A combined maximum-likelihood analysis of the high-energy astrophysical neutrino flux measured with IceCube (1507.03991v2)

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 $\gtrsim30$ 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, $\nu_\mu$-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 $-2.50\pm0.09$ and a flux at 100 TeV of $\left(6.7_{-1.2}^{+1.1}\right)\cdot10^{-18}\,\mathrm{GeV}^{-1}\mathrm{s}^{-1}\mathrm{sr}^{-1}\mathrm{cm}^{-2}$. Under the same assumptions, an unbroken power law with index $-2$ is disfavored with a significance of 3.8 $\sigma$ ($p=0.0066\%$) with respect to the best fit. This significance is reduced to 2.1 $\sigma$ ($p=1.7\%$) if instead we compare the best fit to a spectrum with index $-2$ that has an exponential cut-off at high energies. Allowing the electron neutrino flux to deviate from the other two flavors, we find a $\nu_e$ fraction of $0.18\pm0.11$ 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 $\sigma$ ($p=0.014\%$).

• The paper presents a combined maximum-likelihood analysis that integrates six IceCube data sets to robustly measure the astrophysical neutrino flux.
• The study finds a power-law energy spectrum with a spectral index of -2.50 ± 0.09, challenging the classic unbroken E^-2 assumption.
• It confirms isotropy and equal flavor composition while setting stringent upper limits on prompt atmospheric neutrinos.

Overview of "A combined maximum-likelihood analysis of the high-energy astrophysical neutrino flux measured with IceCube"

The paper "A combined maximum-likelihood analysis of the high-energy astrophysical neutrino flux measured with IceCube" by Aartsen et al. presents a comprehensive statistical paper of the high-energy neutrinos detected using the IceCube Neutrino Observatory. This work synthesizes information from six distinct data sets of neutrino events, incorporating various event topologies and interaction types, to robustly estimate the astrophysical neutrino flux.

Key Findings

• Astrophysical Neutrino Flux: The analysis suggests that the astrophysical neutrino flux is well-described by a power-law energy spectrum with a spectral index of $-2.50 \pm 0.09$. The flux normalization at 100 TeV is calculated as $$6.7_{-1.2}^{+1.1} \times 10^{-18} \, \text{GeV}^{-1} \text{s}^{-1} \text{sr}^{-1} \text{cm}^{-2}$$.
• Isotropy and Flavor Composition: Assuming isotropy and an equal flavor distribution at Earth ($\nu_e:\nu_\mu:\nu_\tau = 1:1:1$), the paper finds a high degree of agreement with the neutrino data. The analysis disfavors an unbroken spectrum $E^{-2}$ with high significance, indicating a possible deviation from standard Fermi acceleration predictions.
• Prompt Atmospheric Neutrinos: The paper finds no evidence for a substantial contribution from prompt atmospheric neutrinos and places upper limits on this background component, slightly higher than previous studies, due to combined systematic uncertainties.

Methodology

The paper employs a maximum-likelihood approach to fit six data sets collected by IceCube, encompassing different periods and conditions. This allows for an integrated analysis that accounts for both systematic and statistical uncertainties across various neutrino events: contained and throughgoing, track-like and shower-like, as well as muon and electron neutrinos. The sampled events covered energies from approximately 25 TeV to 2.8 PeV, analyzed through their energy, zenith angle, and topology.

The fits are conducted under multiple hypotheses regarding energy spectrum shape, isotropy, and flavor composition. Nuisance parameters are introduced to account for uncertainties in cosmic-ray interactions and muon backgrounds, enhancing the robustness of the statistical confidence in their estimates.

Implications and Future Directions

The findings push the understanding and constraints of astrophysical neutrino properties. The spectral steepness raises questions about the mechanisms responsible for neutrino production and propagation in extragalactic sources. The inability to detect a significant prompt atmospheric component suggests either a lower yield than predicted or challenges in observing such neutrinos within current experimental constraints.

The paper hints at future investigations by suggesting that more detailed studies on spectral variations and improved flavor composition measurements could be pivotal. Identification of neutrino sources and their underlying physical processes will benefit from improved statistical and detector techniques, possibly by future observatories with increased sensitivity or complementary methodology.

Conclusion

This paper marks a significant step in neutrino astronomy, tightening constraints on the characteristics of the high-energy astrophysical neutrino flux as observed by IceCube. It provides strong numerical evidence supporting an anisotropic but consistent astrophysical neutrino presence across TeV to PeV scales, challenging some of the prevailing theoretical models and guiding the search for astrophysically relevant phenomena.