Search for Astrophysical Tau Neutrinos in Six Years of High-Energy Starting Events in IceCube

Abstract

The IceCube Neutrino Observatory at the geographic South Pole is a cubic kilometer Cherenkov detector built to measure high-energy neutrinos from cosmic sources. It has reported the detection of a diffuse flux of astrophysical neutrinos in the energy range from $\rm\sim10\,TeV$ to $\rm\sim10\,PeV$ consistent with a neutrino flavor ratio of $\mathrm{\nu_e:\nu_\mu:\nu_\tau\simeq1:1:1}$ as expected from pion decay in astrophysical sources after propagation to Earth. However, no tau neutrino has been identified so far. Its observation would be a smoking gun for astrophysical neutrinos and constrain their possible sources. The double bang channel is most promising for identifying tau neutrino interactions. Its event signature is unique to the tau flavor, linking two consecutive particle showers from the charged current interaction of a tau neutrino with an ice nucleus and the subsequent decay of the produced tau lepton. It can only be well resolved at deposited energies above a few $\rm 100\,TeV$ where the average tau decay length is larger than $\rm 20\,m$. Results are presented from an analysis which uses an optimized direct reconstruction of the double bang event signature using six years of high-energy starting events (HESE) in IceCube. It is the most recent search for tau neutrinos allowing a measurement of the high-energy flavor ratio which, for the first time, is sensitive to the tau neutrino fraction.