arXiv subjects

H. Nunokawa

Publications and source records attributed to H. Nunokawa.

At least 37 records · Page 2Linked to original sources

arXiv2011

Testing for Large Extra Dimensions with Neutrino Oscillations

We consider a model where sterile neutrinos can propagate in a large compactified extra dimension giving rise to Kaluza-Klein (KK) modes and the standard model left-handed neutrinos are confined to a 4-dimensional spacetime brane. The KK modes mix with the standard neutrinos modifying their oscillation pattern. We examine former and current experiments such as CHOOZ, KamLAND, and MINOS to estimate the impact of the possible presence of such KK modes on the determination of the neutrino oscillation parameters and simultaneously obtain limits on the size of the largest extra dimension. We found that the presence of the KK modes does not essentially improve the quality of the fit compared to the case of the standard oscillation. By combining the results from CHOOZ, KamLAND and MINOS, in the limit of a vanishing lightest neutrino mass, we obtain the stronger bound on the size of the extra dimension as ~ 1.0(0.6) micrometer at 99% C. L. for normal (inverted) mass hierarchy. If the lightest neutrino mass turn out to be larger, 0.2 eV, for example, we obtain the bound ~ 0.1 micrometer. We also discuss the expected sensitivities on the size of the extra dimension for future experiments such as Double CHOOZ, T2K and NOvA.

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arXiv2011

Probing Extra Dimensions with Neutrino Oscillations

We consider a model where sterile neutrinos can propagate in a large compactified extra dimension (a) giving rise to Kaluza-Klein (KK) modes and the Standard Model left-handed neutrinos are confined to a 4-dimensional spacetime brane. The KK modes mix with the standard neutrinos modifying their oscillation pattern. We examine current experiments in this framework obtaining stringent limits on a.

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arXiv2009

Resolving CP Violation by Standard and Nonstandard Interactions and Parameter Degeneracy in Neutrino Oscillations

In neutrino oscillation with non-standard interactions (NSI) the system is enriched with CP violation caused by phases due to NSI in addition to the standard lepton Kobayashi-Maskawa phase δ. In this paper we show that it is possible to disentangle the two CP violating effects by measurement of muon neutrino appearance by a near-far two detector setting in neutrino factory experiments. Prior to the quantitative analysis we investigate in detail the various features of the neutrino oscillations with NSI, but under the assumption that only one of the NSI elements, ε_{e μ} or ε_{e τ}, is present. They include synergy between the near and the far detectors, the characteristic differences between the ε_{e μ} and ε_{eτ} systems, and in particular, the parameter degeneracy. Finally, we use a concrete setting with the muon energy of 50 GeV and magnetized iron detectors at two baselines, one at L=3000 km and the other at L=7000 km, each having a fiducial mass of 50 kton to study the discovery potential of NSI and its CP violation effects. We demonstrate, by assuming 4 \times 10^{21} useful muon decays for both polarities, that one can identify non-standard CP violation down to | epsilon_{e μ} | \simeq \text{a few} \times 10^{-3}, and | ε_{e τ} | \simeq 10^{-2} at 3σCL for θ_{13} down to \sin^2 2θ_{13} = 10^{-4} in most of the region of δ. The impact of the existence of NSI on the measurement of δand the mass hierarchy is also worked out.

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arXiv2008

Parameter Degeneracy in Flavor-Dependent Reconstruction of Supernova Neutrino Fluxes

We reexamine the possibility of reconstructing the initial fluxes of supernova neutrinos emitted in a future core-collapse galactic supernova explosion and detected in a Megaton-sized water Cherenkov detector. A novel key element in our method is the inclusion, in addition to the total and the average energies of each neutrino species, of a "pinching" parameter characterizing the width of the distribution as a fit parameter. We uncover in this case a continuous degeneracy in the reconstructed parameters of supernova neutrino fluxes at the neutrinosphere. We analyze in detail the features of this degeneracy and show how it occurs irrespective of the parametrization used for the distribution function. Given that this degeneracy is real we briefly comment on possible steps towards resolving it, which necessarily requires going beyond the setting presented here.

