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Joe Sato

Publications and source records attributed to Joe Sato.

At least 73 records · Page 4Linked to original sources
arXiv2007

Impact of non-standard neutrino interactions on future oscillation experiments

We study the performance of reactor and superbeam neutrino experiments in the presence of non-standard interactions (NSI). We find that for some non-standard terms, reactor and superbeam experiments would yield conflicting result in the theta-13 determination, 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. Throughout our discussion, we pay special attention to the impact of the complex phases of the NSI parameters and to the observations at the near detector.

hep-ph↗
arXiv2007

Neutrino Beams From Electron Capture at High Gamma

We investigate the potential of a flavor pure high gamma electron capture electron neutrino beam directed towards a large water cherenkov detector with 500 kt fiducial mass. The energy of the neutrinos is reconstructed by the position measurement within the detector and superb energy resolution capabilities could be achieved. We estimate the requirements for such a scenario to be competitive to a neutrino/anti-neutrino running at a neutrino factory with less accurate energy resolution. Although the requirements turn out to be extreme, in principle such a scenario could achieve as good abilities to resolve correlations and degeneracies in the search for sin^2(2 theta_13) and delta_CP as a standard neutrino factory experiment.

hep-ph↗
arXiv2007

Relic abundance of dark matter in universal extra dimension models with right-handed neutrinos

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 has. The first KK particle of the right-handed neutrino is a candidate for dark matter in this framework, and its relic abundance is determined by three processes, (1) the decay of the KK photon into the first KK right-handed neutrino in the late universe, (2) production of the first KK right-handed neutrino from the thermal bath in the early universe, and (3) the decay of higher KK right-handed neutrinos into the first KK right-handed neutrino in the late universe. When ordinary neutrino masses are large enough such as the degenerate mass spectrum case, the last process contribute to the abundance significantly, even if the reheating temperature is low. 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.

hep-ph↗
arXiv2007

Laser Irradiated Enhancement of the Atomic Electron Capture Rate in search of New Physics

Electron capture processes are important in the search for new physics. In this context, a high capture rate is desired. We investigate the possibility of enhancing the electron capture rate by irradiating laser beam to ''atom''. The possibility of such enhancement can be understood as a consequence of an enhancement of the electron wave function at the origin, $Ψ(0)$, through an increased effective mass of the electron. We find that an order of magnitude enhancement can be realized by using a laser with intensity on the order of $10^{10}$ W/mm$^2$ and a photon energy on the order of $10^{-3}$ eV.

hep-ph↗
arXiv2007

Solving cosmological problem in universal extra dimension models by introducing Dirac neutrino

Universal extra dimension (UED) models with right-handed neutrinos are studied. The introduction of the neutrinos makes us possible not only to describe Dirac neutrino masses but also to solve the cosmological problem called the KK graviton problem. This problem is essentially caused by the late time decay of a KK photon into a KK graviton and a photon, and it distorts the spectrum of the cosmic microwave background or the diffuse photon. We point out that, once we introduce right-handed neutrinos to UED models, the KK photon decays dominantly into neutrinos and does not emit a photon. We also discuss sub-dominant modes with a photon in the decay quantitatively, and show that their branching ratios are so small that the spectra are not distorted.

hep-ph↗
arXiv2006

Long life stau in the minimal supersymmetric standard model

We study the stau lifetime in a scenario with the LSP taken to be a neutralino and the NLSP being a stau, based on the minimal supersymmetric Standard Model. The mass difference between the LSP and NLSP, $δm$, must satisfy $δm/m_{\tildeχ} \sim$ a few % or less for coannihilation to occur, where $m_{\tildeχ}$ is the neutralino mass. We calculate the stau lifetime from the decay modes $\tildeτ\to \tildeχτ$, $\tildeχν_τπ$, and $\tildeχν_τμ(e)ν_{μ(e)}$ and discuss its dependence on various parameters. We find that the lifetime is in the range $10^{-22}$--$10^{16}$ sec for $10^{-2} \le δm \le 10$ GeV. We also discuss the connection with lepton flavor violation if there is mixing between sleptons.

hep-ph↗
arXiv2005

Enhancement of Line Gamma Ray Signature from Bino-like Dark Matter Annihilation due to CP Violation

We study the line gamma ray signature from the Galactic Center originating from Bino-like dark matter pair annihilation, when CP is not conserved. We consider the case that the Bino is nearly degenerate with a sfermion in mass. Although an indirect detection of Bino-like dark matter is difficult in general, the signal of Bino-like dark matter pair annihialtion in this case is enormously enhanced due to the threshold singularity.

