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Joe Sato

Publications and source records attributed to Joe Sato.

At least 19 recordsLinked to original sources
arXiv2026

Impact of matter effects on the unitarity test of lepton mixing

Testing the unitarity of the lepton mixing matrix, in a manner analogous to the unitarity tests of the CKM matrix in the quark sector, is an important step toward probing physics beyond the standard three-generation framework. In long baseline neutrino oscillation experiments, the formula of the oscillation probabilities can be written as a sum of terms with various combinations of the mixing-matrix elements, and their coefficients depend differently on energy. By observing the spectral information of long baseline experiments such as T2HK and a future neutrino factory at J-PARC with a $\nu_e$ beam, the elements of the mixing matrix can be extracted without assuming a specific parametrization of the mixing matrix. We investigate how such an extraction method can be applied to neutrino oscillations by taking into account matter effects, and discuss how one can test unitarity of the mixing matrix in future long baseline experiments. As a concrete example, we examine the unitarity test by using a four-generation model, where we look at a quantity which should be vanishing in a unitary model. Among possible combinations of measurements, the most powerful test can be provided from the energy spectra of the CP-conjugate appearance channels $\nu_\mu \to \nu_e$ and $\bar{\nu}_\mu \to \bar{\nu}_e$ at T2HK, as well as from the T-conjugate pair $\nu_\mu \to \nu_e$ and $\nu_e \to \nu_\mu$ available at neutrino factories.

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arXiv2025

Unitarity test of lepton mixing via energy dependence of neutrino oscillation

We study the method to test the unitarity of the lepton mixing matrix by using only the long baseline neutrino oscillation experiments, such as the combination of the T2HK experiment and the one with the $\nu_e$ beam from a future neutrino factory at J-PARC. Without a specific parametrization, one can directly extract the elements of the lepton mixing matrix by observing the energy dependence of the oscillation probabilities. A non-trivial test of the unitarity under the three-generation assumption can thus be made possible by examining the orthogonality in a similar manner to the unitarity triangle in the quark sector. As the first trial, we perform the analysis based on the simplified situation where the matter effects in the neutrino oscillation can be neglected. Under this simplified analysis, we demonstrate the observation of the unitarity violation in the $3\times3$ part of the lepton mixing matrix for a parameter set in the four-generation model. The statistically most significant measurement can be provided by the energy dependences of the combination of the CP conjugate modes, $\nu_\mu \to \nu_e$ and $\bar \nu_\mu \to \bar \nu_e$, at T2HK and, independently, by the T conjugate modes, $\nu_\mu \to \nu_e$ and $\nu_e \to \nu_\mu$, with the latter measured at the neutrino factory experiments.

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arXiv2025

Gauge symmetry breaking with $S^2$ extra dimensions

We consider symmetry breaking of arbitrary gauge groups on a six-dimensional space-time which consists of a four-dimensional Minkowski space-time $M^4$ and a two-dimensional sphere $S^2$. We expand the gauge fields in the presence of a non-trivial background unique to $S^2$. We analyze Kaluza-Klein(KK) modes of the gauge fields and derive the mass spectrum of the KK modes. We found that the gauge fields (not) commuting with the background fields (do not) remain symmetry operators in four dimensions. We also discuss the mass spectrum of the extra-dimensional components of the gauge fields and identify a physical scalar $\phi$ and a Nambu-Goldstone mode $\chi$. As a result, we obtain a method to break gauge symmetry due to the nontrivial solution for gauge fields which is a unique feature of $S^2$.

hep-ph↗
arXiv2025

An Analytic Prescription for $t$-channel Singularities

The $t$-channel singularity is a divergence in the scattering amplitude which occurs when a stable particle propagating in $t$-channel scattering process becomes an on-shell state. Such situations appear either in the system of collider experiments or in the context of the cosmological particle production. No scheme which is generally applicable is known. In this work, we propose a new formulation to identify and remove the source of the divergence. The scheme is fully analytical and various applications can be expected. This work provides a valuable tool in this research field.

hep-ph↗
arXiv2024

T violation at a future neutrino factory (Contribution to the 25th International Workshop on Neutrinos from Accelerators)

