Four-Way Neutrino Oscillations

We present a four-neutrino model with three active neutrinos and one sterile neutrino which naturally has maximal $\nu_\mu\to\nu_\tau$ oscillations of atmospheric neutrinos and can explain the solar neutrino and LSND results. The model predicts $\nu_e \to \nu_\tau$ and $\nu_e \to \nu_\mu$ oscillations in long-baseline experiments with $L/E \gg 1$ km/GeV with amplitudes that are determined by the LSND oscillation amplitude and argument controlled by the atmospheric $\delta m^2$.Comment: 21 pages, Latex2.09, uses epsf.sty to include 2 postscript figure

Generalized Neutrino Mixing from the Atmospheric Anomaly

We determine the neutrino mixing and mass parameters that are allowed by the Super-Kamiokande atmospheric neutrino data in a three-neutrino model with one mass-squared difference contributing to the oscillations. We find that although $\nu_\mu\to\nu_\tau$ oscillations are favored, $\nu_\mu\to \nu_e$ oscillations with amplitude as large as 0.18 are allowed even after accounting for the limit from the CHOOZ reactor experiment. The range of allowed parameters permit observable $\nu_\mu\leftrightarrow\nu_e$ and $\nu_e\to\nu_\tau$ oscillations in future long-baseline experiments.Comment: 9 pages, Latex2.09, 2 postscript figures embedded using epsf.st

Neutrino Decay and Atmospheric Neutrinos

We reconsider neutrino decay as an explanation for atmospheric neutrino observations. We show that if the mass-difference relevant to the two mixed states \nu_\mu and \nu_\tau is very small (< 10^{-4} eV^2), then a very good fit to the observations can be obtained with decay of a component of \nu_\mu to a sterile neutrino and a Majoron. We discuss how the K2K and MINOS long-baseline experiments can distinguish the decay and oscillation scenarios.Comment: 9 pages, Revtex, uses epsf.sty, 3 postscript figures. Additions and corrections to references, minor changes in the text and to some number

Extragalactic Sources for Ultra High Energy Cosmic Ray Nuclei

In this article we examine the hypothesis that the highest energy cosmic rays are complex nuclei from extragalactic sources. Under reasonable physical assumptions, we show that the nearby metally rich starburst galaxies (M82 and NGC 253) can produce all the events observed above the ankle. This requires diffusion of particles below $10^{20}$ eV in extragalactic magnetic fields $B \approx 15$ nG. Above $10^{19}$ eV, the model predicts the presence of significant fluxes of medium mass and heavy nuclei with small rate of change of composition. Notwithstanding, the most salient feature of the starburst-hypothesis is a slight anisotropy induced by iron debris just before the spectrum-cutoff.Comment: To appear in Phys. Rev. D, reference adde

High Energy Cosmic Rays from Neutrinos

We discuss recent models in which neutrinos, which are assumed to have mass in the eV range, originate the highest energy cosmic rays by interaction with the enhanced density in the galactic halo of the relic cosmic neutrino background. We make an analytical calculation of the required neutrino fluxes to show that the parameter space for these models is constrained by horizontal air shower searches and by the total number of background neutrinos, so that only models which have fairly unnatural halo sizes and enhanced densities are allowed.Comment: 14 pages, 3 ps figures. To appear in Phys. Rev.

Ultra-High Energy Cosmic Rays from Neutrino Emitting Acceleration Sources?

We demonstrate by numerical flux calculations that neutrino beams producing the observed highest energy cosmic rays by weak interactions with the relic neutrino background require a non-uniform distribution of sources. Such sources have to accelerate protons at least up to 10^{23} eV, have to be opaque to their primary protons, and should emit the secondary photons unavoidably produced together with the neutrinos only in the sub-MeV region to avoid conflict with the diffuse gamma-ray background measured by the EGRET experiment. Even if such a source class exists, the resulting large uncertainties in the parameters involved in this scenario does currently not allow to extract any meaningful information on absolute neutrino masses.Comment: 6 pages, 4 figures, RevTeX styl

Which blazars are neutrino loud?

