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Joe Sato

Publications and source records attributed to Joe Sato.

At least 37 records · Page 2Linked to original sources
arXiv2017

Detecting the $L_μ-L_τ$ gauge boson at Belle II

We discuss the feasibility of detecting the gauge boson of the $U(1)_{L_μ-L_τ}$ symmetry, which possesses a mass in the range between MeV and GeV, at the Belle-II experiment. The kinetic mixing between the new gauge boson $Z'$ and photon is forbidden at the tree level and is radiatively induced. The leptonic force mediated by such a light boson is motivated by the discrepancy in muon anomalous magnetic moment and also the gap in the energy spectrum of cosmic neutrino. Defining the process $e^{+} e^{-} \rightarrow γZ' \rightarrow γν\barν~(missing~energy)$ to be the signal, we estimate the numbers of the signal and the background events and show the parameter region to which the Belle-II experiment will be sensitive. The signal process in the $L_μ-L_τ$ model is enhanced with a light $Z'$, which is a characteristic feature differing from the dark photon models with a constant kinetic mixing. We find that the Belle-II experiment with the design luminosity will be sensitive to the $Z'$ with the mass $M_{Z'} \lesssim 1 $ GeV and the new gauge coupling $g_{Z'} \gtrsim 8\cdot 10^{-4}$, which covers a half of the unconstrained parameter region that explains the discrepancy in muon anomalous magnetic moment. The possibilities to improve the significance of the detection are also discussed.

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↗
arXiv2016

Parametric Resonance in Neutrino Oscillation: A Guide to Control the Effects of Inhomogeneous Matter Density

Effects of the inhomogeneous matter density on the three-generation neutrino oscillation probability are analyzed. Realistic profile of the matter density is expanded into a Fourier series. Taking in the Fourier modes one by one, we demonstrate that each mode has its corresponding target energy. The high Fourier mode selectively modifies the oscillation probability of the low-energy region. This rule is well described by the parametric resonance between the neutrino oscillation and the matter effect. The Fourier analysis gives a simple guideline to systematically control the uncertainty of the oscillation probability caused by the uncertain density of matter. Precise analysis of the oscillation probability down to the low-energy region requires accurate evaluation of the Fourier coefficients of the matter density up to the corresponding high modes.

hep-ph↗
arXiv2016

Improved analyses for $μ^-e^-\rightarrow e^-e^-$ in muonic atoms by contact interactions

The charged lepton flavor violating (CLFV) processes of $μ^-e^-\rightarrow e^-e^-$ decay by four Fermi contact interactions in a muonic atom for various atoms are investigated. The wave functions of bound and scattering state leptons are properly treated by solving Dirac equations with Coulomb interaction of the finite nuclear charge distributions. This new effect contributes significantly in particular for heavier atoms, where the obtained decay rate is about one order of magnitude larger than the previous estimation for $^{208}$Pb. We find that, as the atomic number $Z$ increases, the $μ^-e^-\rightarrow e^-e^-$ decay rates increase more rapidly than the result of the previous work of $Z^3$, suggesting this decay as one of the promising processes to search for CLFV interaction.

hep-ph↗
arXiv2016

MeV scale leptonic force for cosmic neutrino spectrum and muon anomalous magnetic moment

Characteristic patterns of cosmic neutrino spectrum reported by the IceCube collaboration and long-standing inconsistency between theory and experiment in muon anomalous magnetic moment are simultaneously explained by an extra leptonic force mediated by a gauge field with a mass of the MeV scale. With different assumptions for redshift distribution of cosmic neutrino sources, diffuse neutrino flux is calculated with the scattering between cosmic neutrino and cosmic neutrino background through the new leptonic force. Our analysis sheds light on a relation among lepton physics at the three different scales, PeV, MeV, and eV, and provides possible clues to the distribution of sources of cosmic neutrino and also to neutrino mass spectrum.

hep-ph↗
arXiv2015

Improved analysis of the CLFV decay of muonic atoms $μ^-e^-\rightarrow e^-e^-$

Koike et al. proposed the charged lepton flavor violation (CLFV) decay of the muonic atom $μ^-e^-\rightarrow e^-e^-$ as one of the promising processes to search for new physics beyond the standard model. It was found that the attractive interaction of leptons with the nucleus enhances the transition rate of the $μ^-e^-\rightarrow e^-e^-$ process. We report on our improved analysis of this process by taking into account the distortion of the out-going electrons in the nuclear Coulomb potential and the relativistic treatment of the muon and the electrons. As results, we found significant enhancement of the transition rate. The transition rate for $^{208}$Pb becomes about 7 times larger than the previous estimation, which enhances the sensitivity of this process to discover the CLFV interaction. We also report on the energy spectrum of the out-going electron.

