Sterile Neutrino Searches at the IceCube Neutrino Observatory

TL;DR

This paper reports on two analyses using eight years of IceCube data to search for sterile neutrinos through spectral differences in muon neutrino events, finding results consistent with no sterile neutrino presence.

Contribution

The study introduces improved event selection, novel simulation techniques, and updated calibration for sterile neutrino searches at IceCube, covering new parameter space regions.

Findings

Best fit sterile hypothesis at Δm²₄₁=4.47 eV², sin²(2θ₂₄)=0.10, consistent with no sterile neutrino at 8% confidence.

Best fit at sin²(2θ₃₄)=0.40, sin²(2θ₂₄)=0.006, consistent with null hypothesis at 19% confidence.

Analysis of eight years of data with over 300,000 well reconstructed muon neutrino events.

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 $\Delta$m$^2_{41}$ and sin$^2$(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 sin$^2$(2$\theta_{24}$) and sin$^2$(2$\theta_{34}$) in the oscillation averaged out region of high-$\Delta$m$^2_{41}$ ($\Delta$m$^2_{41}$ $\gtrsim$ 10eV$^2$). 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 $\Delta$m$^2_{41}$ = 4.47eV$^2$ and sin$^2$($\theta_{24})$ =0.10, consistent with the no-sterile hypothesis at the 8\% confidence level. The second analysis finds a best fit sterile hypothesis at sin$^2$($\theta_{34})$ = 0.40, sin$^2$($\theta_{24})$ = 0.006, consistent with the null hypothesis at the 19\% confidence level. % and provides a significant improvement in the worlds measurements using muon neutrino disappearance.