arXiv subjects

Atsushi Watanabe

Publications and source records attributed to Atsushi Watanabe.

At least 19 recordsLinked to original sources

arXiv2015

Probing heavy neutrinos in the COMET experiment

We argue that the COMET experiment --- a dedicated experiment for the $μ$-$e$ conversion search --- can be a powerful facility to search for heavy neutrinos in the mass range $1\,{\rm MeV} \lesssim M \lesssim 100\,{\rm MeV}$. The stopped muons captured by the target nuclei or decaying in orbit are efficiently produce heavy neutrinos via the active-sterile mixing. The produced heavy neutrinos then decay to electron-positron pair (plus an active neutrino), which events are clearly seen by the cylindrical drift chamber surrounding the target. The expected sensitivity is comparable to the PS191 bound when the COMET experiment achieves $\sim 10^{17}$ stopping muons in the target.

hep-ph↗

arXiv2015

The spectrum and flavor composition of the astrophysical neutrinos in IceCube

We fit the energy distribution of the IceCube starting events by a model which involves four parameters in the neutrino spectrum, namely three normalizations $n_e,n_μ,n_τ$ and a common power-law index $γ$, with a fixed background simulated by IceCube. It is found that the best fit index is $γ= 2.7$ with $χ^2_{\rm min} = 32.3/24\,{\rm dof}$. As for the two parameter model involving a democratic normalization and an index, the best fit is at $γ= 2.8$ with $χ^2_{\rm min} = 33.9/26\,{\rm dof}$. The flavored model and the democratic model do not have much difference in the quality of the (energy-spectrum) fit. The standard $1:1:1$ composition is not disfavored by the current data.

astro-ph.HE↗

arXiv2014

Dark matter and U(1)' symmetry for the right-handed neutrinos

We consider a U(1)' gauge symmetry acting on three generations of right-handed neutrinos. The U(1)' symmetry is broken at the TeV scale and its remnant discrete symmetry makes one of the right-handed neutrinos stable. As a natural consequence of the anomaly cancellation, the neutrino mass matrix consists of a combination of Type I (TeV scale) seesaw and radiative correction. The stable right-handed neutrino communicates with the Standard Model via s-channel exchange of the Higgs field and the U(1)' gauge boson, so that the observed relic density for dark matter is obtained in a wide range of the parameter space. The experimental signatures in collider and other experiments are briefly discussed.

hep-ph↗

arXiv2013

Reproducing lepton mixing in a texture zero model

We note that the emerging features of lepton mixing can be reproduced if, with inverted neutrino mass ordering, both the smallest neutrino mass and the $ττ$ element of the neutrino mass matrix vanish. Then, the atmospheric neutrino mixing angle is less than maximal and the Dirac phase $δ$ is close to $π$. We derive the correlations among the mixing parameters and show that there is a large cancellation in the effective mass responsible for neutrinoless $ββ$ decay. Three simple seesaw models leading to our scenario are provided.

hep-ph↗

arXiv2013

Heavy neutrino search in accelerator-based experiments

We explore the feasibility of detecting heavy neutrinos by the existing facilities of neutrino experiments. A heavy neutrino in the mass range 1 MeV < M < 500 MeV is produced by pion or kaon decay, and decays to charged particles which leave signals in neutrino detectors. Taking the T2K experiment as a typical example, we estimate the heavy neutrino flux produced in the neutrino beam line. Due to massive nature of the heavy neutrino, the spectrum of the heavy neutrino is significantly different from that of the ordinary neutrinos. While the ordinary neutrinos are emitted to various directions in the laboratory frame due to their tiny masses, the heavy neutrinos tend to be emitted to the forward directions and frequently hit the detector.The sensitivity for the mixing parameters is studied by evaluating the number of signal events in the near detector ND280. For the electron-type mixing, the sensitivity of T2K at 10^{21} POT is found to be better than that of the previous experiment PS191, which has placed the most stringent bounds on the mixing parameters of the heavy neutrinos for 140 MeV< M < 500 MeV.

hep-ph↗

arXiv2012

On the interpretation of IceCube cascade events in terms of the Glashow resonance

