arXiv subjects

Joe Sato

Publications and source records attributed to Joe Sato.

At least 55 records · Page 3Linked to original sources

arXiv2010

A new idea to search for charged lepton flavor violation using a muonic atom

We propose a new process of mu(-) e(-) -> e(-) e(-) in a muonic atom for a quest of charged lepton flavor violation. The Coulomb attraction from the nucleus in a heavy muonic atom leads to significant enhancement in its rate, compared to mu(+) e(-) -> e(+) e(-). The upper limit of the branching ratio is estimated to be of the orders of O[10^(-17)-10^(-18)] for the photonic and the four Fermi interactions from the present experimental constraints. The search for this process could serve complementarily with the other relevant processes to shed lights upon the nature of charged lepton flavor violation.

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arXiv2010

Stau relic density at the Big-Bang nucleosynthesis era consistent with the abundance of the light element nuclei in the coannihilation scenario

We calculate the relic density of stau at the beginning of the Big-Bang Nucleosynthesis (BBN) era in the coannihilation scenario of minimal supersymmetric standard model (MSSM). In this scenario, stau can be long-lived and form bound states with nuclei. We put constraints on the parameter space of MSSM by connecting the calculation of the relic density of stau to the observation of the light elements abundance, which strongly depends on the relic density of stau. Consistency between the theoretical prediction and the observational result, both of the dark matter abundance and the light elements abundance, requires the mass difference between the lighter stau and the lightest neutralino to be around 100MeV, the stau mass to be 300 -- 400 GeV, and the mixing angle of the left and right-handed staus to be $\sinθ_τ = (0.65 \textrm{--} 1)$.

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arXiv2010

The Universal Extra Dimensional Model with $S^2/Z_2$ extra-space

We propose a new Universal Extra Dimensional model that is defined on a six-dimensional spacetime which has a two-sphere orbifold $S^2/Z_2$ as an extra-space. We specify our model by choosing the gauge symmetry as SU(3)$\timesSU(2)\timesU(1)_Y \timesU(1)_X$, introducing field contents in six-dimensions as their zero modes correspond to the Standard model particles, and determining a boundary condition of these fields on orbifold $S^2/Z_2$. A background gauge field that belongs to U(1)$_X$ is introduced there, which is necessary to obtain massless chiral fermions in four-dimensional spacetime. We then analyze Kaluza-Klein(KK) mode expansion of the fields in our model and derive the mass spectrum of the KK particles. We find that the lightest KK particles are the 1st KK particle of massless gauge bosons at tree level. We also discuss the KK parity of the KK modes in our model and confirm the stability of the lightest KK particle which is important for dark matter physics.

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arXiv2009

Higgs Production via Gluon Fusion in a Six Dimensional Universal Extra Dimension Model on S^2/Z_2

We investigate Higgs boson production process via gluon fusion at LHC in our six dimensional universal extra dimension model compactified on a spherical orbifold S^2/Z_2. We find a striking result that Higgs production cross section in our model is predicted to be 30(10)% enhancement comparing to the predictions of the Standard Model (the minimal universal extra dimension model) for the compactification scale of order 1 TeV.

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arXiv2009

Productions of second Kaluza-Klein gauge bosons in the minimal universal extra dimension model at LHC

We calculate the production rates of the second Kaluza-Klein (KK) photon $γ^{(2)}$ and Z boson $Z^{(2)}$ at the LHC including all significant processes in the minimal Universal Extra Dimension (MUED) model. For discrimination of the MUED model from other TeV scale models at the LHC, $γ^{(2)}$ and $Z^{(2)}$ play a crucial role. In order to discuss the discrimination and calculate their production rates, we derive KK number violating operators including the contribution of the top Yukawa coupling. Using these operators, we accurately calculate branching ratios of second KK particles. In addition we find that these KK number violating operators provide new processes for $γ^{(2)}$ and $Z^{(2)}$ productions, such as cascade decay from second KK quarks produced through these operators. They have large contributions to their total production rates. In particular, these production processes give the dominant contribution for $γ^{(2)}$ production for $1/R \gtrsim 800$ GeV. As a result, with an integrated luminosity of 100 fb$^{-1}$, the number of produced $γ^{(2)}$ and $Z^{(2)}$ are estimated as 10$^6$ - 10$^2$ for the compactification scale between 400 GeV and 2000 GeV.

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arXiv2009

Model building by coset space dimensional reduction in ten-dimensions with direct product gauge symmetry

We investigate ten-dimensional gauge theories whose extra six-dimensional space is a compact coset space, $S/R$, and gauge group is a direct product of two Lie groups. We list up candidates of the gauge group and embeddings of $R$ into them. After dimensional reduction of the coset space,we find fermion and scalar representations of $G_{\mathrm{GUT}} \times U(1)$ with $G_{\mathrm{GUT}}=SU(5), SO(10)$ and $E_6$ which accomodate all of the standard model particles. We also discuss possibilities to generate distinct Yukawa couplings among the generations using representations with a different dimension for $G_{\mathrm{GUT}}=SO(10)$ and $E_6$ models.