Protons accelerated in the cores of active galactic nuclei can effectively produce neutrinos only if the soft radiation background in the core is sufficiently high. We find restrictions on the spectral properties and luminosity of blazars under which they can be strong neutrino sources. We analyze the possibility that neutrino flux is highly beamed along the rotation axis of the central black hole. The enhancement of neutrino flux compared to GeV gamma-ray flux from a given source makes the detection of neutrino point sources more probable. At the same time the smaller open angle reduces the number of possible neutrino-loud blazars compared to the number of gamma-ray loud ones. We present the table of 15 blazars which are the most likely candidates for the detection by future neutrino telescopes.Comment: 9 pages, 5 figures, version to be published in PR

Relic Neutrino Absorption Spectroscopy

Resonant annihilation of extremely high-energy cosmic neutrinos on big-bang relic anti-neutrinos (and vice versa) into Z-bosons leads to sizable absorption dips in the neutrino flux to be observed at Earth. The high-energy edges of these dips are fixed, via the resonance energies, by the neutrino masses alone. Their depths are determined by the cosmic neutrino background density, by the cosmological parameters determining the expansion rate of the universe, and by the large redshift history of the cosmic neutrino sources. We investigate the possibility of determining the existence of the cosmic neutrino background within the next decade from a measurement of these absorption dips in the neutrino flux. As a by-product, we study the prospects to infer the absolute neutrino mass scale. We find that, with the presently planned neutrino detectors (ANITA, Auger, EUSO, OWL, RICE, and SalSA) operating in the relevant energy regime above 10^{21} eV, relic neutrino absorption spectroscopy becomes a realistic possibility. It requires, however, the existence of extremely powerful neutrino sources, which should be opaque to nucleons and high-energy photons to evade present constraints. Furthermore, the neutrino mass spectrum must be quasi-degenerate to optimize the dip, which implies m_{nu} >~ 0.1 eV for the lightest neutrino. With a second generation of neutrino detectors, these demanding requirements can be relaxed considerably.Comment: 19 pages, 26 figures, REVTeX

Confusing the extragalactic neutrino flux limit with a neutrino propagation limit

We study the possible suppression of the extragalactic neutrino flux due to a nonstandard interaction during its propagation. In particular, we study neutrino interaction with an ultra-light scalar field dark matter. It is shown that the extragalactic neutrino flux may be suppressed by such an interaction, leading to a new mechanism to reduce the ultra-high energy neutrino flux. We study both the cases of non-self-conjugate as well as self-conjugate dark matter. In the first case, the suppression is independent of the neutrino and dark matter masses. We conclude that care must be taken when explaining limits on the neutrino flux through source acceleration mechanisms only, since there could be other mechanisms for the reduction of the neutrino flux.Comment: 15 pages, 4 figures. Important changes implemented. Abstract modified. Conclusions remain. To be published in JCA

FORTE satellite constraints on ultra-high energy cosmic particle fluxes

The FORTE (Fast On-orbit Recording of Transient Events) satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a dual polarization antenna. We investigate the possible RF signature of ultra-high energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in ice. We calculate the sensitivity of the FORTE satellite to ultra-high energy (UHE) neutrino fluxes at different energies beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff. Some constraints on supersymmetry model parameters are also estimated due to the limits that FORTE sets on the UHE neutralino flux. The FORTE database consists of over 4 million recorded events to date, including in principle some events associated with UHE neutrinos. We search for candidate FORTE events in the period from September 1997 to December 1999. The candidate production mechanism is via coherent VHF radiation from a UHE neutrino shower in the Greenland ice sheet. We demonstrate a high efficiency for selection against lightning and anthropogenic backgrounds. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the corresponding particle fluxes assuming this event represents our background level.Comment: added a table, updated references and Figure 8, this version is submitted to Phys. Rev.