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arXiv2008

Probing Nonstandard Neutrino Physics at T2KK

Having a far detector in Korea for the J-PARC neutrino beam in addition to one at Kamioka has been shown to be a powerful way to lift neutrino parameter ($Δm^2$ and mixing angles) degeneracies. In this talk, I report the sensitivity of the same experimental setup to nonstandard neutrino physics, such as quantum decoherence, violation of Lorentz symmetry (with/without CPT invariance), and nonstandard neutrino interactions with matter. In many cases, two detector setup is better than one detector setup at SK. This observation makes another support for the two detector setup.

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arXiv2007

Probing Non-Standard Neutrino Interactions with Neutrino Factories

We discuss the sensitivity reach of a neutrino factory measurement to non-standard neutrino interactions (NSI), which may exist as a low-energy manifestation of physics beyond the Standard Model. We use the muon appearance mode ν_e --> ν_μand consider two detectors, one at 3000 km and the other at 7000 km. Assuming the effects of NSI at the production and the detection are negligible, we discuss the sensitivities to NSI and the simultaneous determination of θ_{13} and δby examining the effects in the neutrino propagation of various systems in which two NSI parameters ε_{αβ} are switched on. The sensitivities to off-diagonal ε's are found to be excellent up to small values of θ_{13}. We demonstrate that the two-detector setting is powerful enough to resolve the θ_{13}-NSI confusion problem. We believe that the results obtained in this paper open the door to the possibility of using neutrino factory as a discovery machine for NSI while keeping its primary function of performing precision measurements of the lepton mixing parameters.

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arXiv2006

Determining Neutrino Mass Hierarchy by Precision Measurements in Electron and Muon Neutrino Disappearance Experiments

Recently a new method for determining the neutrino mass hierarchy by comparing the effective values of the atmospheric Δm^2 measured in the electron neutrino disappearance channel, Δm^2(ee), with the one measured in the muon neutrino disappearance channel, Δm^2(μμ), was proposed. If Δm^2(ee) is larger (smaller) than Δm^2(μμ) the hierarchy is of the normal (inverted) type. We re-examine this proposition in the light of two very high precision measurements: Δm^2(μμ) that may be accomplished by the phase II of the Tokai-to-Kamioka (T2K) experiment, for example, and Δm^2(ee) that can be envisaged using the novel Mossbauer enhanced resonant \barν_e absorption technique. Under optimistic assumptions for the systematic uncertainties of both measurements, we estimate the parameter region of (θ_13, δ) in which the mass hierarchy can be determined. If θ_13 is relatively large, sin^2 2θ_13 \gsim 0.05, and both of Δm^2(ee) and Δm^2(μμ) can be measured with the precision of \sim 0.5 % it is possible to determine the neutrino mass hierarchy at > 95% CL for 0.3 π\lsim δ\lsim 1.7 πfor the current best fit values of all the other oscillation parameters.

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arXiv2005

Angra Neutrino Project: status and plans

We present the status and plans of the Angra Project, a new nuclear reactor neutrino oscillation experiment, proposed to be built in Brazil at the Angra dos Reis nuclear reactor complex. This experiment is aimed to measure theta_13, the last unknown of the three neutrino mixing angles. Combining a high luminosity design, very low background from cosmic rays and careful control of systematic errors at the 1% level, we propose a high sensitivity multi-detector experiment, able to reach a sensitivity to antineutrino disappearance down to sin^2(2*theta_13) = 0.006 in a three years running period, improving present limits constrained by the CHOOZ experiment by more than an order of magnitude.

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arXiv2005

Reactor measurement of θ_{12}; Secret of the power

We demonstrate enormous power of dedicated reactor experiment for θ_{12} with a detector placed at around the first oscillation maximum, which we call "SADO". It allows determination of sin^2θ_{12} to the accuracy of \simeq 2% at 1 σCL, which surpasses all the method so far proposed. Unlike reactor θ_{13} experiments, the requirement for the systematic error is very mild, \simeq 4%, which makes it an even more feasible experiment. If we place a detector at \sim 60 km away from the Kashiwazaki-Kariwa nuclear power plant, 0.5 kt yr exposure of SADO is equivalent to \sim 100 kt yr exposure of KamLAND assuming the same systematic error.

