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Toru Goto

Publications and source records attributed to Toru Goto.

At least 19 recordsLinked to original sources
arXiv2023

Flavor physics in SU(5) GUT with scalar fields in the 45 representation

We study a realistic SU(5) grand unified model, where a 45 representation of scalar fields is added to the Georgi-Glashow model in order to realize the gauge coupling unification and the masses and mixing of quarks and leptons. The gauge coupling unification together with constraints from proton decay implies mass splittings in scalar representations. We assume that an SU(2) triplet component of the 45 scalar, which is called $S_3$ leptoquark, has a TeV-scale mass, and color-sextet and color-octet ones have masses of the order of $10^6$ GeV. We calculate one-loop beta functions for Yukawa couplings in the model, and derive the low-energy values of the $S_3$ Yukawa couplings which are consistent with the grand unification. We provide predictions for lepton-flavor violation and lepton-flavor-universality violation induced by the $S_3$ leptoquark, and find that current and future experiments have a chance to find a footprint of our SU(5) model.

hep-ph↗
arXiv2018

Gluino-mediated electroweak penguin with flavor-violating trilinear couplings

In light of a discrepancy of the direct $CP$ violation in $K\toππ$ decays, $\varepsilon'/\varepsilon_K$, we investigate gluino contributions to the electroweak penguin, where flavor violations are induced by squark trilinear couplings. Top-Yukawa contributions to $ΔS = 2$ observables are taken into account, and vacuum stability conditions are evaluated in detail. It is found that this scenario can explain the discrepancy of $\varepsilon'/\varepsilon_K$ for the squark mass smaller than 5.6 TeV. We also show that the gluino contributions can amplify $\mathcal{B}(K \to πν\overlineν)$, $\mathcal{B}(K_S \to μ^+ μ^-)_{\rm eff}$ and $ΔA_{\rm CP}(b\to sγ)$. Such large effects could be measured in future experiments.

hep-ph↗
arXiv2015

Lepton flavor violating $Z$-boson couplings from non-standard Higgs interactions

The Standard Model predicts that the Higgs boson couples to the fermions in the mass eigenstates. We consider the effects of lepton flavor violating (LFV) $Z$ boson couplings in the case where the Higgs boson has flavor non-diagonal Yukawa interactions with the muon and the tau lepton generated from physics beyond the Standard Model. We list the formulae of the couplings of the effective interactions among the $τ$ lepton, the muon and the $Z$ boson. Using these formulae, we calculate the branching fractions of various leptonic and hadronic LFV $τ$ decays, and the LFV $Z$ boson decay: $Z\to τμ$. Although the $Z$-boson contributions to LFV tau decays cannot be ignored in terms of the counting of operator dimensions or chirality flipptings, it turns out that they are not very significant for $τ\to3μ$ and $τ\toμρ$ decays. We also calculate the branching fractions of the processes, $τ\to μπ$, $τ\to μη^{(\prime)}$ and $τ\toμa_1$, which are dominated by the $Z$-boson exchanges due to the spin and the parity of the hadrons.

hep-ph↗
arXiv2014

Lepton flavor violation in the supersymmetric seesaw model after the LHC 8 TeV run

We study the lepton flavor violation in the supersymmetric seesaw model taking into account recent experimental improvements, especially for the Higgs boson mass measurement, direct searches of superpartners and the rare decay of B_s -> mu+ mu- at the LHC, the neutrino mixing angle of theta_{13} at the neutrino experiments, and the search of mu -> e gamma at the MEG experiment. We obtain the latest constraints on the parameters in the supersymmetry breaking terms and study the effect on the lepton flavor violating decays of tau -> mu gamma and mu -> e gamma. In particular, we consider two kinds of assumption on the structures in the Majorana mass matrix and the neutrino Yukawa matrix. In the case of the Majorana mass matrix proportional to the unit matrix, allowing non-vanishing CP violating parameters in the neutrino Yukawa matrix, we find that the branching ratio of tau -> mu gamma can be larger than 10^{-9} within the improved experimental limit of mu -> e gamma. We also consider the neutrino Yukawa matrix that includes the mixing only in the second and third generations, and find that a larger branching ratio of tau -> mu gamma than 10^{-9} is possible with satisfying the recent constraints.

hep-ph↗
arXiv2011

Tau and muon lepton flavor violations in the littlest Higgs model with T-parity

Lepton flavor violation in tau and mu processes is studied in the littlest Higgs model with T-parity. We consider various asymmetries defined in polarized tau and mu decays. Correlations among branching ratios and asymmetries are shown in the following lepton flavor violation processes: mu+ --> e+ gamma, mu+ --> e+ e+ e-, mu- A --> e- A (A = Al, Ti, Au and Pb), tau+ --> mu+ gamma, tau+ --> mu+ mu+ mu-, tau+ --> mu+ e+ e-, tau+ --> mu+ P (P = pi0, eta and eta'), tau+ --> mu+ V (V = rho0, omega and phi), tau+ --> e+ gamma, tau+ --> e+ e+ e-, tau+ --> e+ mu+ mu-, tau+ --> e+ P, tau+ --> e+ V, tau+ --> mu+ mu+ e- and tau+ --> e+ e+ mu-. It is shown that large parity asymmetries and time-reversal asymmetries are allowed in mu+ --> e+ e+ e-. For tau lepton flavor violation processes, sizable asymmetries are possible reflecting characteristic chirality structure of lepton flavor violating interactions in this model.

