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H. Nunokawa

Publications and source records attributed to H. Nunokawa.

At least 55 records · Page 3Linked to original sources
arXiv2001

Probing Flavor Changing Neutrino Interactions Using Neutrino Beams from a Muon Storage Ring

We discuss the capabilities of a future neutrino factory based on intense neutrino beams from a muon storage ring to explore the non-standard neutrino matter interactions, which are assumed to be sub-leading effects in the standard mass induced neutrino oscillations. The conjunction of these two mechanisms will magnify fake CP violating effect in the presence of matter which is not coming from the CP phase in the neutrino mixing matrix. We show that such fake CP violation can be observed in neutrino factory experiments by measuring the difference between the neutrino and anti-neutrino probabilities. In order to perform such test, we consider three neutrino flavors, admitting the mixing parameters in the range consistent with the oscillation solution to the atmospheric and the solar neutrino problems, as well as with the constraints imposed by the reactor neutrino data. We show that with a 10 kt detector with 5 years of operation, a stored muon energy E_μ\ge 20 GeV, 2 \times 10^{20} muon decays per year, and a baseline L \sim 732 km, such a neutrino facility can probe the non-standard flavor changing neutrino interactions down to the level of (10^{-3}-10^{-2})~G_F, in both ν_μ\to ν_τ/ \bar ν_μ\to \bar ν_τand ν_e \to ν_τ/\bar ν_e \to \bar ν_τmodes.

hep-ph↗
arXiv2001

Testing the principle of equivalence by supernova neutrinos

We study the possible impact of the neutrino oscillation which could be induced by a tiny violation of equivalence principle (VEP) for neutrinos emitted from supernova driven by gravitational collapse. Due to the absence of any significant indication of neutrino oscillation in the SN1987A data, we obtain sever bounds on relevant VEP parameters δγ\lsim O(10^{-31}) for massless or degenerated neutrinos and δγ\lsim O(10^{-16})\times [Δm^2/10^{-5} eV^2] for massive neutrinos.

hep-ph↗
arXiv2000

Supernova Neutrino Oscillations

We discuss some possible influence of neutrino oscillations on physics of supernova mainly focusing on the observations of $\barν_e$, and present some analysis of SN1987A data in the light of three neutrino mixing scheme.

hep-ph↗
arXiv2000

Violation of Equivalence Principle and Solar Neutrinos

We have updated the analysis for the solution to the solar neutrino problem by the long-wavelength neutrino oscillations induced by a tiny breakdown of the weak equivalence principle of general relativity, and obtained a very good fit to all the solar neutrino data.

hep-ph↗
arXiv2000

On the massless "just-so" solution to the solar neutrino problem

We study the effect of the non-resonant, vacuum oscillation-like neutrino flavor conversion induced by non-standard flavor changing and non-universal flavor diagonal neutrino interactions with electrons in the sun. We have found an acceptable fit for the combined analysis for the solar experiments total rates, the Super-Kamiokande (SK) energy spectrum and zenith angle dependence. Phenomenological constraints on non-standard flavor changing and non-universal flavor diagonal neutrino interactions are considered.

hep-ph↗
arXiv2000

Three Generation Long-wavelength Vacuum Oscillation Solution to the Solar Neutrino Problem

We investigate the current status of the long-wavelength vacuum oscillation solution to the solar neutrino problem and to what extent the presence of a third neutrino can affect and modify it. Assuming that the smaller mass squared difference that can induce such oscillations, $Δm^2_{12}$, is in the range $10^{-11}-10^{-8}$ eV$^2$ and the larger one, $Δm^2_{23}$, in the range relevant to atmospheric neutrino observations, we analyze the most recent solar neutrino data coming from Homestake, SAGE, GALLEX, GNO and Super-Kamiokande experiments in the context of three neutrino generations. We include in our vacuum oscillation analysis the MSW effect in the Sun, which is relevant for some of the parameter space scrutinized. We have also performed, as an extreme exercise, the fit without Homestake data. % While we found that the MSW effect basically does not affect the best fitted parameters, it significantly modifies the allowed parameter space for $Δm^2_{12}$ larger than $\sim 3 \times 10^{-10}$ eV$^2$, in good agreement with the result obtained by A. Friedland in the case of two generations. % Although the presence of a third neutrino does not essentially improve the quality of the fit, the solar neutrino data alone can give an upper bound on $θ_{13}$, which is constrained to be less than $\sim 60^\circ$ at 95 % C.L.

hep-ph↗
arXiv2000

Status of the solution to the solar neutrino problem based on non-standard neutrino interactions

We analyze the current status of the solution to the solar neutrino problem based both on: a) non-standard flavor changing neutrino interactions (FCNI) and b) non-universal flavor diagonal neutrino interactions (FDNI). We find that FCNI and FDNI with matter in the sun as well as in the earth provide a good fit not only to the total rate measured by all solar neutrino experiments but also to the day-night and seasonal variations of the event rate, as well as the recoil electron energy spectrum measured by the SuperKamiokande collaboration. This solution does not require massive neutrinos and neutrino mixing in vacuum. Stringent experimental constraints on FCNI from bounds on lepton flavor violating decays and on FDNI from limits on lepton universality violation rule out $ν_e \to ν_μ$ transitions induced by New Physics as a solution to the solar neutrino problem. However, a solution involving $ν_e \to ν_τ$ transitions is viable and could be tested independently by the upcoming $B$-factories if flavor violating tau decays would be observed at a rate close to the present upper bounds.