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arXiv2005

Reactor Measurement of theta_12; Principles, Accuracies and Physics Potentials

We discuss reactor measurement of θ_{12} which has a potential of reaching the ultimate sensitivity which surpasses all the methods so far proposed. The key is to place a detector at an appropriate baseline distance from the reactor neutrino source to have an oscillation maximum at around a peak energy of the event spectrum in the absence of oscillation. By a detailed statistical analysis the optimal distance is estimated to be \simeq (50-70) km x [8 x 10^{-5} eV^2/Δm^2_{21}], which is determined by maximizing the oscillation effect in the event number distribution and minimizing geo-neutrino background contamination. To estimate possible uncertainty caused by surrounding nuclear reactors in distance of \sim 100 km, we examine a concrete example of a detector located at Mt. Komagatake, 54 km away from the Kashiwazaki-Kariwa nuclear power plant in Japan, the most powerful reactor complex in the world. The effect turns out to be small. Under a reasonable assumption of systematic error of 4% in the experiment, we find that sin^2{θ_{12}} can be determined to the accuracy of \simeq 2% (\simeq 3%), at 68.27% CL for 1 degree of freedom, for 60 GW_th kton yr (20 GW_th kton yr) operation. We also discuss implications of such an accurate measurement of θ_{12}.

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arXiv2003

Discriminating among Earth composition models using geo-antineutrinos

It has been estimated that the entire Earth generates heat corresponding to about 40 TW (equivalent to 10,000 nuclear power plants) which is considered to originate mainly from the radioactive decay of elements like U, Th and K, deposited in the crust and mantle of the Earth. Radioactivity of these elements produce not only heat but also antineutrinos (called geo-antineutrinos) which can be observed by terrestrial detectors. We investigate the possibility of discriminating among Earth composition models predicting different total radiogenic heat generation, by observing such geo-antineutrinos at Kamioka and Gran Sasso, assuming KamLAND and Borexino (type) detectors, respectively, at these places. By simulating the future geo-antineutrino data as well as reactor antineutrino background contributions, we try to establish to which extent we can discriminate among Earth composition models for given exposures (in units of kt$\cdot$ yr) at these two sites on our planet. We use also information on neutrino mixing parameters coming from solar neutrino data as well as KamLAND reactor antineutrino data, in order to estimate the number of geo-antineutrino induced events.

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arXiv2003

Status of a hybrid three-neutrino interpretation of neutrino data

We reanalyze the non-standard interaction (NSI) solutions to the solar neutrino problem in the light of the latest solar, atmospheric and reactor neutrino data. We show that such solutions, although preferred by the solar data and consistent with the oscillation description of the atmospheric neutrino data, are ruled out by the first results of the KamLAND reactor experiment, at more than 3_sigma.

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arXiv2003

Probing the LSND scale and four neutrino scenarios with a neutrino telescope

We show in this paper that the observation of the angular distribution of upward-going muons and cascade events induced by atmospheric neutrinos at the TeV energy scale, which can be performed by a kilometer-scale neutrino telescope, such as the IceCube detector, can be used to probe a large neutrino mass splitting, $| Δm^2 | \sim (0.5-2.0)$ eV$^2$, implied by the LSND experiment and discriminate among four neutrino mass schemes. This is due to the fact that such a large mass scale can promote non negligible muon neutrino to electron/tau neutrino (and/or anti-muon neutrino to anti-electron/anti-tau neutrino) conversions at these energies by the MSW effect as well as vacuum oscillation, unlike what is expected if all the neutrino mass splittings are small.

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arXiv2003

Determining the oscillation parameters by Solar neutrinos and KamLAND

The neutrino oscillation experiment KamLAND has given the first evidence for disappearance of $\bar ν_e$ coming from nuclear reactors. We have combined their data with all the solar neutrino data assuming two flavor neutrino mixing and obtained allowed parameter regions which are compatible with the so-called large mixing angle MSW solution to the solar neutrino problem. The allowed regions in the plane of mixing angle and mass squared difference are now split into two islands at 99% C.L. We have speculated how these two islands can be separated in the near future. We have shown that a 50% reduction of the error on SNO neutral-current measurement can be important in establishing in each of these islands the true values of these parameters lie. We also have simulated KamLAND positron energy spectrum after 1 year of data taking, assuming the current best fitted values of the oscillation parameters, combined it the with current solar neutrino data and showed how these two split islands can be modified.