hep-ph↗
arXiv2010

Lepton electric dipole moments in supersymmetric type II seesaw model

We study the lepton electric dipole moments in the framework of the supersymmetric type II seesaw model where the exchange of heavy SU(2)_W triplets generates small neutrino masses. We show that the CP violating phase of the bilinear soft supersymmetry breaking term associated with the SU(2)_W triplets contributes to lepton electric dipole moments mainly through threshold corrections to the gaugino masses at the seesaw scale. As a consequence, the ratio of the electric dipole moments of the muon and the electron is the same as the ratio of their masses in a wide region of parameter space.

hep-ph↗
arXiv2008

Patterns of flavor signals in supersymmetric models

Quark and lepton flavor signals are studied in four supersymmetric models, namely the minimal supergravity model, the minimal supersymmetric standard model with right-handed neutrinos, SU(5) supersymmetric grand unified theory with right-handed neutrinos and the minimal supersymmetric standard model with U(2) flavor symmetry. We calculate b --> s(d) transition observables in B_d and B_s decays, taking the constraint from the B_s--B_s-bar mixing recently observed at Tevatron into account. We also calculate lepton flavor violating processes mu --> e gamma, tau --> mu gamma and tau --> e gamma for the models with right-handed neutrinos. We investigate possibilities to distinguish the flavor structure of the supersymmetry breaking sector with use of patterns of various flavor signals which are expected to be measured in experiments such as MEG, LHCb and a future Super B Factory.

hep-ph↗
arXiv2004

Neutralino polarization effect in the squark cascade decay at LHC

We study the effect of the neutralino polarization in a squark cascade decay $\tilde{q} \to q \tildeχ^0_2 \to q l^\pm \tilde{l}^\mp \to q l^\pm \tilde{l}^\mp \to q l^\pm l^\mp \tildeχ^0_1$. Charge asymmetry in the lepton-jet invariant mass distribution appears depending on the chirality structure of the sfermion-fermion-neutralino coupling. With use of the Monte Carlo simulation, we show that the asymmetry is measurable in LHC. We also show that the distribution of the charge asymmetry is sensitive to the (s)lepton flavor.

hep-ph↗
arXiv2004

Exploring Flavor Structure of SUSY Breaking from B Decays and the Unitarity Triangle

We study effects of SUSY particles in the unitarity triangle analysis and the CP asymmetries in b -> s decays. We consider three different SUSY models, the minimal supergravity, SU(5) SUSY GUT with right-handed neutrinos, and MSSM with U(2) flavor symmetry, and show that the patterns and correlations of deviations from the standard model will be useful to discriminate the different SUSY models in future B experiments.

hep-ph↗
arXiv2003

Exploring flavor structure of supersymmetry breaking from rare B decays and unitarity triangle

We study effects of supersymmetric particles in various rare B decay processes as well as in the unitarity triangle analysis. We consider three different supersymmetric models, the minimal supergravity, SU(5) SUSY GUT with right-handed neutrinos, and the minimal supersymmetric standard model with U(2) flavor symmetry. In the SU(5) SUSY GUT with right-handed neutrinos, we consider two cases of the mass matrix of the right-handed neutrinos. We calculate direct and mixing-induced CP asymmetries in the b to s gamma decay and CP asymmetry in B_d to phi K_S as well as the B_s--anti-B_s mixing amplitude for the unitarity triangle analysis in these models. We show that large deviations are possible for the SU(5) SUSY GUT and the U(2) model. The pattern and correlations of deviations from the standard model will be useful to discriminate the different SUSY models in future B experiments.

hep-ph↗
arXiv2002

Exploring Flavor Structure of Supersymmetry Breaking at B factories

We investigate flavor physics at present and future B factories in order to distinguish supersymmetric models. We evaluate CP asymmetries in various B decay modes, Delta M(Bd), Delta M(Bs), and epsilon_K in three supersymmetric models, i.e. the minimal supergravity, the SU(5) SUSY GUT with right handed neutrinos, and a supersymmetric model with U(2) flavor symmetry. The allowed regions of Delta M(Bs)/Delta M(Bd) and CP asymmetries in B to J/psi K_S and b to s gamma are different for the three models so that it is possible to distinguish the three models by precise determinations of these observables in near future experiments.

