Unfolding of the atmospheric neutrino flux spectrum with the new program TRUEE and IceCube

Abstract

High energy charged cosmic rays of unidentified origin arrive constantly at the Earth’s atmosphere. This particles reach energies up to $10^{20}$ eV and are assumed to be accelerated by extragalactic sources. To track back the origin of cosmic rays the extragalactic high energy neutrinos have to be detected on Earth as their simultaneous production is predicted by theories. The neutrinos produced in the Earth’s atmosphere represent the main background for extragalactic neutrinos but at high energies their flux is expected to give way to the extragalactic flux. Therefore, the determination of the atmospheric neutrino flux spectrum can shed light on the contribution of extragalactic neutrinos to the measured flux spectrum. IceCube is a cubic kilometer large neutrino telescope located at the geographic South Pole and is well suited for the detection of high energy neutrinos. The data used for this work was taken while the experiment was under construction and thus the measurement proceeded with the partially finished detector IceCube 59. For the determination of the neutrino energy a sophisticated algorithm is needed to estimate the energy distribution from the measured observables. For this purpose, TRUEE, the new unfolding program is developed based on the proven unfolding program RUN (Blobel, 1984). The software provides a large set of extensions to enable a comfortable and user-friendly unfolding analysis. In this work the new software TRUEE is introduced and applied on the IceCube 59 data to estimate the flux spectrum of the atmospheric neutrinos. The software provides reliable results and the energy range of the spectrum could be extended. The systematic uncertainties of the simulation have a large impact and have to be reduced to make conclusions about the neutrino flux origin.