A COMBINED MAXIMUM-LIKELIHOOD ANALYSIS OF THE HIGH-ENERGY ASTROPHYSICAL NEUTRINO FLUX MEASURED WITH ICECUBE

By I. C. M. Aartsen, K. Abraham, M. Ackermann, J. Adams, J. Aguilar, M. Ahlers, M. Ahrens, D. Altmann, T. Anderson, M. Archinger and 302 more. 2015

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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, 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 ) × 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 3.8σ (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%).

Subfields: Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/2661a7b91b6f19b857b129087baffa6a5759f9ee

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Citations in the corpus

• KM3MeT 2.0 Letter of intent for ARCA and ORCA (2016, methodology): “The best constraints on an (assumed diffuse) astrophysical spectrum come from a maximum likelihood analysis using both HESE and other event samples [14], finding a neutrino flux proportional to E−2.”
• Open Questions in Cosmic-Ray Research at Ultrahigh Energies (2019, methodology): “…redshift evolution of the number density of sources, etc.), which can be significant; • Flavor is a difficult property to measure in neutrino telescopes (Aartsen et al., 2015a,b; Vincent et al., 2016; Bustamante and Ahlers, 2019); improved methods of flavor identification might be needed to…”
• All-sky Search for Time-integrated Neutrino Emission from Astrophysical Sources with 7 yr of IceCube Data (2016, methodology): “The second interesting event is a straight down-going starting track at 430 TeV deposited energy(Aartsen et al. 2015f).”
• OBSERVATION AND CHARACTERIZATION OF A COSMIC MUON NEUTRINO FLUX FROM THE NORTHERN HEMISPHERE USING SIX YEARS OF ICECUBE DATA (2016, methodology): “With this result we have further established the observation of an astrophysical neutrino signal (Aartsen et al. 2013a, 2014b, 2015c) in a second, largely independent detection channel.”
• Extending the Search for Muon Neutrinos Coincident with Gamma-Ray Bursts in IceCube Data (2017, methodology): “Combined with the previously published four-year northern hemisphere track (Aartsen et al. 2015d) and three-year all-sky cascade(Aartsen et al. 2016a) analyses, the stacked analysis has a final significance of p=0.60, consistent with the background-only hypothesis.”
• IceCube-Gen2: the window to the extreme Universe (2020, methodology): “data [37, 43, 196] indicate consistency with the benchmark prediction from complete pion decay of νe ∶ νμ ∶ ντ = 1 ∶ 2 ∶ 0 which is transformed to approximately 1:1:1 by neutrino oscillations over astronomical distances [197].”