hep-ph↗
arXiv2002

Exploring Flavor Structure of Supersymmetry Breaking at B factories

We investigate quark flavor signals in three different supersymmetric models, the minimal supergravity, the SU(5) SUSY GUT with right handed neutrinos, and the minimal supersymmetric standard model with U(2) flavor symmetry, in order to study physics potential of the present and future $B$ factories. We evaluate CP asymmetries in various B decay modes, $Δm_{B_s}$, $Δm_{B_d}$, and $ε_K$. The allowed regions of the CP asymmetry in $B\to J/ψK_S$ and $Δm_{B_s}/Δm_{B_d}$ are different for the three models so that precise determinations of these observables in near future experiments are useful to distinguish the three models. We also investigate possible deviations from the standard model predictions of CP asymmetries in other B decay modes. In particular, a large deviation is possible for the U(2) model. The consistency check of the unitarity triangle including $B\to ππ,ρπ,D^{(*)}K^{(*)},D^{(*)}π,D^{*}ρ$, and so on, at future high luminosity $e^+e^-$ $B$ factories and hadronic $B$ experiments is therefore important to distinguish flavor structures of different supersymmetric models.

hep-ph↗
arXiv2001

Lepton flavor violating process in a supersymmetric grand unified theory with right-handed neutrino

Motivated from the recent results of neutrino oscillation experiment, we investigated lepton flavor violating (LFV) processes in a SU(5) supersymmetric grand unified theory with right-handed neutrino. The current experimental upper bound for $μ\to eγ$ process gives already some constraint on the model. Correlation between $μ\to e γ$ and the SUSY contribution to the muon anomalous magnetic moment is also discussed. Future LFV experiments will give considerable impacts on this type of SUSY GUT models equipped with seesaw neutrino mass generation.

hep-ph↗
arXiv2001

Muon anomalous magnetic moment, lepton flavor violation, and flavor changing neutral current processes in SUSY GUT with right-handed neutrino

Motivated by the large mixing angle solutions for the atmospheric and solar neutrino anomalies, flavor changing neutral current processes and lepton flavor violating processes as well as the muon anomalous magnetic moment are analyzed in the framework of SU(5) SUSY GUT with right-handed neutrino. In order to explain realistic mass relations for quarks and leptons, we take into account effects of higher dimensional operators above the GUT scale. It is shown that the supersymmetric (SUSY) contributions to the CP violation parameter in $K^0-\bar{K}^0$ mixing, $ε_K$, the $μ\to e γ$ branching ratio, and the muon anomalous magnetic moment become large in a wide range of parameter space. We also investigate correlations among these quantities. Within the current experimental bound of $\text{B}(μ\to e γ)$, large SUSY contributions are possible either in the muon anomalous magnetic moment or in $ε_K$. In the former case, the favorable value of the recent muon anomalous magnetic moment measurement at the BNL E821 experiment can be accommodated. In the latter case, the allowed region of the Kobayashi-Maskawa phase can be different from the prediction within the Standard Model (SM) and therefore the measurements of the CP asymmetry of $B\to J/ψK_S$ mode and $Δm_{B_s}$ could discriminate this case from the SM. We also show that the $τ\to μγ$ branching ratio can be close to the current experimental upperbound and the mixing induced CP asymmetry of the radiative B decay can be enhanced in the case where the neutrino parameters correspond to the Mikheyev-Smirnov-Wolfenstein small mixing angle solution.

hep-ph↗
arXiv2000

Neutrino Yukawa couplings and FCNC processes in B decays in SUSY-GUT

Flavor changing neutral current and lepton flavor violating processes are studied in the SU(5) SUSY-GUT with right-handed neutrino supermultiplets. Using input parameters motivated by neutrino oscillation, it is shown that the time-dependent CP asymmetry of $b \to s γ$ can be as large as 20%. We also show that the $B_s$-$\bar{B}_s$ mixing can be significantly different from the standard model prediction.

hep-ph↗
arXiv2000

Neutrino oscillation, SUSY GUT and B decay

Effects of supersymmetric particles on flavor changing neutral current and lepton flavor violating processes are studied in the supersymmetric SU(5) grand unified theory with right-handed neutrino supermultiplets. Using input parameters motivated by neutrino oscillation, it is shown that the time-dependent CP asymmetry of radiative B decay can be as large as 30% when the $τ\to μγ$ branching ratio becomes close to the present experimental upper bound. We also show that the $B_s$--$\bar{B}_s$ mixing can be significantly different from the presently allowed range in the standard model.

hep-ph↗
arXiv1999

New constraint on the minimal SUSY GUT model from proton decay

We present results of reanalysis on proton decay in the minimal SU(5) SUSY GUT model. Unlike previous analyses, we take into account a Higgsino dressing diagram of dimension 5 operator with right-handed matter fields ($RRRR$ operator). It is shown that this diagram gives a significant contribution for $p \to K^{+}\barν_τ$. Constraints on the colored Higgs mass $M_C$ and the sfermion mass $m_{\tilde{f}}$ from Super-Kamiokande limit become considerably stronger than those in the previous analyses: $M_C > 6.5\times 10^{16} \gev$ for $m_{\tilde{f}} < 1 \tev$. The minimal model with $m_{\tilde{f}} \lsim 2 \tev$ is excluded if we require the validity of this model up to the Planck scale.

hep-ph↗