No abstract

Evidence for an extraterrestrial flux of high-energy neutrinos has now been found in multiple searches with the IceCube detector. The first solid evidence was provided by a search for neutrino events with deposited energies 30 TeV and interaction vertices inside the instrumented volume. Recent analyses suggest that the extraterrestrial flux extends to lower energies and is also visible with throughgoing, ν µ-induced tracks from the Northern hemisphere. Here, we combine the results from six different IceCube searches for astrophysical neutrinos in a maximum-likelihood analysis. The combined event sample features high-statistics samples of shower-like and track-like events. The data are fit in up to three observables: energy, zenith angle and event topology. Assuming the astrophysical neutrino flux to be isotropic and to consist of equal flavors at Earth, the all-flavor spectrum with neutrino energies between 25 TeV and 2.8 PeV is well described by an unbroken power law with best-fit spectral index −2.50 ± 0.09 and a flux at 100 TeV of 6.7 +1.1 −1.2 • 10 −18 GeV −1 s −1 sr −1 cm −2. Under the same assumptions, an unbroken power law with index −2 is disfavored with a significance of 3.8 σ (p = 0.0066%) with respect to the best fit. This significance is reduced to 2.1 σ (p = 1.7%) if instead we compare the best fit to a spectrum with index −2 that has an exponential cutoff at high energies. Allowing the electron neutrino flux to deviate from the other two flavors, we find a ν e fraction of 0.18 ± 0.11 at Earth. The sole production of electron neutrinos, which would be characteristic of neutron-decay dominated sources, is rejected with a significance of 3.6 σ (p = 0.014%).

The MiniBooNE experiment at Fermilab reports results from an analysis of νe appearance data from 12.84 × 10 20 protons on target in neutrino mode, an increase of approximately a factor of two over previously reported results. A νe charged-current quasielastic event excess of 381.2 ± 85.2 events (4.5σ) is observed in the energy range 200 < E QE ν < 1250 MeV. Combining these data with theνe appearance data from 11.27 × 10 20 protons on target in antineutrino mode, a total νe plus νe charged-current quasielastic event excess of 460.5 ± 99.0 events (4.7σ) is observed. If interpreted in a two-neutrino oscillation model, νµ → νe, the best oscillation fit to the excess has a probability of 21.1%, while the background-only fit has a χ 2 probability of 6 × 10 −7 relative to the best fit. The MiniBooNE data are consistent in energy and magnitude with the excess of events reported by the Liquid Scintillator Neutrino Detector (LSND), and the significance of the combined LSND and MiniBooNE excesses is 6.0σ. A two-neutrino oscillation interpretation of the data would require at least four neutrino types and indicate physics beyond the three neutrino paradigm. Although the data are fit with a two-neutrino oscillation model, other models may provide better fits to the data.Evidence for short-baseline neutrino anomalies at an L/E ν ∼ 1 m/MeV, where E ν is the neutrino energy and L is the distance that the neutrino traveled before detection, comes from both neutrino appearance and disappearance experiments. The appearance anomalies include the excess of ν e andν e charge-current quasielastic (CCQE) events observed by the LSND [1] and MiniBooNE [2,3] experiments, while the disappearance anomalies, although not completely consistent, include the deficit of ν e andν e events observed by reactor [4] and radioactive-source experiments [5]. As the masses and mixings within the 3-generation neutrino matrix have been attached to solar and long-baseline neutrino experiments, more exotic models are typically used to explain these anomalies, including, for example, 3+N neutrino oscillation models involving three active neutrinos and N additional sterile neutrinos [6][7][8][9][10][11][12][13][14], resonant neutrino oscillations [15], Lorentz violation [16], sterile neutrino decay [17], sterile neutrino nonstandard interactions [18], and sterile neutrino extra dimensions [19]. This Letter presents improved MiniBooNE ν e andν e appearance results, assuming two-neutrino oscillations with probability arXiv:1805.12028v2 [hep-ex]

We report on the observation of two neutrino-induced events which have an estimated deposited energy in the IceCube detector of 1.04±0.16 and 1.14±0.17 PeV, respectively, the highest neutrino energies observed so far. These events are consistent with fully contained particle showers induced by neutral-current ν(e,μ,τ) (ν(e,μ,τ)) or charged-current ν(e) (ν(e)) interactions within the IceCube detector. The events were discovered in a search for ultrahigh energy neutrinos using data corresponding to 615.9 days effective live time. The expected number of atmospheric background is 0.082±0.004(stat)(-0.057)(+0.041)(syst). The probability of observing two or more candidate events under the atmospheric background-only hypothesis is 2.9×10(-3) (2.8σ) taking into account the uncertainty on the expected number of background events. These two events could be a first indication of an astrophysical neutrino flux; the moderate significance, however, does not permit a definitive conclusion at this time.

This paper explores the use of L/E oscillation probability distributions to compare experimental measurements and to evaluate oscillation models. In this case, L is the distance of neutrino travel and E is a measure of the interacting neutrino's energy. While comparisons using allowed and excluded regions for oscillation model parameters are likely the only rigorous method for these comparisons, the L/E distributions are shown to give qualitative information on the agreement of an experiment's data with a simple two-neutrino oscillation model. In more detail, this paper also outlines how the L/E distributions can be best calculated and used for model comparisons. Specifically, the paper presents the L/E data points for the final MiniBooNE data samples and, in the Appendix, explains and corrects the mistaken analysis published by the ICARUS collaboration.