Master thesis defence by Ida Storehaug

Title: Improving the Atmospheric Neutrino Flux Estimation in IceCube  

In neutrino oscillation studies in IceCube, the neutrinos making up the signal are of atmospheric origin. An accurate and precise estimate of the atmospheric neutrino flux is therefore crucial in analyses. In recent IceCube analyses, the uncertainty on the atmospheric neutrino flux is the third-largest source of systematic uncertainty, and is expected to increase with the IceCube Upgrade. This thesis presents work on developing a more accurate and precise atmospheric neutrino estimate, based on an analytic solution of the cascade equation with MCEq. One of the key considerations is the re-interactions of hadrons in the air shower. As part of this work, the event generator PYTHIA has been extended to be used as a hadronic interaction model for air showers. The final result of this work is the development of a new treatment of the atmospheric neutrino flux within low energy analyses in IceCube. A tau neutrino appearance analysis with three years of IceCube data is repeated with the developed method of estimating the atmospheric neutrino flux and its uncertainty. When no other sources of uncertainty are taken into account, the systematic uncertainty on the physics value is improved by around 35%.