hep-ph↗
arXiv2014

Mind the gap on Icecube: Cosmic neutrino spectrum and muon anomalous magnetic moment in the gauged L_μ - L_τ model

The energy spectrum of cosmic neutrinos, which was recently reported by the IceCube collaboration, shows a gap between 400 TeV and 1 PeV. An unknown neutrino interaction mediated by a field with a mass of the MeV scale is one of the possible solutions to this gap. We examine if the leptonic gauge interaction L_μ - L_τ can simultaneously explain the two phenomena in the lepton sector: the gap in the cosmic neutrino spectrum and the unsettled disagreement in muon anomalous magnetic moment. We illustrate that there remains the regions in the model parameter space, which account for both the problems. Our results also provide a hint for the distance to the source of the high-energy cosmic neutrinos.

hep-ph↗
arXiv2014

A way to crosscheck $μ$-$e$ conversion in the case of no signals of $μ\to e γ$ and $μ\to 3e$

We consider the case that $μ$-$e$ conversion signal is discovered but other charged lepton flavor violating (cLFV) processes will never be found. In such a case, we need other approaches to confirm the $μ$-$e$ conversion and its underlying physics without conventional cLFV searches. We study R-parity violating (RPV) SUSY models as a benchmark. We briefly review that our interesting case is realized in RPV SUSY models with reasonable settings according to current theoretical/experimental status. We focus on the exotic collider signatures at the LHC ($pp \to μ^- e^+$ and $pp \to jj$) as the other approaches. We show the correlations between the branching ratio of $μ$-$e$ conversion process and cross sections of these processes. It is first time that the correlations are graphically shown. We exhibit the RPV parameter dependence of the branching ratio and the cross sections, and discuss the feasibility to determine the parameters.

hep-ph↗
arXiv2014

One loop radiative correction to Kaluza-Klein masses in $S^2/Z_2$ universal extra dimensional model

We investigate a radiative correction to the masses of Kaluza-Klein(KK) modes in a universal extra dimensional model which are defined on a six-dimensional spacetime with extra space as a two-sphere orbifold $S^2/Z_2$. We first define the Feynman rules which are necessary for the calculation. We then calculate the one-loop diagrams which contribute to the radiative corrections to the KK masses, and obtain one-loop corrections to masses for fermions, gauge bosons and scalar bosons. We estimate the one-loop corrections to KK masses for the first KK modes of standard model particles as a function of momentum cut-off scale, and we determine the lightest KK particle which would be a promising candidate of a dark matter.

hep-ph↗
arXiv2014

Big-bang nucleosynthesis through bound-state effects with a long-lived slepton in the NMSSM

We show that the Li problems can be solved in the next-to-minimal supersymmetric standard model where the slepton as the next-to-lightest SUSY particle is very long-lived. Such a long-lived slepton induces exotic nuclear reactions in big-bang nucleosynthesis, and destroys and produces the $^7$Li and $^6$Li nuclei via bound state formation. We study cases where the lightest SUSY particle is singlino-like neutralino and bino-like neutralino to present allowed regions in the parameter space which is consistent with the observations on the dark matter and the Higgs mass.

hep-ph↗
arXiv2013

A first evidence of the CMSSM is appearing soon

We explore the coannihilation region of the constrained minimal supersymmetric standard model (CMSSM) being consistent with current experimental/observational results. The requirements from the experimental/observational results are the 125GeV Higgs mass and the relic abundances of both the dark matter and light elements, especially the lithium-7. We put these requirements on the caluculated values, and thus we obtain allowed region. Then we give predictions to the mass spectra of the SUSY particles, the anomalous magnetic moment of muon, branching fractions of the B-meson rare decays, the direct detection of the neutralino dark matter, and the number of SUSY particles produced in 14TeV run at the LHC experiment. Comparing these predictions with current bounds, we show the feasibility of the test for this scenario in near future experiment.

hep-ph↗
arXiv2013

Updated results for minimal dilaton model

Both the ATLAS and CMS experiments at the LHC have reported the observation of the particle of mass around 125GeV which is consistent to the Standard Model (SM) Higgs boson, but with an excess of events beyond the SM expectation in the diphoton decay channel at each of them. There still remains room for a logical possibility that we are not seeing the SM Higgs but something else. Here we introduce the minimal dilaton model in which the LHC signals are explained by an extra singlet scalar of the mass around 125GeV that slightly mixes with the SM Higgs heavier than 600GeV. When this scalar has a vacuum expectation value well beyond the electroweak scale, it can be identified as a linearly realized version of a dilaton field. Though the current experimental constraints from the Higgs search disfavors such a region, the singlet scalar model itself still provides a viable alternative to the SM Higgs in interpreting its search results.