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arXiv2009

Model building by Coset space dimensional reduction scheme using ten-dimensional coset spaces

We investigate the gauge-Higgs unification models within the scheme of the coset space dimensional reduction, beginning with a gauge theory in a fourteen-dimensional spacetime where extra-dimensional space has the structure of a ten-dimensional compact coset space. We found seventeen phenomenologically acceptable models through an exhaustive search for the candidates of the coset spaces, the gauge group in fourteen dimension, and fermion representation. Of the seventeen, ten models led to $\mathrm{SO}(10) (\times \mathrm{U}(1))$ GUT-like models after dimensional reduction, three models led to $\mathrm{SU}(5) \times \mathrm{U}(1)$ GUT-like models, and four to $\mathrm{SU}(3) \times \mathrm{SU}(2) \times \mathrm{U}(1) \times \mathrm{U}(1)$ Standard-Model-like models. The combinations of the coset space, the gauge group in the fourteen-dimensional spacetime, and the representation of the fermion contents of such models are listed.

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arXiv2009

Production Rate of Second KK Gauge Bosons in UED Models at LHC

We calculate the production rates of the second Kaluza-Klein (KK) photon $γ^{(2)}$ and Z boson $Z^{(2)}$ at the LHC including all significant processes in the minimal universal extra dimension (MUED) model. For discrimination of the MUED model from other TeV scale models in hadron collider experiments, $γ^{(2)}$ and $Z^{(2)}$ play a crucial role. In order to discuss the discrimination and calculate their production rates, we derive effective Lagrangian containing KK number violating operators. We show that KK number violating processes are extremely important for the compactification scale larger than 800 GeV. We find that, with an integrated luminosity of 100 fb$^{-1}$, $γ^{(2)}$ and $Z^{(2)}$ are produced 10$^6$ - 10$^2$ for the compactification scale between 400 GeV and 2000 GeV.

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arXiv2009

Model building by coset space dimensional reduction in eight-dimensions

We investigate gauge-Higgs unification models in eight-dimensional spacetime where extra-dimensional space has the structure of a four-dimensional compact coset space. The combinations of the coset space and the gauge group in the eight-dimensional spacetime of such models are listed. After the dimensional reduction of the coset space, we identified $\mathrm{SO}(10)$, $\mathrm{SO}(10) \times \mathrm{U}(1)$ and $\mathrm{SO}(10) \times \mathrm{U}(1) \times \mathrm{U}(1)$ as the possible gauge groups in the four-dimensional theory that can accomodate the Standard Model and thus is phenomenologically promising. Representations for fermions and scalars for these gauge groups are tabulated.

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arXiv2009

Standard(-like) Model from an SO(12) Grand Unified Theory in six-dimensions with $S_2$ extra-space

We analyze a gauge-Higgs unification model which is based on a gauge theory defined on a six-dimensional spacetime with an $S^2$ extra-space. We impose a symmetry condition for a gauge field and non-trivial boundary conditions of the $S^2$. We provide the scheme for constructing a four-dimensional theory from the six-dimensional gauge theory under these conditions. We then construct a concrete model based on an SO(12) gauge theory with fermions which lie in a 32 representation of SO(12), under the scheme. This model leads to a Standard-Model(-like) gauge theory which has gauge symmetry SU(3) $\times$ SU(2)$_L$ $\times$ U(1)$_Y$($\times$ U(1)$^2$) and one generation of SM fermions, in four-dimensions. The Higgs sector of the model is also analyzed, and it is shown that the electroweak symmetry breaking and the prediction of W-boson and Higgs-boson masses are obtained.

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arXiv2009

Fourier Analysis of the Parametric Resonance in Neutrino Oscillations

Parametric enhancement of the appearance probability of the neutrino oscillation under the inhomogeneous matter is studied. Fourier expansion of the matter density profile leads to a simple resonance condition and manifests that each Fourier mode modifies the energy spectrum of oscillation probability at around the corresponding energy; below the MSW resonance energy, a large-scale variation modifies the spectrum in high energies while a small-scale one does in low energies. In contrast to the simple parametric resonance, the enhancement of the oscillation probability is itself an slow oscillation as demonstrated by a numerical analysis with a single Fourier mode of the matter density. We derive an analytic solution to the evolution equation on the resonance energy, including the expression of frequency of the slow oscillation.

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arXiv2008

Measuring Lepton Flavour Violation at LHC with Long-Lived Slepton in the Coannihilation Region

When the mass difference between the lightest slepton, the NLSP, and the lightest neutralino, the LSP, is smaller than the tau mass, the lifetime of the lightest slepton increases in many orders of magnitude with respect to typical lifetimes of other supersymmetric particles. These small mass differences are possible in the MSSM and, for instance, they correspond to the coannihilation region of the CMSSM for $M_{1/2} \gsim 700$ GeV. In a general gravity-mediated MSSM, where the lightest supersymmetric particle is the neutralino, the lifetime of the lightest slepton is inversely proportional to the square of the intergenerational mixing in the slepton mass matrices. Such a long-lived slepton would produce a distinctive signature at LHC and a measurement of its lifetime would be relatively simple. Therefore, the long-lived slepton scenario offers an excellent opportunity to study lepton flavour violation at ATLAS and CMS detectors in the LHC and an improvement of the leptonic mass insertion bounds by more than five orders of magnitude would be possible.