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arXiv2002

Constraining the absolute neutrino mass scale and Majorana CP violating phases by future neutrinoless double beta decay experiments

Assuming that neutrinos are Majorana particles, in a three generation framework, current and future neutrino oscillation experiments can determine six out of the nine parameters which fully describe the structure of the neutrino mass matrix. We try to clarify the interplay among the remaining parameters, the absolute neutrino mass scale and two CP violating Majorana phases, and how they can be accessed by future neutrinoless double beta ($0νββ$) decay experiments, for the normal as well as for the inverted order of the neutrino mass spectrum. Assuming the oscillation parameters to be in the range presently allowed by atmospheric, solar, reactor and accelerator neutrino experiments, we quantitatively estimate the bounds on $m_0$, the lightest neutrino mass, that can be infered if the next generation $0νββ$ decay experiments can probe the effective Majorana mass ($m_{ee}$) down to $\sim$ 1 meV. In this context we conclude that in the case neutrinos are Majorana particles: (a) if $m_0 \gsim 300$ meV, {\em i.e.}, within the range directly attainable by future laboratory experiments as well as astrophysical observations, then $m_{ee} \gsim 30$ meV must be observed; (b) if $m_0 < 300$ meV, results from future $0νββ$ decay experiments combined with stringent bounds on the neutrino oscillation parameters, specially the solar ones, will place much stronger limits on the allowed values of $m_0$ than these direct experiments.

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arXiv2002

Measuring the Spectra of High Energy Neutrinos with a Kilometer-Scale Neutrino Telescope

We investigate the potential of a future kilometer-scale neutrino telescope such as the proposed IceCube detector in the South Pole, to measure and disentangle the yet unknown components of the cosmic neutrino flux, the prompt atmospheric neutrinos coming from the decay of charmed particles and the extra-galactic neutrinos, in the 10 TeV to 1 EeV energy range. Assuming a power law type spectra, $dϕ_ν/dE_ν\sim αE_ν^β$, we quantify the discriminating power of the IceCube detector and discuss how well we can determine magnitude ($α$) as well as slope ($β$) of these two components of the high energy neutrino spectrum, taking into account the background coming from the conventional atmospheric neutrinos.

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arXiv2002

Probing supernova physics with neutrino oscillations

We point out that solar neutrino oscillations with large mixing angle as evidenced in current solar neutrino data have a strong impact on strategies for diagnosing collapse-driven supernova (SN) through neutrino observations. Such oscillations induce a significant deformation of the energy spectra of neutrinos, thereby allowing us to obtain otherwise inaccessible features of SN neutrino spectra. We demonstrate that one can determine temperatures and luminosities of non-electron flavor neutrinos by observing bar{nu}_{e} from galactic SN in massive water Cherenkov detectors by the charged current reactions on protons.

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arXiv2002

Global Analysis of the post-SNO Solar Neutrino Data for Standard and Non-Standard Oscillation Mechanisms

What can we learn from solar neutrino observations? Is there any solution to the solar neutrino anomaly which is favored by the present experimental panorama? After SNO results, is it possible to affirm that neutrinos have mass? In order to answer such questions we analyze the current available data from the solar neutrino experiments, including the recent SNO result, in view of many acceptable solutions to the solar neutrino problem based on different conversion mechanisms, for the first time, using the same statistical procedure. This allows us to do a direct comparison of the goodness of the fit among different solutions, from which we can discuss and conclude on the current status of each proposed dynamical mechanism. These solutions are based on different assumptions: (a) neutrino mass and mixing, (b) non-vanishing neutrino magnetic moment, (c) the existence of non-standard flavor-changing and non-universal neutrino interactions and (d) the tiny violation of the equivalence principle. We investigate the quality of the fit provided by each one of these solutions not only to the total rate measured by all the solar neutrino experiments but also to the recoil electron energy spectrum measured at different zenith angles by the Super-Kamiokande collaboration. We conclude that several non-standard neutrino flavor conversion mechanisms provide a very good fit to the experimental data which is comparable with (or even slightly better than) the most famous solution to the solar neutrino anomaly based on the neutrino oscillation induced by mass.

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