GUT

The ratio of the partial widths of some dimension-5 proton decay modes can be predicted without detailed knowledge of SUSY particle masses, and thus allows us to experimentally test various SUSY GUT models without discovering SUSY particles. In this paper, we study the ratio of the partial widths of the $p\to K^0μ^+$ and $p\to K^+\barν_μ$ decays in the minimal renormalizable SUSY $SO(10)$ GUT, under only a plausible assumption that the 1st and 2nd generation left-handed squarks are mass-degenerate. In the model, we expect that the Wilson coefficients of dimension-5 operators responsible for these modes are on the same order and that the ratio of $p\to K^0μ^+$ and $p\to K^+\barν_μ$ partial widths is $O(0.1)$. Hence, we may be able to detect both $p\to K^0μ^+$ and $p\to K^+\barν_μ$ decays at Hyper-Kamiokande, thereby gaining a hint for the minimal renormalizable SUSY $SO(10)$ GUT. Moreover, since this partial width ratio is quite suppressed in the minimal $SU(5)$ GUT, it allows us to distinguish the minimal renormalizable SUSY $SO(10)$ GUT from the minimal $SU(5)$ GUT. In the main body of the paper, we perform a fitting of the quark and lepton masses and flavor mixings with the Yukawa couplings of the minimal renormalizable $SO(10)$ GUT, and derive a concrete prediction for the partial width ratio based on the fitting results. We find that the partial width ratio generally varies in the range 0.05-0.6, confirming the above expectation.

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arXiv2020

On $θ_{23}$ Octant Measurement in $3+1$ Neutrino Oscillations in T2HKK

It has been pointed out that the mixing of an eV-scale sterile neutrino with active flavors can lead to loss of sensitivity to the $θ_{23}$ octant (sign of $\sin^2θ_{23}-1/2$) in long baseline experiments, because the main oscillation probability $P_0=4\sin^2θ_{23}\sin^2θ_{13}\sin^2Δ_{31}$ can be degenerate with the sum of the interferences with the solar oscillation amplitude and an active-sterile oscillation amplitude in both neutrino and antineutrino oscillations, depending on CP phases. In this paper, we show that the above degeneracy is resolved by measuring the same beam at different baseline lengths. We demonstrate that Tokai-to-Hyper-Kamiokande-to-Korea (T2HKK) experiment (one 187~kton fiducial volume water Cerenkov detector is placed at Kamioka, $L=295$~km, and another detector is put in Korea, $L\sim1000$~km) exhibits a better sensitivity to the $θ_{23}$ octant in those parameter regions where the experiment with two detectors at Kamioka is insensitive to it. Therefore, if a hint of sterile-active mixings is discovered in short baseline experiments, T2HKK is a better option than the plan of placing two detectors at Kamioka. We also consider an alternative case where one detector is placed at Kamioka and a different detector is at Oki Islands, $L=653$~km, and show that this configuration also leads to a better sensitivity to the $θ_{23}$ octant.

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arXiv2019

Detectable dimension-6 proton decay in SUSY SO(10) GUT at Hyper-Kamiokande

In the minimal SUSY SU(5) GUT with $O(1)$ TeV SUSY particles and $O(1)$ or below self-coupling for the GUT-breaking Higgs field, the width of the dimension-6 proton decay is suppressed below the reach of Hyper-Kamiokande. In this paper, we point out that a SUSY SO(10) GUT which adopts only ${\bf 45}_{\rm H}+{\bf 16}_{\rm H}+{\bf \overline{16}}_{\rm H}$ GUT-breaking Higgs fields leads to an enhanced dimension-6 proton decay width detectable at Hyper-Kamiokande. The enhancement is because the SU(5)-breaking VEV of ${\bf 45}_{\rm H}$ arises due to Planck-suppressed terms, $W\propto ({\bf 45}_{\rm H}^2)^2/M_*+{\bf 45}_{\rm H}^4/M_*$, and is therefore substantially larger than the other VEVs that conserve SU(5). As a result, the $({\bf 3},{\bf 2},1/6)$ GUT gauge boson mass is about $1/5$ smaller than the $({\bf 3},{\bf 2},-5/6)$ GUT gauge boson mass and can induce a fast dimension-6 proton decay. Through a numerical analysis on threshold corrections of the GUT gauge bosons and the physical components of the GUT-breaking Higgs fields, we confirm that the dimension-6 proton decay can be within the reach of Hyper-Kamiokande.

