Sterile neutrino searches at the Icecube Neutrino Observatory

Abstract

The IceCube Neutrino Observatory is capable of performing a unique search for sterile neutrinos through the exploitation of a matter enhanced resonant neutrino oscillation phenomena. As atmospheric muon neutrinos pass the dense material within the Earth, neutral current elastic forward scattering is predicted to induce a transition into a sterile state. This thesis presents two 3+1 sterile neutrino analyses by searching for spectral differences in the reconstructed energy and zenith direction of muon neutrino events, indicative of a transition into a sterile state. The first search probes the parameter space [delta]m²₄₁ and sin²(2[theta]₂₄) with relevant sensitivity to the global best fit region for a 3+1 sterile neutrino hypothesis. The second search performs a scan through sin²(2[theta]₂₄) and sin²([theta]₃₄) in the oscillation averaged out region of high-[delta]m²₄₁ ([theta]²₄₁ >~ 10 eV²). The analyses are performed using an improved event selection, which was found to extract 305,891 well reconstructed muon neutrino events with a sample purity above 99.9%, from eight years of IceCube data. Novel simulation techniques, along with updated calibration, and a re-assessment of the systematic uncertainties are also discussed. The first analysis finds a best fit sterile hypothesis point at [theta]²₄₁ = 4.47eV² and sin²([theta]₂₄ = 0.10, consistent with the no-sterile hypothesis at the 8% confidence level. The second analysis finds a best fit sterile hypothesis at sin²([theta]₃₄ = 0.40, sin²([theta]₃₄ =0.006, consistent with the null hypothesis at the 19% confidence level.