Through Iron & Ice: Searching for Sterile Neutrinos at the IceCube Neutrino Observatory

Abstract

Despite the rapid progression in our understanding of neutrinos over the last half century, much is left unknown about their properties. This leaves neutrinos as the most promising portal for Beyond Standard Model (BSM) physics, and neutrinos have already provided fruitful surprises.

A number of neutrino experiments in the last three decades have observed anomalous oscillation signals consistent with a mass-squared splitting of Δ𝑚2 ∼ 1 eV^2, motivating the existence and search for sterile neutrinos. On the other hand, other experiments have failed to see such a signal.

In this thesis, we present two analyses. The first is an update to the sterile neutrino global fits with the inclusion of recent experimental data. We find that the 3+1 model provides a better fit to the global data set compared to the null, with an improvement of Δ𝜒2 = 51 with the addition of only 3 degrees of freedom, corresponding to 6.6𝜎. While a substantial improvement, we also find a irreconcilable tension between the data sets of 5.1𝜎, calculated using the parameter goodness-of-fit test. This motivates the exploration of expanded models: a 3+2 model, and a 3+1+Decay model. In the 3+2 model, we find negligible improvement to the fit, and an even worse tension of 5.5𝜎. In the more exotic 3+1+Decay model, we find the tension reduced to 3.6𝜎. While a substantial improvement compared to the 3+1 model with the introduction of only one additional parameter, the tension is still too large to assuage concerns.

The second analysis is the results of an expanded IceCube sterile neutrino search. A previous sterile neutrino search found no evidence for sterile neutrinos, finding a p-value of 8%. Of the three sterile mixing angles, 𝜃_14,𝜃_24, and 𝜃_34, only 𝜃_24 was fitted for, as 𝜃_14 was negligible and 𝜃_34 = 0 was considered a conservative assumption. We present results of an analysis where we include 𝜃_34 to the fitted model. Both a frequentist and Bayesian analysis were conducted, with fits done in terms of the mass-squared splitting Δ𝑚^2_41 and the mixing matrix parameters |𝑈_𝜇4|^2 and |𝑈_𝜏4|^2. The frequentist analysis finds a best fit at Δ𝑚^2_41 = 5.0 eV^2, |𝑈_𝜇4|^2 = 0.04, and |𝑈_𝜏4|^2 = 0.006, with a p-value of 5.2% assuming Wilks’ Theorem with 3 degrees of freedom. Pseudoexperiments are indicating a smaller p-value 2.7%. The Bayesian analysis finds a similar best fit point at Δ𝑚^2_41 = 5.0 eV^2, |𝑈_𝜇4|^2 = 0.02, and |𝑈_𝜏4|^2 = 0.006, with a Bayes factor indicating a “Very Strong” preference for this sterile hypothesis over the null hypothesis.