Flavor Ratio of Astrophysical Neutrinos above 35 TeV in IceCube (1502.03376v1)

Abstract: A diffuse flux of astrophysical neutrinos above $100\,\mathrm{TeV}$ has been observed at the IceCube Neutrino Observatory. Here we extend this analysis to probe the astrophysical flux down to $35\,\mathrm{TeV}$ and analyze its flavor composition by classifying events as showers or tracks. Taking advantage of lower atmospheric backgrounds for shower-like events, we obtain a shower-biased sample containing 129 showers and 8 tracks collected in three years from 2010 to 2013. We demonstrate consistency with the $(f_e:f_{\mu}:f_\tau)\oplus\approx(1:1:1)\oplus$ flavor ratio at Earth commonly expected from the averaged oscillations of neutrinos produced by pion decay in distant astrophysical sources. Limits are placed on non-standard flavor compositions that cannot be produced by averaged neutrino oscillations but could arise in exotic physics scenarios. A maximally track-like composition of $(0:1:0)\oplus$ is excluded at $3.3\sigma$, and a purely shower-like composition of $(1:0:0)\oplus$ is excluded at $2.3\sigma$.

Analysis of the Flavor Ratio of High-Energy Astrophysical Neutrinos in IceCube

The paper "Flavor Ratio of Astrophysical Neutrinos above 35 TeV in IceCube" discusses the investigation of the astrophysical neutrino flux detected using the IceCube Neutrino Observatory, extending the previous analysis down to an energy threshold of 35 TeV. This analysis specifically focuses on identifying the flavor composition of the neutrino flux, providing crucial insights into the high-energy processes occurring in the universe.

Key Findings

The authors leverage the IceCube detector's ability to differentiate between neutrino interaction topologies, distinguishing between shower-like (cascade) and track-like events. The analysis employs a sample consisting of 129 shower-like events and 8 track-like events collected over three years (2010-2013). The paper reinforces the expectation of a flavor ratio at Earth of approximately (1:1:1), originating from distant astrophysical sources where neutrinos undergo flavor oscillations during propagation.

1. Astrophysical Neutrino Flux and Spectrum:
   • The paper constrains the spectral index of the astrophysical neutrino flux, finding a best-fit value of approximately 2.6. This deviates from the canonical index of 2 suggesting that the source mechanisms or propagation effects may differ from initial assumptions.
   • A purely power-law spectrum is favored over one with a high-energy cutoff.
2. Flavor Composition Results:
   • The analysis places bounds on flavor compositions, excluding a purely muon neutrino dominated composition (0:1:0) at 3.3σ and purely electron neutrino composition (1:0:0) at 2.3σ confidence levels.
   • Constraints are consistent with expectations arising from standard neutrino oscillations translating a source composition of (1:2:0) to (1:1:1) at Earth.
3. Atmospheric Neutrino Backgrounds:
   • The observed events were not compatible with being solely induced by atmospheric neutrino backgrounds, including prompt neutrinos (from charm decays), thereby confirming the astrophysical nature of the signal.

Implications

The results provide a compelling validation for neutrino oscillation physics over astronomical distances. The consistency of the Earth-based flavor composition with theoretical predictions provides further constraints on exotic physics scenarios, such as new interaction mechanisms or decay channels, that would result in non-standard flavor ratios.

Furthermore, these results contribute to the broader understanding of cosmic ray sources and high-energy astrophysical phenomena, as the detailed neutrino spectrum and composition can provide hints about the acceleration mechanisms and environments from which these particles originate.

Future Directions

Future studies could further refine the flavor composition analysis with improved statistics and enhanced detector capabilities. This includes utilizing advanced veto techniques to reduce the atmospheric background and exploring multi-PeV energy ranges. Exploration of tau neutrino identification methods could also resolve the intrinsic degeneracy between tau and electron neutrino signatures in current analyses, providing a complete view of high-energy neutrino astrophysics.

Such advancements will enable more stringent tests of the neutrino oscillation framework and may potentially uncover evidence for physics beyond the Standard Model if deviations from expected flavor ratios are observed.