The IceCube experiment (IC) has recently observed 2 cascade events with energies between 1 and 10 PeV. This energy combined with the fact that no muon-track events are observed may be interpreted as a cosmogenic $\bar ν_e$ interacting in IC via the Glashow resonance (GR) $\bar ν_e e \to W^-\to$ (hadrons or $\bar ν_e e$). We point out a unique, background-free signature of the GR, a single isolated muon unaccompanied by any shower activity from the interaction $\bar ν_e e \to W^-\to \bar ν_μμ^-$, and propose it as a test of this interpretation. We calculate the event numbers and find that a single such event is expected over about a three-year period in IC. We also show that, if event rates remain at their current levels then, even with the GR, standard cosmogenic fluxes cannot easily explain the observations. Moreover, if muon-tracks remain conspicuous by their absence, then new physics needs to be invoked. As example scenarios in conformity with the observations, we calculate event rates for neutrino decay and Lorentz-invariance violation.

hep-ph↗

arXiv2012

Atmospheric Sterile Neutrinos

We study production of sterile neutrinos in the atmosphere and their detection at Super-Kamiokande. A sterile neutrino in the mass range $1\,{\rm MeV} \lesssim M_N \lesssim 105\,{\rm MeV}$ is produced by muon or pion decay, and decays to an electron-positron pair and an active neutrino. Such a decay of the sterile neutrino leaves two electron-like Cherenkov rings in the detector. We estimate the sterile neutrino flux from the well-established active neutrino fluxes and study the number of the decay events in the detector. The upper bounds for the active-sterile mixings are obtained by comparing the $2e$-like events from the sterile neutrino decays and the observed data by Super-Kamiokande. The upper bound for the muon type mixing $Θ_μ$ is found to be $|Θ_μ|^2 \lesssim 5 \times 10^{-5}$ for $20 \,{\rm MeV} \lesssim M_N \lesssim 80\,{\rm MeV}$, which is significantly loosened compared to the previous estimation. We demonstrate that the opening angle and the total energy of the rings may serve as diagnostic tools to discover the sterile neutrinos in further data accumulation and future upgraded facilities. The directional asymmetry of the events is a sensitive measure of the diminishment of the sterile neutrino flux due to the decays on the way to the detector.

hep-ph↗

arXiv2012

Relating large $U_{e3}$ to the ratio of neutrino mass-squared differences

The non-zero and sizable value of $U_{e3}$ puts pressure on flavor symmetry models which predict an initially vanishing value. Hence, the tradition of relating fermion mixing matrix elements with fermion mass ratios might need to be resurrected. We note that the recently observed non-vanishing value of $U_{e3}$ can be related numerically to the ratio of solar and atmospheric mass-squared differences. The most straightforward realization of this can be achieved with a combination of texture zeros and a vanishing neutrino mass. We analyze the implications of some of these possibilities and construct explicit flavor symmetry models that predict these features.

hep-ph↗

arXiv2011

The Glashow resonance at IceCube: signatures, event rates and $pp$ vs. $pγ$ interactions

We revisit the signatures of the Glashow resonance process $\barν_e e \to W$ in the high-energy astrophysical neutrino observatory IceCube. We note that in addition to the standard hadronic and electromagnetic showers produced by an incoming neutrino at the resonance energy of $E_ν\approx 6.3$ PeV, there are two clear signals of the process: the "pure muon" from $\barν_e e \to \barν_μμ$ and the "contained lollipop" from $\barν_e e \to \barν_ττ$. The event rate and the signal-to-background ratio (the ratio of the resonant to concurrent non-resonant processes) are calculated for each type of interaction, based on current flux limits on the diffuse neutrino flux. Because of the low background in the neighborhood of the resonance, the observation of only one pure muon or contained lollipop event essentially signals discovery of the resonance, even if the expected event numbers are small. We also evaluate the total event rates of the Glashow resonance from the extra-galactic diffuse neutrino flux and emphasize its utility as a discovery tool to enable first observations of such a flux. We find that one can expect 3.6 (0.65) events per year for a pure $pp$ ($pγ$) source, along with an added contribution of 0.51 (0.21) from non-resonant events. We also give results as a function of the ratio of $pp$ vs $pγ$ sources.

