Astrophysical tau neutrinos in IceCube

The IceCube neutrino observatory at the South Pole has confirmed the existence of a diffuse astrophysical neutrino flux. The flavor composition of astrophysical neutrinos carries information on the environments at the sites of cosmic particle acceleration as well as potential imprints of new physics acting during neutrino propagation. To measure the flavor composition the observation of the long-elusive tau neutrinos is required. Starting at an energy of ~ O(100 TeV) a tau neutrino charged current interaction can produce a double cascade topology, where the two energy depositions from the tau creation and the tau decay vertices are resolvable. This topology together with the well-established track and single cascade topology is used to measure the flavor composition on Earth. In this work, high-energy events starting in IceCube's detector volume are classified algorithmically into the three topologies. In the dataset with a livetime of 7.5 years, two events are classified as double cascades for the first time, yielding multi-TeV tau-neutrino candidates. The properties of the two tau-neutrino candidates are investigated in an a-posteriori analysis. The statistical method is improved by performing a log-likelihood-ratio test using multi-dimensional probability densities. One of the double cascades is consistent with being a misclassified single cascade, while the second double cascade is found to have a misclassification probability of only 3%. The measured flavor composition ${\nu }_e$ : ${\nu }_{\mu }$ : ${\nu }_{\tau }$ = 0.20 : 0.39 : 0.42 is consistent with astrophysical neutrinos and with previously published results. The astrophysical tau-neutrino flux is measured to $\frac{d{\Phi }_{\nu \tau }}{d{E}_{\nu }}$ = 3.0${}_{-1.8}^{+2.2}$$\cdot (\frac{E}{100TeV}{)}^{-2.87}$$\cdot$10${}^{-18}$$\cdot$GeV${}^{-1}$ cm${}^{-2}$ s${}^{-1}$ sr${}^{-1}$ with spectral index $\gamma$ = 2.87${}_{-0.20}^{+0.21}$, yielding the first non-zero results for the tau normalization. The absence of an astrophysical tau-neutrino flux is disfavored at 2.8 $\sigma$.