hep-ph↗
arXiv2013

Minimal Dilaton Model

We construct a minimal calculable model of a light dilaton based on the scenario where only top and Higgs sectors are involved in a quasiconformal dynamics. The model consistently accommodates the electroweak precision tests even when the Higgs boson is very heavy, thereby allowing one to consider the possibility that the particle at around 125GeV, discovered at the LHC experiments, is identified as the light dilaton rather than the Higgs boson. We find that the current LHC data allow distinct parameter regions where the observed particle is either mostly the Higgs boson or the dilaton.

hep-ph↗
arXiv2012

Big-bang nucleosynthesis with a long-lived CHAMP including He4 spallation process

We propose helium-4 spallation processes induced by long-lived stau in supersymmetric standard models, and investigate an impact of the processes on light elements abundances. We show that, as long as the phase space of helium-4 spallation processes is open, they are more important than stau-catalyzed fusion and hence constrain the stau property. This talk is based on works (Jittoh et al., 2011).

hep-ph↗
arXiv2012

Allowed slepton intergenerational mixing in light of light element abundances

We studied allowed region on the intergenerational mixing parameters of sleptons from a viewpoint of big-bang nucleosynthesis in a slepton-neutralino coannihilation scenario. In this scenario, $^7$Li and $^6$Li problems can be solved by considering exotic reactions caused by bound-state effects with a long-lived slepton. Light element abundances are calculated as functions of the relic density and lifetime of the slepton which considerably depend on the intergenerational mixing parameters. Compared with observational light element abundances, we obtain allowed regions on the intergenerational mixing. Ratio of selectron component to stau component, $c_e$, is allowed in $2\times 10^{-11} \lesssim c_e \lesssim 2\times 10^{-9}$ with solving both the $^7$Li and $^6$Li problems. Similarly, the ratio for smuon, $c_μ$, is allowed in $%2\times 10^{-7} \lesssim c_μ \lesssim 5\times 10^{-5}$ for mass difference between slepton and neutralino, which is smaller than muon mass, and $ %10^{-11}\lesssim c_μ \lesssim 2\times 10^{-10}$ for the mass difference in range between muon mass and 125 MeV. We also discuss collider signatures of the slepton decays. We find characteristic double peaks in momentum distribution of event number of the slepton decays with allowed mixing parameters. Discoveries of the double peaks at future collider experiments should confirm our scenario.

hep-ph↗
arXiv2011

Gauge-Higgs unification models in six dimensions with $S^2/Z_2$ extra space and GUT gauge symmetry

In this article, we review gauge-Higgs unification models based on gauge theories defined on six-dimensional spacetime with $S^2/Z_2$ topology in the extra spatial dimensions. On the extra $S^2/Z_2$ space, non-trivial boundary conditions are imposed. This review considers two scenarios for constructing a four-dimensional theory from the six-dimensional model. One scheme utilizes the SO(12) gauge symmetry with a special symmetry condition imposed on the gauge field, whereas the other employs the E$_6$ gauge symmetry without requiring the additional symmetry condition. Both models lead to a Standard Model-like gauge theory with the SU(3) $\times$ SU(2)$_L$ $\times$ U(1)$_Y$($\times$ U(1)$^2$) symmetry and SM fermions in four dimensions. The Higgs sector of the model is also analyzed. The electroweak symmetry breaking can be realized, and the weak gauge boson and Higgs boson masses are obtained.

hep-ph↗
arXiv2011

Correlation between flavour violating decay of long-lived slepton and tau in the coannihilation scenario with Seesaw mechanism

We investigate flavour violating decays of the long-lived lightest slepton and the tau lepton in the coannihilation region of the Minimal Supersymmetric Standard Model with a Seesaw mechanism to generate neutrino masses. We consider a situation where the mass difference between the lightest neutralino, as the Lightest Supersymmetric particle (LSP), and the lightest slepton, as the Next-to-LSP, is smaller than the mass of tau lepton. In this situation, the lifetime of the lightest slepton is very long and it is determined by lepton flavour violating (LFV) couplings because the slepton mainly consists of the lighter stau and the flavour conserving 2-body decay is kinematically forbidden. We show that the lifetime can change many orders of magnitude by varying the Yukawa couplings entering the Seesaw mechanism. We also show that branching ratio of LFV tau decays are strongly correlated with the lightest slepton lifetime. Therefore the branching ratios of LFV tau decays can be determined or constrained by measuring the slepton lifetime at the LHC experiment.

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