astro-ph.HE↗

arXiv2011

Breaking Tri-bimaximal Mixing and Large $θ_{13}$

The long baseline neutrino experiment, T2K, and the reactor experiment, Double Chooz will soon present new data. If we expect $\sinθ_{13}$ to be 0.1-0.2, which is close to the present experimental upper bound, we should not persist in the paradigm of the tri-bimaximal mixing. We discuss breaking the tri-bimaximal mixing by adding a simple mass matrix, which could be derived from some non-Abelian discrete symmetries. It is found that $\sinθ_{13}=0.1-0.2$ is expected in our model independent analysis of the generalized mass matrix for the normal or inverted hierarchical neutrino mass spectrum. On the other hand, $\sin^2θ_{23}$ and $\sin^2θ_{12}$ are expected to be not far from 1/2 and 1/3, respectively. As a typical example, we also discuss the $A_4$ flavor model with the 1 and 1' flavons, which break the tri-bimaximal mixing considerably. In this modified version of the Altarelli and Feruglio model, $\sinθ_{13}$ is predicted to be around 0.15 in the case of the normal hierarchical neutrino masses $m_3\gg m_2, m_1$, and 0.2 in the case of the inverted hierarchy $m_3\ll m_2, m_1$. The form of the neutrino mass matrix looks rather interesting --- it is suggestive of other discrete symmetries as well.

hep-ph↗

arXiv2011

Neutrino masses and mixing from S4 flavor twisting

We discuss a neutrino mass model based on the S4 discrete symmetry where the symmetry breaking is triggered by the boundary conditions of the bulk right-handed neutrino in the fifth spacial dimension. While the symmetry restricts bare mass parameters to flavor-diagonal forms, the viable mixing angles emerge from the wave functions of the Kaluza-Klein modes which carry symmetry breaking effect. The magnitudes of the lepton mixing angles, especially the reactor angle is related to the neutrino mass patterns and the model will be tested in future neutrino experiments, e.g., an early (late) discovery of the reactor angle favors the normal (inverted) hierarchy. The size of extra dimension has a connection to the possible mass spectrum; a small (large) volume corresponds to the normal (inverted) mass hierarchy.

hep-ph↗

arXiv2010

Seesaw in the bulk

A five-dimensional seesaw framework is analyzed with the lepton-number-violating propagator of bulk right-handed neutrinos. That can bypass summing up the effects of heavy Majorana particles whose masses and wavefunctions are not exactly known. The propagator method makes it easier to evaluate the seesaw-induced neutrino mass for various boundary conditions of bulk neutrinos and in a general background geometry, including the warped extra dimension. It is also found that the higher-dimensional seesaw gives a natural framework for the inverse seesaw suppression of low-energy neutrino masses.

hep-ph↗

arXiv2010

Ultra-high neutrino fluxes as a probe for non-standard physics

We examine how light neutrinos coming from distant active galactic nuclei (AGN) and similar high energy sources may be used as tools to probe non-standard physics. In particular we discuss how studying the energy spectra of each neutrino flavour coming from such distant sources and their distortion relative to each other may serve as pointers to exotic physics such as neutrino decay, Lorentz symmetry violation, pseudo-Dirac effects, CP and CPT violation and quantum decoherence. This allows us to probe hitherto unexplored ranges of parameters for the above cases, for example lifetimes in the range $ 10^{-3}-10^{4} $ s/eV for the case of neutrino decay. We show that standard neutrino oscillations ensure that the different flavours arrive at the earth with similar shapes even if their flavour spectra at source may differ strongly in both shape and magnitude. As a result, observed differences between the spectra of various flavours at the detector would be signatures of non-standard physics altering neutrino fluxes during propagation rather than those arising during their production at source. Since detection of ultra-high energy (UHE) neutrinos is perhaps imminent, it is possible that such differences in spectral shapes will be tested in neutrino detectors in the near future. To that end, using the IceCube detector as an example, we show how our results translate to observable shower and muon-track event rates.

