Abstract
The IceCube experiment has recently released 3 years of data of the first ever detected high-energy (≳30TeV) neutrinos, which are consistent with an extraterrestrial origin. In this talk, we compute the compatibility of the observed track-to-shower ratio with possible combinations of neutrino flavors with relative proportion (αe:αμ:ατ)⊕. Although this observation is naively favored for the canonical (1 : 1 : 1)⊕ at Earth, once we consider the IceCube expectations for the atmospheric muon and neutrino backgrounds, this flavor combination presents some tension with data. We find that, for an astrophysical neutrino Eν−2 energy spectrum, (1 : 1 : 1)⊕ at Earth is currently disfavored at 92% C.L. We discuss the trend of this result by comparing the results with the 2-year and 3-year data. We obtain the best-fit for (1 : 0 : 0)⊕ at Earth, which cannot be achieved from any flavor ratio at sources with averaged oscillations during propagation. Although it is not statistically significant at present, if confirmed, this result would suggest either a misunderstanding of the expected background events, or a misidentification of tracks as showers, or even more compellingly, some exotic physics which deviates from the standard scenario.
Highlights
The first evidence for a high-energy neutrino flux of extraterrerstrial origin was obtained with a 2-year search in the IceCube neutrino detector, from May 2010 to May 2012 [1, 2]
For the fraction of background showers and tracks in the 30 TeV − 2 PeV energy range, we use the numbers quoted by the IceCube collaboration: tracks account for 69% of the conventional atmospheric neutrino event rate, 19% of the prompt atmospheric neutrino event rate and 90% of the events induced by atmospheric muons [3]
We note that the best-fit point is (1 : 0 : 0)⊕, which cannot be obtained from any flavor ratio at sources assuming averaged oscillations during propagation
Summary
The first evidence for a high-energy neutrino flux of extraterrerstrial origin was obtained with a 2-year search in the IceCube neutrino detector, from May 2010 to May 2012 [1, 2] In this period, 28 veto-passing events were recorded (7 tracks and 21 showers) with deposited energies between ∼30 TeV and ∼1 PeV. For the 2-year (3-year) data, IceCube expects to see 8.6 (12.1) tracks from the background [2, 3], whereas only 7 (9) tracks have been observed above ∼30 TeV in deposited energy This would imply that the astrophysical component overwhelmingly produces showers inside the detector. As a result of (photo)hadronic interactions, astrophysical neutrinos are commonly modeled as the decay products of pions, kaons and secondary muons, and the expectation for the neutrino flavor ratio at the source is (αe,S : αμ,S : ατ,S) =(1 : 2 : 0)S.
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