Flavor Ratio of Astrophysical Neutrinos above 35 TeV in IceCube

TL;DR

This study extends IceCube's analysis of astrophysical neutrinos to energies above 35 TeV, examining their flavor composition and testing standard oscillation predictions against exotic physics scenarios.

Contribution

It provides the first detailed flavor ratio measurement of astrophysical neutrinos above 35 TeV, using a novel shower-biased sample to improve flavor discrimination.

Findings

Consistent with the expected (1:1:1) flavor ratio at Earth.

Excludes maximally track-like and shower-like compositions at significant confidence levels.

Places constraints on non-standard neutrino flavor scenarios.

Abstract

A diffuse flux of astrophysical neutrinos above $100\,\mathrm{TeV}$ has been observed at the IceCube Neutrino Observatory. Here we extend this analysis to probe the astrophysical flux down to $35\,\mathrm{TeV}$ and analyze its flavor composition by classifying events as showers or tracks. Taking advantage of lower atmospheric backgrounds for shower-like events, we obtain a shower-biased sample containing 129 showers and 8 tracks collected in three years from 2010 to 2013. We demonstrate consistency with the $(f_e:f_{\mu}:f_\tau)_\oplus\approx(1:1:1)_\oplus$ flavor ratio at Earth commonly expected from the averaged oscillations of neutrinos produced by pion decay in distant astrophysical sources. Limits are placed on non-standard flavor compositions that cannot be produced by averaged neutrino oscillations but could arise in exotic physics scenarios. A maximally track-like composition of $(0:1:0)_\oplus$ is excluded at $3.3\sigma$, and a purely shower-like composition of $(1:0:0)_\oplus$ is excluded at $2.3\sigma$.