hep-ph↗

arXiv2010

Diffuse Ultra-High Energy Neutrino Fluxes and Physics Beyond the Standard Model

We study spectral distortions of diffuse ultra-high energy (UHE) neutrino flavour fluxes resulting due to physics beyond the Standard Model (SM). Even large spectral differences between flavours at the source are massaged into a common shape at earth by SM oscillations, thus, any significant observed spectral differences are an indicator of new physics present in the oscillation probability during propagation. Neutrino decay and Lorentz symmetry violation (LV) are examples, and result in significant distortion of the fluxes and the well-known bounds on them, which may allow UHE detectors to probe lifetimes, the mass hierarchy and LV parameters over a broad range.

hep-ph↗

arXiv2009

Geometry-free neutrino masses in curved spacetime

The seesaw-induced neutrino mass is discussed in a generic class of curved spacetime, including the flat and warped extra dimensions. For Majorana masses in the bulk and on the boundary, the exact forms of seesaw-induced masses are derived by using the Kaluza-Klein mode expansion and the lepton number violating correlator for bulk fermion. It is found that the neutrino mass is determined without the knowledge of wave functions and whole background geometry when the metric factor is fixed on the boundary, e.g. by solving the hierarchy problem.

hep-ph↗

arXiv2009

The Role and Detectability of the Charm Contribution to Ultra High Energy Neutrino Fluxes

It is widely believed that charm meson production and decay may play an important role in high energy astrophysical sources of neutrinos, especially those that are baryon-rich, providing an environment conducive to pp interactions. Using slow-jet supernovae (SJS) as an example of such a source, we study the detectability of high-energy neutrinos, paying particular attention to those produced from charmed-mesons. We highlight important distinguishing features in the ultra-high energy neutrino flux which would act as markers for the role of charm in the source. In particular, charm leads to significant event rates at higher energies, after the conventional (pi, K) neutrino fluxes fall off. We calculate event rates both for a nearby single source and for diffuse SJS fluxes for an IceCube-like detector. By comparing muon event rates for the conventional and prompt fluxes in different energy bins, we demonstrate the striking energy dependence in the rates induced by the presence of charm. We also show that it leads to an energy dependant flux ratio of shower to muon events, providing an additional important diagnostic tool for the presence of prompt neutrinos. Motivated by the infusion of high energy anti-electron neutrinos into the flux by charm decay, we also study the detectability of the Glashow resonance due to these sources.

hep-ph↗

arXiv2009

Hybrid textures in minimal seesaw mass matrices

In the context of minimal seesaw framework, we study the implications of Dirac and Majorana mass matrices in which two rigid properties coexist, namely, equalities among mass matrix elements and texture zeros. In the first part of the study, we discuss general possibilities of the Dirac and Majorana mass matrices for neutrinos with such hybrid structures. We then classify the mass matrices into realistic textures which are compatible with global neutrino oscillation data and unrealistic ones which do not comply with the data. Among the large number of general possibilities, we find that only 6 patterns are consistent with the observations at the level of the most minimal number of free parameters. These solutions have only 2 adjustable parameters, so that all the mixing angles can be described in terms of the two mass differences or pure numbers. We analyze these textures in detail and discuss their impacts for future neutrino experiments and for leptogenesis.

hep-ph↗

arXiv2008

Minimal Seesaw Textures with Two Heavy Neutrinos

We systematically analyze the Dirac and the Majorana mass matrices in seesaw models with two heavy right-handed neutrinos. We perform thorough classification of the vanishing matrix elements which are compatible with the results from the current neutrino oscillation experiments. We include the possibility of a non-diagonal Majorana mass matrix which leads to new solutions viable with data. In a basis where the Majorana mass matrix is diagonal, these solutions imply a Dirac matrix with specific relationships amongst its elements. We find that at the level of total 4 zeros together in the Dirac and the Majorana sectors, the mass matrices are almost consistent with the data but one mixing angle is predicted to be unsuitable. At the next level, i.e. with total 3 zeros, only seven patterns of mass matrices describe the experimental data well. The seven solutions have testable predictions for the future neutrino experiments. In particular, each solution has definite predictions about the observation of the 1-3 leptonic mixing angle and the effective mass measured in neutrino-less double beta decay. The solutions of the mass matrices contain novel texture forms and provide new insights into the lepton-generation structure. We also discuss possible connections between these textures and the tri-bimaximal mixing to search for symmetry principles behind the mass matrix structure.

hep-ph↗