We study the possibility of measuring T (time reversal) violation in a future long baseline neutrino oscillation experiment. By assuming a neutrino factory as a staging scenario of a muon collider at the J-PARC site, we find that the $\nu_e \to \nu_\mu$ oscillation probabilities can be measured with good accuracy at the Hyper-Kamiokande detector. By comparing with the probability of the time-reversal process, $\nu_\mu \to \nu_e$, measured at the T2K/T2HK, one can determine the CP phase $\delta$ in the neutrino mixing matrix if $|\sin(\delta)|$ is large enough. The determination of $\delta$ can be made with poor knowledge of the matter density of the earth as T violation is almost insensitive to the matter effects. The comparison of CP and T-violation measurements, {\it \`a la} the CPT theorem, provides us with a non-trivial check of the three neutrino paradigm based on the quantum field theory. This proceeding is based on JHEP 12 (2024), 014 [arXiv:hep-ph/2407.05807].

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arXiv2024

Extracting Signal Electron Trajectories in the COMET Phase-I Cylindrical Drift Chamber Using Deep Learning

We present a pioneering approach to tracking analysis within the COMET Phase-I experiment, which aims to search for the charged lepton flavor violating $\mu\to e$ conversion process in a muonic atom, at J-PARC, Japan. This paper specifically introduces the extraction of signal electron trajectories in the COMET Phase-I cylindrical drift chamber (CDC) amidst a high background hit rate, with more than $40\,\%$ occupancy of the total CDC cells, utilizing deep learning techniques of semantic segmentation. Our model achieved remarkable results, with a purity rate of $98\,\%$ and a retention rate of $90\,\%$ for CDC cells with signal hits, surpassing the design-goal performance of $90\,\%$ for both metrics. This study marks the initial application of deep learning to COMET tracking, paving the way for more advanced techniques in future research.

hep-ex↗
arXiv2024

T violation at a future neutrino factory

We study the possibility of measuring T (time reversal) violation in a future long baseline neutrino oscillation experiment. By assuming a neutrino factory as a staging scenario of a muon collider at the J-PARC site, we find that the ${\nu}_e \to {\nu}_{\mu}$ oscillation probabilities can be measured with a good accuracy at the Hyper-Kamiokande detector. By comparing with the probability of the time-reversal process, ${\nu}_{\mu} \to {\nu}_e$, measured at the T2K/T2HK experiments, one can determine the CP phase $\delta$ in the neutrino mixing matrix if $| \sin(\delta)|$ is large enough. The determination of $\delta$ can be made with poor knowledge of the matter density of the earth as T violation is almost insensitive to the matter effects. The comparison of CP and T-violation measurements, ${\it \`a\ la}$ the CPT theorem, provides us with a non-trivial check of the three neutrino paradigm based on the quantum field theory.

hep-ph↗
arXiv2023

Contribution of Majoron to Hubble tension in gauged U(1)$_{L_\mu-L_\tau}$ Model

In this paper, we analyze parameter regions that can alleviate the Hubble tension in the U(1)$_{L_\mu - L_\tau}$ model with the broken lepton number U(1)$_L$ symmetry. As new particles, this model has a U(1)$_{L_\mu - L_\tau}$ gauge boson $Z'$ and a Majoron $\phi$, which can affect the early universe and the effective number of neutrino species $N_{\rm eff}$. If $Z'$ and $\phi$ simultaneously exist in the early universe, $Z'\,$-$\,\phi$ interaction processes such as $Z'\nu_\alpha \leftrightarrow \phi\bar{\nu}_\beta$ occur. The comparison of $N_{\rm eff}$ between the cases with and without the $Z'\,$-$\,\phi$ interaction processes shows that these processes make a small contribution of $\mathcal{O}(10^{-4})$ to $N_{\rm eff}$, and it does not need to be considered for the alleviation of the Hubble tension. Based on these facts, we calculated $N_{\rm eff}$ for various Majoron parameters without the $Z'\,$-$\,\phi$ interaction processes to search parameters that could alleviate the Hubble tension. As a result, we found that the U(1)$_{L_\mu - L_\tau}$ gauge boson and Majoron can alleviate the Hubble tension in some parameter regions, and there is a non-trivial synergy contribution between $Z'$ and $\phi$. Moreover, the parameter region with a lighter mass $m_\phi \lesssim 2$ MeV and a larger coupling $\lambda \gtrsim 10^{-8}$ is excluded because it predicts too large $N_{\rm eff}$, i.e. $N_{\rm eff} \gtrsim 3.5$. The favored and restricted regions of the Majoron parameters depend on the $Z'$ parameters because of the presence of the $Z'$ contribution and synergy one.