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arXiv2019

Proton Lifetime Upper Bound in Non-SUSY SU(5) GUT

In preparation for upcoming nucleon decay searches at Hyper-Kamiokande, it is important to derive a theoretical upper bound on the proton lifetime in a general class of grand unified theory (GUT) models. In this paper, we make an attempt along this direction for non-SUSY SU(5) models, under the mild restrictions that only one or two SM-decomposed multiplets are singularly light, and that the SU(5) gauge theory is asymptotically free and thus there are no too large representations in the model. We derive criteria for SM-decomposed multiplets that potentially enhance the proton lifetime when they are singularly light. We perform a numerical analysis on the proton lifetime and show that some choices of singularly light multiplets can provide a testable upper bound on the proton lifetime.

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arXiv2019

A Grand Unified Parity Solution to Strong CP Problem

A beyond the standard model theory that respects parity symmetry at short distances is known to provide a solution to the strong CP problem without the need for an axion, while keeping the CKM phase unconstrained. In this paper we present a supersymmetric SO(10) grand unified embedding of this idea with Yukawa couplings generated by {\bf 10}, ${\bf \overline{126}}$ and {\bf 120} Higgs fields. This model is known to provide a unified description of masses and mixings of quarks and leptons. When CP symmetry is imposed on this model, the discrete gauge subgroup C of SO(10) combines with it to generate an effective parity symmetry, leading to hermitian quark mass matrices. Imposing an additional discrete symmetry, $G$, we show that there are no other tree level sources of $θ$ in the model; $G$ also guarantees that the one- and two-loop contributions to $θ$ vanish. We then show that the leading three-loop effects and the effect of higher-dimensional operators invariant under $G$ give rise to $θ$ near the current experimental bound, making the model testable in the current searches for neutron electric dipole moment.

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arXiv2019

Electron g-2 with flavor violation in MSSM

The muon $g-2$ anomaly is a long-standing discrepancy from its standard model prediction. The recent improved measurement of the fine structure constant makes the electron $g-2$ anomaly very interesting for both sign and magnitude in comparison to the muon $g-2$ anomaly. In order to explain both muon and electron $g-2$ anomalies, we introduce flavor violation in the minimal supersymmetric standard model (MSSM) as a low energy theory. The lepton flavor violating process $τ\to eγ$ is one the major constraints to explain both $g-2$ anomalies simultaneously emerging from flavor violating terms. We show various mass spectra of sleptons, neutralinos, and charginos, consistent with the LHC results, to explain both anomalies after satisfying the flavor violation constraint.

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arXiv2019

Yukawa Unification with Four Higgs Doublets in Supersymmetric GUT

We discuss the Yukawa coupling unification, a very interesting prediction of the grand unified theory, in the context of scenarios with more than one pair of Higgs doublet since the current LHC constraint has become a problem for the Yukawa unification scenarios with just one pair of Higgs doublet. More than one pair of Higgs doublets can easily arise in missing partner mechanism which solves the doublet-triplet splitting problem. In such a scenario, the Yukawa unification occurs at a medium $\tanβ$ value, e.g., $\sim$ 30, which corresponds to much smaller threshold corrections compared to usual large $\tanβ$ scenario for $t-b-τ$ unification in the context of SO(10) and $b-τ$ unification in the context of SU(5) models. Further, we show that an additional Higgs doublet pair lowers the sensitivity of the radiative symmetry breaking of the electroweak vacuum.

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arXiv2018

Nearly right unitarity triangle and CP phase in quark and lepton flavor mixings

The nearly right unitarity triangle can be simply obtained if a quark mass matrix is written as a linear combination of real rank-1 matrices and the coefficient which gives the mass of the second generation is pure imaginary. Supposing that the source of the CP violation is in the Yukawa coupling to an additional Higgs field which provides a factor for the strange quark and muon mass ratio, we obtain that the angle $α= ϕ_2$ of the unitarity triangle shifts from $90^{\rm o}$ by units of $V_{ub}/V_{us}$, and is postdicted as $α\simeq 87^{\rm o}$ or $91^{\rm o}$. The Dirac CP phase $δ$ which appears in the three-flavor neutrino oscillations is obtained to be $|δ| \simeq 80^{\rm o}$ if the neutrino mass matrix gives tri-bimaximal-like mixing form. If the factor 3 for the muon and strange quark mass is considered in a simple manner in the quark-lepton unification, we obtain three distinctive prediction of $δ$ for nearly right-angled phase as $|δ| \simeq 70^{\rm o}$, $90^{\rm o}$, or $110^{\rm o}$ in an idealistic orthogonal structure of the neutrino mass matrix. The deviation from $90^{\rm o}$ is roughly given by $\arcsin(1/3)$ by inputting the experimental measurements.

