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Toshihiko Ota

Publications and source records attributed to Toshihiko Ota.

50 records · Page 3Linked to original sources
arXiv2006

Breaking of L_mu - L_tau Flavor Symmetry, Lepton Flavor Violation and Leptogenesis

A supersymmetric see-saw model obeying the flavor symmetry L_mu - L_tau, which naturally predicts quasi-degenerate neutrinos, is investigated. Breaking of the symmetry is introduced in the Dirac mass matrix because it is the most economic choice in the sense that all interesting low and high energy phenomenology is made possible: we analyze the predictions for the low energy neutrino observables, for leptogenesis and for lepton flavor violating decays such as mu \to e gamma, where the SPS benchmark points for the SUSY parameters are used. It is outlined how these decays in connection with the requirement of successful leptogenesis and with correlations between the neutrino observables depend on the way the symmetry is broken.

hep-ph↗
arXiv2006

Lepton flavor violation in Higgs boson decays under the rare tau decay results

We study lepton flavor violation (LFV) associated with tau leptons in the framework of the two Higgs doublet model, in which LFV couplings are introduced as a deviation from Model II Yukawa interaction. Parameters of the model are constrained from experimental results and also from requirements of theoretical consistencies such as vacuum stability and perturbative unitarity. Current data for rare tau decays provide substantial upper limits on the LFV Yukawa couplings in the large $\tanβ$ region ($\tanβ\gtrsim 30 $), which are comparable with predictions in fundamental theories. Here $\tanβ$ is the ratio of vacuum expectation values of the two Higgs doublets. We show that a search for the LFV decays $ϕ^{0} \to τ^\pm μ^\mp$ $(τ^\pm e^\mp)$ of neutral Higgs bosons ($ϕ^{0} =h,H$ and $A$) at future collider experiments can be useful to further constrain the LFV couplings especially in the relatively small $\tanβ$ region ($\tanβ\lesssim 30 $), where rare tau decay data cannot give any strong limit.

hep-ph↗
arXiv2005

Enhancement of lepton flavor violation in a model with bi-maximal mixing at the grand unification scale

We study phenomenological predictions in the scenario with the quasi-degenerate relation among neutrino Dirac masses, m_D1 simeq m_D2 < m_D3, assuming the bi-maximal mixing at the grand unification scale in supersymmetric standard models with right-handed neutrinos. A sufficient lepton number asymmetry can be produced for successful leptogenesis. The lepton flavor violating process mu to e gamma can be enhanced due to the Majorana phase, so that it can be detectable at forthcoming experiments. The processes tau to e gamma and tau to mu gamma are suppressed because of the structure of neutrino Dirac masses, and their branching ratios are smaller than that of mu to e gamma.

hep-ph↗
arXiv2005

Measurement of lepton flavor violating Yukawa couplings at ILC

We discuss lepton flavor violation (LFV) associated with tau leptons in the general framework of the two Higgs doublet model, in which LFV couplings are introduced as a deviation from Model II Yukawa interaction. Parameters of the model are constrained from experimental results and also from requirements of theoretical consistencies such as vacuum stability and perturbative unitarity. Current data for LFV rare tau decays provide substantial upper limits on the LFV Yukawa couplings in the large $\tanβ$ region, which are comparable with predictions in fundamental theories. Here $\tanβ$ is the ratio of vacuum expectation values of the two Higgs doublets. A search for the LFV decays $ϕ^{0} \to τ^\pm μ^\mp$ $(τ^\pm e^\mp)$ of neutral Higgs bosons ($ϕ^{0} =h,H$ and $A$) at future collider experiments can be useful to further constrain the LFV couplings, especially in the relatively small $\tanβ$ region ($\tanβ\lesssim 30 $), where rare tau decay data cannot give any strong limit.

hep-ph↗
arXiv2005

Phase effects from the general neutrino Yukawa matrix on lepton flavor violation

We examine contributions from Majorana phases to lepton flavor violating processes in the framework of the minimal supersymmetric standard model with heavy right-handed neutrinos. All phases in the complex neutrino Yukawa matrix are taken into account in our study. We find that in the scenario with universal soft-breaking terms sizable phase effects can appear on the lepton flavor violating processes such as $μ\to e γ$, $τ\to e γ$, and $τ\to μγ$. In particular, the branching ratio of $μ\to e γ$ can be considerably enhanced due to the Majorana phases, so that it can be much greater than that of $τ\to μγ$.

hep-ph↗
arXiv2005

Lepton Flavor Violation in a Long Baseline Experiment

We evaluate the size of a coupling for a lepton flavor violating interaction with neutrinos (nLFV) in the minimal supersymmetric standard model with right-handed neutrinos (MSSMRN). It is expected that the nLFV interactions are detected in a long baseline neutrino oscillation experiment. We here deal with $μ^{-} \to e^{-} \barν_{e} ν_τ$ in $ν_μ \to ν_τ$ channel at a neutrino factory. First, we show the numerical study on feasibility to observe such interaction and point out that we can search for them with some oscillation channels. Next, we make a numerical calculation for the size of the nLFV coupling in the framework of the MSSMRN. A long baseline experiment is sensitive not only to the lepton mixings in the standard model but also to new physics effect.

hep-ph↗
arXiv2005

Signature of the Minimal Supersymmetric Standard Model with Right-Handed Neutrinos in Long Baseline Experiments

