Unfolding the Atmospheric Neutrino Flux Using the IceCube/DeepCore Neutrino Telescope

Abstract

Abstract Cosmic rays in the form of high energy particles, strike the Earth every second. Since the first discoveries of the cosmic ray flux in the early nineteenth century, the origin and properties of the cosmic rays have been the subject of much investigation and debate. Cosmic ray interactions in the upper atmosphere create particle showers, containing several species of particles. The daughter particles generated in these showers carry with them information about the interaction dynamics and properties of the parent cosmic rays. One group of daughter particles are the neutrinos, which are left handed leptons included in the standard model of particle physics. While they were previously believed to be massless, progress over the past two decades have established their mass through observation of oscillations, particularly in atmospheric neutrinos. This work presents an investigation of the atmospheric neutrino flux measured with the IceCube South Pole Neutrino Observatory, using data from 5 seasons of IceCube operation. Modeling of the atmospheric neutrino flux is a multi dimensional problem and at the time of publication the uncertainties on both theoretical predictions and measurements are of such scale that all models and measurements are in agreement. This work aims to improve the precision in measurement in order to give better discriminating power between models. The analysis presented herein performs an unfolding in three dimensions, energy, zenith angle and particle identification channel. The method is as model independent as possible and utilizes iterative Bayesian unfolding to measure the unfolded event rate by detection volume for two particle groups: \nu_{\mu}^{\mathrm{cc}}+\bar{\nu}_{\mu}^{\mathrm{cc}} constituting the muon neutrino charged current interactions, and \nu_{\mathrm{rest}} constituting all other flavor and interaction types. A total of 204847 neutrino candidate events are observed and unfolded. Great care is taken to avoid bias in the unfolding via a series of closure tests. The unfolded results yield true interaction rates on the scale of 10^{-10}[\mathrm{m^{-3}}\mathrm{s^{-1}]}. The unfolded results are generally in good agreement with previous unfolded data from Super Kamiokande, but do show some tension with current predictions - particularly in the low energy region below 80GeV and in the upgoing direction. The relative uncertainty on the unfolded result lies between 3\mathrm{\%} and 13\% for the energy spectrum, with low uncertainty particularly in the region of interest to neutrino oscillation measurements. For the \cos(\theta_{z}) spectrum the relative uncertainty lies between 4% and 7.5% in the upgoing region, while it rises to 18.5\mathrm{\%} in the downgoing region. Even with these uncertainties, the results presented in this dissertation constitute the most precise measurement of the atmospheric neutrino flux at the time of publication.