A combined maximum-likelihood analysis of the high-energy astrophysical neutrino flux measured with IceCube
(2015) Astrophysical Journal — Vol. 809 (2015)
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- 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 $\gtrsim 30$ 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, νμ-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 $({6.7}_{-1.2}^{+1.1})\times {10}^{-18}\,{\mathrm{GeV}}^{-1}\,{{\rm{s}}}^{-1}\,{\mathrm{sr}}^{-1}\,{\mathrm{cm}}^{-2}$. Under the same assumptions, an unbroken power law with index −2 is disfavored with a significance of 3.8σ (p = 0.0066%) with respect to the best fit. This significance is reduced to 2.1σ (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 νe 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 3.6σ (p = 0.014%).
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de Wasseige, G., IceCube, & et al. (2015). A combined maximum-likelihood analysis of the high-energy astrophysical neutrino flux measured with IceCube. Astrophysical Journal, 809. https://doi.org/10.1088/0004-637X/809/1/98 (Original work published 2015)