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↗
arXiv2005

Nonexponential decay of an unstable quantum system: Small-$Q$-value s-wave decay

We study the decay process of an unstable quantum system, especially the deviation from the exponential decay law. We show that the exponential period no longer exists in the case of the s-wave decay with small $Q$ value, where the $Q$ value is the difference between the energy of the initially prepared state and the minimum energy of the continuous eigenstates in the system. We also derive the quantitative condition that this kind of decay process takes place and discuss what kind of system is suitable to observe the decay.

quant-ph↗
arXiv2003

R-Parity Violation in a SUSY GUT Model and Radiative Neutrino Masses

Within the framework of an SU(5) SUSY GUT model, a mechanism which effectively induces R-parity-violating terms below the unification energy scale M_X is proposed. The model has matter fields \bar{5}_{L(+)}+10_{L(-)} and Higgs fields H_{(-)} and \bar{H}_{(+)} in addition to the ordinary Higgs fields H_{(+)} and \bar{H}_{(-)} which contribute to the Yukawa interactions, where (\pm) denote the transformation properties under a discrete symmetry Z_2. The Z_2 symmetry is only broken by the μ-term \bar{H}_{(+)}H_{(-)} softly, so that the \bar{5}_{(+)}\leftrightarrow \bar{H}_{(+)} \leftrightarrow \bar{H}_{(-)} mixing appears at μ< m_{SB}, i.e. R-parity violating terms \bar{5}_L\bar{5}_L 10_L are effectively induced from the Yukawa interactions \bar{H}_{(-)}\bar{5}_{L(+)}10_{L(-)}, so that the effective coupling constants λ_{ijk} of ν_{Li} e_{Lj} e_{Rk}^c and ν_{Li} d_{Rj}^c d_{Lk} are proportional to the mass matrices (M_e^*)_{jk} and (M_d^\dagger)_{jk}, respectively. The parameter regions which are harmless for the proton decay are investigated. Possible forms of the radiatively induced neutrino mass matrix are also investigated.

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↗
arXiv2002

An SU(5) SUSY Model with R-Parity Violation and Radiatively Induced Neutrino Masses

The radiatively induced neutrino mass matrix is investigated on the basis of an SU(5) SUSY model. In order to evade the proton decay, an ansatz based on a discrete symmetry $Z_2$ is assumed: although, at the unification scale, we have two types of superfields $Ψ_{L(\pm)}=\bar{5}_{L(\pm)}+10_{L(\pm)}$, which are transformed as $Ψ_{L(\pm)} \to \pm Ψ_{L(\pm)}$ under the discrete symmetry Z$_2$, the particles $Ψ_{L(+)}$ are decoupled after the SU(5) symmetry is broken, so that our quarks and leptons belong to Ψ_{L(-)}. The R-parity-violating terms for our quarks and leptons Ψ_{L(-)} are basically forbidden under the symmetry Z_2. However, we assume that mixings between members of Ψ_{L(+)} and those of Ψ_{L(-)} are in part caused after SU(5) is broken. As a result, the R-parity-violating interactions are in part allowed, so that the neutrino masses are radiatively generated, while the proton decay due to the R-parity violating terms is still forbidden because the term d_R^c d_R^c u_R^c has z=-1.

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↗
arXiv2000

Neutrino Masses and Lepton-Flavor Violation in Supersymmetric Models with lopsided Froggatt-Nielsen charges

We analyze in detail lepton-flavor violation (LFV) in the charged-lepton sector such as $μ\to e γ$, $τ\to μγ$, $μ\to eee$ and the $μ\to e$ conversion in nuclei, within the framework of supersymmetric models with lopsided Froggatt--Nielsen charges, in which the large mixing in the neutrino sector as well as small mixings in the quark sector can be naturally accommodated. We show that the present experimental limits on the LFV processes already exclude some of the models. The future proposed search for LFV, especially in muon processes, can provide a significant probe to this framework. We also stress the importance of the measurement of $U^{MNS}_{e3}$ in neutrino experiments, and the fact that the KamLAND experiment could play a significant role to test a certain class of models.

hep-ph↗