hep-ph↗
arXiv1999

Flavor changing models with strictly massless neutrinos

Theoretical models which present flavor changing neutrino interactions and simultaneously prevent this particle from acquiring any mass exist. We discuss some of them and their predictions for neutrino oscillations in matter which can account for the solar neutrino anomaly as well as the zenith-angle dependence of the deficit of atmospheric neutrinos observed by the SuperKamiokande experiment without invoking, therefore, neutrino masses nor mixing.

hep-ph↗
arXiv1999

Current Status of the Resonant Spin-Flavor Precession Solution to the Solar Neutrino Problem

We discuss the current status of the resonant spin-flavor precession (RSFP) solution to the solar neutrino problem. We perform a fit to all the latest solar neutrino data for various assumed magnetic field profiles in the sun. We show that the RSFP can account for all the solar neutrino experiments, giving as good fit as other alternative solutions such as MSW or Just so, and therefore can be a viable solution to the solar neutrino problem

hep-ph↗
arXiv1999

Atmospheric neutrino observations and flavor changing interactions

Flavor changing (FC) neutrino-matter interactions can account for the zenith-angle dependent deficit of atmospheric neutrinos observed in the SuperKamiokande experiment, without directly invoking neither neutrino mass, nor mixing. We find that FC $ν_μ$-matter interactions provide a good fit to the observed zenith angle distributions, comparable in quality to the neutrino oscillation hypothesis. The required FC interactions arise naturally in many attractive extensions of the Standard Model.

hep-ph↗
arXiv1999

Type-II Supernovae and Neutrino Magnetic Moment

The present solar and atmospheric neutrino data together with the LSND results and the presence of hot dark matter (HDM) suggest the existence of a sterile neutrino at the eV scale. We have reanalysed the effect of resonant {\sl sterile} neutrino conversions induced by neutrino magnetic moments in a type-II supernova. We analyse the implications of $ν_e-ν_s$ and $\barν_e-\barν_s$ ($ν_s$ denotes sterile neutrino) conversions for the supernova shock re-heating, the detected $\barν_e$ signal from SN1987A and the $r$-process nucleosynthesis hypothesis. Using reasonable magnetic field profiles we determine the sensitivity of these three arguments to the relevant neutrino parameters, i.e. the value of the transition magnetic moment and the $ν_e-ν_s$ mass difference $Δm^2 \equiv Δm^2_{LSND/HDM}$

astro-ph↗
arXiv1998

Active-active and active-sterile neutrino oscillation solutions to the atmospheric neutrino anomaly

We perform a fit to the full data set corresponding to 33.3 kt-yr of data of the Super-Kamiokande experiment as well as to all other experiments in order to compare the two most likely solutions to the atmospheric neutrino anomaly in terms of oscillations in the $ν_μ\to ν_τ$ and $ν_μ\to ν_s$ channels. Using state-of-the-art atmospheric neutrino fluxes we have determined the allowed regions of oscillation parameters for both channels. We find that the $Δm^2$ values for the active-sterile oscillations (both for positive and negative $Δm^2$) are higher than for the $ν_μ\to ν_τ$ case, and that the increased Super-Kamiokande sample slightly favours $ν_μ\to ν_τ$ oscillations over oscillations into a sterile species $ν_s$, $ν_μ\to ν_s$, and disfavours $ν_μ\to ν_e$. We also give the zenith angle distributions predicted for the best fit points in each of the possible oscillation channels. Finally we compare our determinations of the atmospheric neutrino oscillation parameters with the expected sensitivities of future long-baseline experiments K2K, MINOS, ICARUS, OPERA and NOE.

hep-ph↗
arXiv1998

Pulsar Velocities without Neutrino Mass

We show that pulsar velocities may arise from anisotropic neutrino emission induced by resonant conversions of massless neutrinos in the presence of a strong magnetic field. The main ingredient is a small violation of weak universality and neither neutrino masses nor magnetic moments are required.

astro-ph↗
arXiv1998

Update on Atmospheric Neutrinos

We discuss the impact of recent experimental results on the determination of atmospheric neutrino oscillation parameters. We use all published results on atmospheric neutrinos, including the preliminary large statistics data of Super-Kamiokande. We re-analyze the data in terms of both $ν_μ\to ν_τ$ and $ν_μ\to ν_e$ channels using new improved calculations of the atmospheric neutrino flux. We compare the sensitivity attained in atmospheric neutrino experiments with those of accelerator and reactor neutrino oscillation searches, including the recent Chooz experiment. We briefly comment on the implications of atmospheric neutrino data in relation to future searches for neutrino oscillations with long baselines, such as the K2K, MINOS, ICARUS and NOE experiments.

hep-ph↗