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Characteristics of the diffuse astrophysical electron and tau neutrino flux with six years of IceCube high energy cascade data
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Characteristics of the diffuse astrophysical electron and tau neutrino flux with six years of IceCube high energy cascade data

IceCube Collaboration, M. G Aartsen, M Ackermann, J Adams, J. A Aguilar, M Ahlers, M Ahrens, C Alispach, K Andeen, T Anderson, …
arXiv.org
10 Aug 2020
DOI: https://doi.org/10.48550/arxiv.2001.09520
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Abstract

Physics - Cosmology and Nongalactic Astrophysics Physics - High Energy Astrophysical Phenomena
Phys. Rev. Lett. 125, 121104 (2020) We report on the first measurement of the astrophysical neutrino flux using particle showers (cascades) in IceCube data from 2010 -- 2015. Assuming standard oscillations, the astrophysical neutrinos in this dedicated cascade sample are dominated ($\sim 90 \%$) by electron and tau flavors. The flux, observed in the sensitive energy range from $16\,\mathrm{TeV}$ to $2.6\,\mathrm{PeV}$, is consistent with a single power-law model as expected from Fermi-type acceleration of high energy particles at astrophysical sources. We find the flux spectral index to be $\gamma=2.53\pm0.07$ and a flux normalization for each neutrino flavor of $\phi_{astro} = 1.66^{+0.25}_{-0.27}$ at $E_{0} = 100\, \mathrm{TeV}$, in agreement with IceCube's complementary muon neutrino results and with all-neutrino flavor fit results. In the measured energy range we reject spectral indices $\gamma\leq2.28$ at $\ge3\sigma$ significance level. Due to high neutrino energy resolution and low atmospheric neutrino backgrounds, this analysis provides the most detailed characterization of the neutrino flux at energies below $\sim100\,{\rm{TeV}}$ compared to previous IceCube results. Results from fits assuming more complex neutrino flux models suggest a flux softening at high energies and a flux hardening at low energies (p-value $\ge 0.06$). The sizable and smooth flux measured below $\sim 100\,{\rm{TeV}}$ remains a puzzle. In order to not violate the isotropic diffuse gamma-ray background as measured by the Fermi-LAT, it suggests the existence of astrophysical neutrino sources characterized by dense environments which are opaque to gamma-rays.

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