hep-ph↗
arXiv2023

Model Building by Coset Space Dimensional Reduction Scheme Using Twelve-Dimensional Coset Spaces

We investigate the twelve-dimensional gauge-Higgs unification models with an eight-dimensional coset space. For each model, we apply the coset space dimensional reduction procedure and examine the particle contents of the resulting four-dimensional theory. Then, some twelve-dimensional SO(18) gauge theories lead to models of the SO(10)\times U(1) grand unified theory in four dimensions, where fermions of the Standard Model appear in multiple generations along with scalars that may break the electroweak symmetry. The representations of the obtained scalars and fermions are summarized.

hep-ph↗
arXiv2022

Asymmetric Mediator in Scotogenic Model

The scotogenic model is the Standard Model (SM) with Z_2 symmetry and the addition of Z_2 odd right-handed Majorana neutrinos and SU(2)_L doublet scalar fields. We have extended the original scotogenic model by an additional Z_2 odd singlet scalar field that plays a role in dark matter. In our model, the asymmetries of the lepton and Z_2 odd doublet scalar are simultaneously produced through CP-violating right-handed neutrino decays. While the former is converted into baryon asymmetry through the sphaleron process, the latter is relaid to the DM density through the decay of SU(2)_L doublet scalar that is named "asymmetric mediator". In this way, we provide an extended scotogenic model that predicts the energy densities of baryon and dark matter being in the same order of magnitude, and also explains the low-energy neutrino masses and mixing angles.

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arXiv2022

Determination of coupling patterns by parallel searches for $\mu^-\to e^+$ and $\mu^-\to e^-$ in muonic atoms

We investigate a possibility that the $\mu^-\to e^+$ conversion is discovered prior to the $\mu^-\to e^-$ conversion, and its implications to the new physics search. We focus on the specific model including the mixing of the $SU(2)_L$ doublet- and singlet-type scalar leptoquarks, which induces not only the lepton flavor violation but also the lepton number violation. Such a structure is motivated by R-parity violating (RPV) supersymmetric models, where a sbottom mediates the conversion processes. We formulate the $\mu^-\to e^+$ rate in analogy with the muon capture in a muonic atom, and numerically evaluate it using several target nuclei. The lepton flavor universality test of pion decay directly limits the $\mu^-\to e^+$ rate, and the maximally allowed $\mu^-\to e^+$ branching ratio is $\sim 10^{-18}$ under the various bounds on RPV parameters. We show that either $\mu^-\to e^-$ or $\mu^-\to e^+$ signals can be discovered in near future experiments. This indicates that parallel searches for these conversions will give us significant information on the pattern of coupling constants.

hep-ph↗
arXiv2021

Resolving the Hubble tension in a U(1)$_{L_\mu-L_\tau}$ model with Majoron

In this paper, we explore possibilities of resolving the Hubble tension and $(g-2)_{\mu}$ anomaly simultaneously in a U(1)$_{L_\mu - L_\tau}$ model with Majoron. We only focus on a case where the Majoron $\phi$ does not exist at the beginning of the universe and is created by neutrino inverse decay $\nu\nu\to \phi$ after electron-positron annihilation. In this case, contributions of the new gauge boson $Z'$ and Majoron $\phi$ to the effective number of neutrino species $N_{\rm eff}$ can be calculated in separate periods. These contribution are labelled $N'_{\rm eff}$ for the U(1)$_{L_\mu - L_\tau}$ gauge boson and $\Delta N_{\rm eff}^\prime$ for the Majoron. The effective number $N_{\rm eff} = N'_{\rm eff} + \Delta N_{\rm eff}^\prime$ is evaluated by the evolution equations of the temperatures and the chemical potentials of light particles in each period. As a result, we found that the heavier $Z'$ mass $m_{Z^\prime}$ results in the smaller $N_{\mathrm{eff}}^\prime$ and requires the larger $\Delta N_{\mathrm{eff}}^\prime$ to resolve the Hubble tension. Therefore, compared to previous studies, the parameter region where the Hubble tension can be resolved is slightly shifted toward the larger value of $m_{Z^\prime}$.

hep-ph↗
arXiv2021

Sensitivity of indirect detection of Neutralino dark matter by Sommerfeld enhancement mechanism

