The Case of the Missing Neutrino: Astrophysical Messengers of Planck-Scale Physics

Abstract

In recent decades, particle physics has made significant advances in our understanding of high-energy phenomena, both on theoretical and experimental fronts, culminating in the current Standard Model (SM) of particle physics. However, the SM is not a self-contained theory and leaves many questions unanswered; in particular, it falls short of incorporating dark matter or gravitational forces. Among the implications for this is that the SM is a low-energy, effective limit of a more fundamental theory that includes a quantum description of gravity. However, without the means to probe such extreme scales, we have no way of knowing what such a theory would look like.

It turns out that neutrinos can give us some clues. For one, they are the only SM particle that exhibit non-SM effects in the form of lepton-flavor-violating processes. To first order, this can happen because they have a mass, although we still do not know how this mass can arise. However, higher-order effects can also contribute if the underlying theory exhibits spontaneous CPT- and Lorentz-symmetry breaking. The effective theory that arises from this can lead to the presence of observables that exhibit low-energy manifestations of these symmetry violations, implicating neutrinos and their coincidentally-produced astrophysical messengers. These are ideal candidates, because when it comes to the origin and nature of the astrophysical neutrino flux, there are many questions waiting to be answered. These neutrinos must come from highly energetic sources, where hints of new physics could be hiding in wait. Could higher-order oscillation effects point us in the right direction towards a better understanding of high-energy neutrinos?

In this thesis, I cover some of the work that I have done in examining effective theories of high-energy phenomena. In an effort to characterize the astrophysical neutrinos that IceCube has observed, I discuss what we can learn about the physics that happens during neutrino production, detection, and anything that goes on in between. In so doing, I present the newest IceCube results testing the Standard Model Extension (SME), which extend the constraints for a dimension-6 coefficient beyond the Planck scale for the first time in an atmospheric and astrophysical study.