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arXiv2008

Fourier Analysis of the Parametric Resonance of the Neutrino Oscillation in the Presence of Inhomogeneous Matter

We study the parametric resonance of the neutrino oscillation through the matter whose density varies spatially. The Fourier analysis of the matter effect enables us to clarify the parametric resonance condition, which is summarized in a frequency matching between the neutrino oscillation and the spatial variation of the matter density. As a result, the n-th Fourier mode of a matter density profile modifies the energy spectrum of the nu_mu -> nu_e appearance probability at around the n-th dip.

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arXiv2008

Big-bang nucleosynthesis and the relic abundance of dark matter in a stau-neutralino coannihilation scenario

A scenario of the Big-Bang Nucleosynthesis is analyzed within the Minimal Supersymmetric Standard Model which is consistent with a stau-neutralino coannihilation scenario to explain the relic abundance of dark matter. We find that we can account for the possible descrepancy of the abundance of $\mathrm{^{7}Li}$ between the observation and the prediction of the Big-Bang Nucleosynthesis by taking the mass of the neutralino as $300 \mathrm{GeV}$ and the mass difference between the stau and the neutralino as $(100 -- 120) MeV$. We can therefore simultaneously explain the abundance of the dark matter and that of $\mathrm{^{7}Li}$ by these values of parameters. The lifetime of staus in this scenario is predicted to be $O(100 -- 1000) sec$.

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arXiv2008

Universal Extra Dimension models with right-handed neutrinos

Relic abundance of dark matter is investigated in the framework of universal extra dimension (UED) models with right-handed neutrinos. These models are free from the KK graviton problem in the minimal UED model. The first KK particle of the right-handed neutrino is a dark matter candidate in this framework. When ordinary neutrino masses are large enough such as the degenerate mass spectrum case, the dark matter relic abundance can increase significantly. The scale of the extra dimension consistent with cosmological observations can be 500 GeV in the minimal setup of UED models with right-handed neutrinos.

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arXiv2008

Non-standard neutrino interactions in reactor and superbeam experiments

The formalism of non-standard four-fermion interactions provides a convenient, model-independent way of parameterizing a wide class of ``new physics'' scenarios. In this article, we study the performance of reactor and superbeam neutrino experiments in the presence of such non-standard interactions (NSI). Due to interference between the standard and non-standard amplitudes, sizeable effects are to be expected if the NSI parameters are close to their current upper limits. We derive approximate formulas for the relevant oscillation probabilities including NSI, and show how the leading effects can be understood intuitively even without any calculations. We will present a classification of all possible NSI according to their impact on reactor and superbeam experiments, and it will turn out that these experiments are highly complementary in terms of their sensitivity to the non-standard parameters. The second part of the paper is devoted to detailed numerical simulations, which will demonstrate how a standard oscillation fit of the mixing angle theta-13 may fail if experimental data is affected by NSI. We find that for some non-standard terms, reactor and superbeam experiments would yield seemingly conflicting results, while in other cases, they may agree well with each other, but the resulting value for theta-13 could be far from the true value. This offset may be so large that the true theta-13 is even ruled out erroneously. In the last section of the paper, we demonstrate that reactor and superbeam data can actually establish the presence of non-standard interactions. Throughout our discussion, we pay special attention to the impact of the complex phases, and of the near detectors.

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arXiv2007

Possible solution to the $^7$Li problem by the long lived stau

Modification of standard big-bang nucleosynthesis is considered in the minimal supersymmetric standard model to resolve the excessive theoretical prediction of the abundance of primordial lithium 7. We focus on the stau as a next-lightest superparticle, which is long lived due to its small mass difference with the lightest superparticle. It provides a number of additional decay processes of $\mathrm{^{7}Li}$ and $\mathrm{^{7}Be}$. A particularly important process is the internal conversion in the stau-nucleus bound state, which destroys the $\mathrm{^{7}Li}$ and $\mathrm{^{7}Be}$ effectively. We show that the modification can lead to a prediction consistent with the observed abundance of $\mathrm{^{7}Li}$.

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arXiv2007

The neutrino masses and the change of allowed parameter region in universal extra dimension models

Relic abundance of dark matter is investigated in the framework of universal extra dimension models with right-handed neutrinos. These models are free from the serious Kaluza-Klein (KK) graviton problem that the original universal extra dimension model possesses. The first KK particle of the right-handed neutrino is a candidate for dark matter in this framework. When ordinary neutrino masses are large enough such as the degenerate mass spectrum case, the dark matter relic abundance can change significantly. The scale of the extra dimension consistent with cosmological observations can be 500 GeV in the minimal setup of universal extra dimension models with right-handed neutrinos.

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