We have investigated neutralino dark matter in the framework of minimal supersymmetric Standard Model focusing on the coannihilatioin region. In this region, where the particle whose mass is tightly degenerated with the neutralino dark matter exists, we can solve the Lithium problem in the case of lepton flavor being violated. It turns out that Sommerfeld enhancement is important in the coannihilation region so that the dark matter signal becomes large enough to be observed by the current sensitivity of indirect experiments.

hep-ph↗
arXiv2019

Low scale seesaw models for low scale $U(1)_{L_μ-L_τ}$ symmetry

We propose models for neutrino masses and mixing in the framework of low scale $U(1)_{L_μ-L_τ}$ gauge extension of the standard model. The models are designed to spontaneously break $U(1)_{L_μ-L_τ}$ so that the $U(1)_{L_μ-L_τ}$ gauge boson acquires an MeV scale mass, which is required to solve the long-standing problem of muon anomalous magnetic moment. Tiny neutrino masses are obtained by simultaneously invoking the linear and the inverse seesaw mechanism, and we succeed in realizing two types of one-zero textures in the active neutrino mass matrix. Both of the obtained textures favor inverted neutrino mass ordering and are testable in next generation experiments of neutrinoless double beta decay. We also show that some of extra scalar bosons can have MeV scale masses and would have significant impacts on observations of high energy cosmic neutrinos.

hep-ph↗
arXiv2019

Momentum distribution of the electron pair from the charged lepton flavor violating process $μ^-e^-\to e^-e^-$ in muonic atoms with a polarized muon

The $μ^-e^-\to e^-e^-$ process in a muonic atom is one of the promising probes to study the charged lepton flavor violation (CLFV). We have investigated the angular distribution of electrons from the polarized muon of the atomic bound state. The parity violating asymmetric distribution of electrons is analyzed by using lepton wave functions under the Coulomb interaction of a finite nuclear charge distribution. It is found that the asymmetry parameters of electrons are very sensitive to the chiral structure of the CLFV interaction and the contact/photonic interaction. Therefore, together with the atomic number dependence of the decay rate studied in our previous work, the angular distribution of electrons from a polarized muon should be a very useful tool to constrain the model beyond the standard model.

hep-ph↗
arXiv2018

Big-bang nucleosynthesis and Leptogenesis in CMSSM

We have investigated the constrained minimal supersymmetric standard model with three right-handed Majorana neutrinos whether there still is a parameter region which is consistent with all existing experimental data/limits such as Leptogenesis and the dark matter abundance and we also can solve the Lithium problem. Using Casas-Ibarra parameterization, we have found that a very narrow parameter space of the complex orthogonal matrix elements where the lightest slepton can have a long lifetime, that is necessary for solving the Lithium problem. Further, under this condition, there is a parameter region that can give an explanation for the experimental observations. We have studied three cases of the right-handed neutrino mass ratio \mbox{\em (i)} $M_{2}=2 \times M_{1}$, \mbox{\em (ii)} $M_{2}=4 \times M_{1}$, \mbox{\em (iii)} $M_{2}=10 \times M_{1}$ while $M_{3}=40 \times M_{1}$ is fixed. We have obtained the mass range of the lightest right-handed neutrino mass that lies between $10^9$ GeV and $10^{11}$ GeV. The important result is that its upper limit is derived by solving the Lithium problem and the lower limit comes from Leptogenesis. Calculated low-energy observables of these parameter sets such as BR($μ\to e γ$) is not yet restricted by experiments and will be verified in the near future.

hep-ph↗
arXiv2017

Improved analysis for $μ^-e^-\to e^-e^-$ in muonic atoms by photonic interaction

Studies of the charged lepton flavor violating process of $μ^-e^-\to e^-e^-$ in muonic atoms by the four Fermi interaction [Y. Uesaka \textit{et al}., Phys. Rev. D {\bf 93}, 076006 (2016)] are extended to include the photonic interaction. The wave functions of a muon and electrons are obtained by solving the Dirac equation with the Coulomb interaction of a finite nuclear charge distribution. We find suppression of the $μ^-e^-\to e^-e^-$ rate over the initial estimation for the photonic interaction, in contrast to enhancement for the four Fermi interaction. It is due to the Coulomb interaction of scattering states and relativistic lepton wave functions. This finding suggests that the atomic number dependence of the $μ^-e^-\to e^-e^-$ rate could be used to distinguish between the photonic and the four Fermi interactions.

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