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arXiv2016

Relation between proton decay and PMNS phase in the minimal SUSY $SO(10)$ GUT

Proton decay is one of the most important predictions of the grand unified theory (GUT). In the supersymmetric (SUSY) GUT, proton decays via the dimension-five operators need to be suppressed. In the $SO(10)$ model where ${\bf 10}+\overline{\bf 126}$ Higgs fields couple to fermions, neutrino oscillation parameters including the CP-violating Pontecorvo-Maki-Nakagawa-Sakata (PMNS) phase can be related to the Yukawa couplings to generate the dimension-five operators in the unified framework. We show how the suppressed proton decay depends on the PMNS phase, and stress the importance of the precise measurements of the PMNS phase as well as the neutrino 23-mixing angle. These become especially important if the SUSY particles are found around less than a few TeV at LHC and proton decays are observed at Hyper-Kamiokande and DUNE experiments in the near future.

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arXiv2016

Revisiting fermion mass and mixing fits in the minimal SUSY $SO(10)$ GUT

Supersymmetric $SO(10)$ grand unified models with renormalizable Yukawa couplings involving only ${\bf 10}$ and $\overline{\bf 126}$ Higgs fields have been shown to realize the fermion masses and mixings economically. In previous works, the sum rule of the fermion mass matrices are given by inputting the quark matrices, and the neutrino mixings are predicted in this framework. Now the three neutrino mixings have been measured, and in this paper, we give the sum rule by inputting the lepton mass matrices, which makes clear certain features of the solution, especially if the vacuum expectation values of ${\bf 126}+ \overline{\bf126}$ ($v_R$) are large and the right-handed neutrinos are heavy. We perform the $χ^2$ analyses to fit the fermion masses and mixings using the sum rule. In previous works, the best fit appears at $v_R \sim 10^{13}$ GeV, and the fit at the large $v_R$ scale ($\sim 10^{16}$ GeV) has been less investigated. Our expression of the sum rule has a benefit to understand the flavor structure in the large $v_R$ solution. Using the fit results, we perform the calculation of the $μ\to eγ$ process and the electric dipole moment of electron, and the importance of $v_R$ dependence emerges in low energy phenomena. We also show the prediction of the CP phase in the neutrino oscillations, which can be tested in the near future.

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arXiv2016

Is Electroweak Symmetry Breaking Still Natural in the MSSM?

The absence of any signal of supersymmetry (SUSY) at the LHC has raised the SUSY particle mass scale compared to $Z$ boson mass $M_Z$. We investigate the naturalness of the electroweak symmetry breaking after considering radiative symmetry breaking along with 125 GeV Higgs mass. We find that the important quantity to measure the naturalness of the hierarchy between the SUSY scale and $M_Z$ is the separation between the radiative symmetry breaking scale, i.e., where $m_{H_u}^2+μ^2$ turns negative for large $\tanβ$ case ($μ$ is the Higgsino mass and $m_{H_u}$ is the SUSY breaking up-type Higgs boson mass) and the average stop mass. Using this measure, one can show that the electroweak symmetry breaking can be natural even if $μ$ is large contrary to the prevailing claim that $μ$ is needed to be small to maintain the naturalness.

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arXiv2015

Enhancement of Br(B_d -> mu^+mu^-)/Br(B_s -> mu^+mu^-) in Supersymmetric Unified Models

We explain the 2.3 sigma deviation in the recent measurements of the neutral B mesons decay into muon pairs from the standard model prediction in the framework of supersymmetric grand unified models using anti-symmetric coupling as a new source of flavor violation. We show a correlation between the B_d -> mu^+mu^- decay and the CP phase in the B_d -> J/psi K decay and that their deviations from the standard model predictions can be explained after satisfying constraints arising from various hadronic and leptonic rare decay processes, B-bar{B}, K-bar{K} oscillations data and electric dipole moments of electron and neutron. The allowed parameter space is typically represented by pseudoscalar Higgs mass m_A < 1 TeV and tan beta_H (= {v_u}/{v_d}) < 20 for squark and gluino masses around 2 TeV.

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