The effective interactions which violate a lepton flavor accompanied with neutrinos (nLFV) are considered. Such a new physics effect is expected to be measured in future neutrino oscillation experiments with long baseline. They are induced by radiative correction in the framework of the minimal supersymmetric standard model with right-handed neutrinos. We numerically evaluate the size of the couplings for nLFV interactions in this framework. The slepton mixing is not only the origin of the lepton flavor violation in the charged lepton sector (cLFV) but also that of the nLFV. We find that the nLFV couplings are strongly correlated with the corresponding cLFV process, and they are constrained at $\mathcal{O}(10^{-5})$ times smaller than the standard four-Fermi couplings.

hep-ph↗
arXiv2004

A Study of Lepton Flavor Violating $μN (e N) \to τX$ Reactions in Supersymmetric Models

We study a lepton flavor violating $μN \to τX$ reaction in deep inelastic scattering region in supersymmetric models. The contribution from the Higgs boson mediation could be important for this reaction. For that case, the cross section is constrained by the experimental limit of the pseudo-scalar coupling from $τ\to μη$ decays. We find that at a muon energy ($E_μ$) higher than 50 GeV, the predicted cross section increases significantly due to the contribution from sea $b$-quarks. As a result, with $10^{20}$ muons per year, at most a number of $\mathcal{O}(10^4)$ is expected for $μN \to τX$ events at $E_μ$= 300 GeV, whereas $\mathcal{O}(10^2)$ events are given at $E_μ= 50$ GeV. Furthermore, the $μN \to τX$ phenomenology, in particular that for the signal and backgrounds, is briefly discussed. Another promising possibility to search for the $e N \to τX$ reaction at an electron-positron linear collider is also discussed. Searches for these reactions would be competitive to studies of rare tau decays and have potential to improve sensitivities to lepton flavor violation significantly.

hep-ph↗
arXiv2004

Lepton Flavor Violation in the Higgs Boson Decay at a Linear Collider

We study possibility of observing the process $h^{0} \to τ^{\pm} μ^{\mp}$ at a linear collider. The branching ratio is constrained to be of the order of $10^{-4}$ by the $τ^{-} \to μ^{-} η$ result. Supersymmetric standard models can reproduce such amount of the branching ratio by taking a specific parameter set. The Higgsstrahlung process $e^{+}e^{-} \to Zh^{0}$ is preferable because of its simple kinematic structure, then, the signal process is $e^{+} e^{-} \to Z h^{0} \to Z τ^{\pm} μ^{\mp}$. The most serious background comes from the process, $e^{+} e^{-} \to Zh^{0} \to Zτ^{\pm} τ^{\mp} \to τ^{\pm} μ^{\mp} ν\barν$. We estimate the significance of the signal, taking into account the background reduction.

hep-ph↗
arXiv2004

Search for Lepton Flavor Violation in the Higgs Boson Decay at a Linear Collider

We discuss possibility of direct search for lepton flavor violation (LFV) in Yukawa interaction by measuring the branching ratio for the decay of the lightest Higgs boson ($h^0$) into a $τ$-$μ$ pair at a linear collider. We study the significance of the signal process, $e^+e^- \to Z^\ast \to Z h^0 \to Z τ^\pm μ^\mp$, against the backgrounds such as $e^{+}e^{-} \to Z τ^{+}τ^{-} \to Z τ^{\pm}μ^{\mp}+$ missings. After taking appropriate kinematic cuts, the number of the background event is considerably reduced, so that the signal can be visible when the branching ratio of $h^0 \to τ^\pm μ^\mp$ is larger than about $10^{-4}$. In a Minimal Supersymmetric Standard Model scenario, the effective coupling of $h^0 τ^\pm μ^\mp$ can be generated at loop level due to the slepton mixing. When supersymmetric mass parameters are larger than TeV scales, the branching ratio can be as large as several times $10^{-4}$. Therefore, the signal can be marginally visible at a LC. In the general two Higgs doublet model, the possible maximal value for the branching ratio of $h^0 \to τ^\pm μ^\mp$ can reach to a few times $10^{-3}$ within the available experimental bound, so that we can obtain larger significance.

hep-ph↗
arXiv2002

Yet another correlation in the analysis of CP violation using a neutrino oscillation experiment

We investigate the effect induced by variations in the density profile of the Earth's interior using a long-baseline neutrino oscillation experiment. At first, we point out two facts. (i) The most essential part of the matter profile is the first Fourier mode of the matter profile function; and (ii) The Earth models based on the study of seismology include a large uncertainty for the first Fourier mode. Next, we show that there is a strong correlation between the average density value and the first Fourier coefficient in the analysis of oscillation probability. This means that the matter profile effect induces added uncertainty for the average matter parameter. %although the average density can be determined at a few percent accuracy. Taking into account this extra uncertainty, we make numerical calculations for the sensitivity to CP violation search and show that CP sensitivity is impaired by this added uncertainty within a large $U_{e3}$ region.

hep-ph↗
arXiv2001

Oscillation enhanced search for new interaction with neutrinos

We discuss the measurement of new physics in long baseline neutrino oscillation experiments. Through the neutrino oscillation, the probability to detect the new physics effects such as flavor violation is enhanced by the interference with the weak interaction. We carefully explain the situations that the interference can take place. Assuming a neutrino factory and an upgraded conventional beam, we estimate the feasibility to observe new physics numerically and point out that we can search new interactions using some channels, for example $ν_μ \to ν_μ$, in these experiments. We also discuss several models which induce the effective interactions interfering with the weak interaction, and show that some new physics effects are large enough to be observed in the oscillation enhanced way.

hep-ph↗