Characterizing the Atmospheric Neutrino Spectrum with the IceCube Neutrino Observatory

  • Author / Creator
    Wood, Tania
  • Neutrinos, one of nature's fundamental particles, have been demonstrated to oscillate (change flavour) from their point of production to detection; implying neutrinos must have mass and hence providing the first evidence for physics beyond the Standard Model of Particle Physics. Atmospheric neutrinos, produced in cosmic ray interactions with the Earth's atmosphere, played a crucial role in the first measurements of neutrino oscillations, thereby launching the current era of precision measurements of neutrino properties. These enigmatic particles from our atmosphere continue to comprise an important part of the global neutrino effort, acting as a primary signal for some experiments, and a key background consideration for others. In particular, advancing our understanding of the production mechanisms, and hence the resultant flux, of atmospheric neutrinos has become increasingly important as planning for the next generation of particle astrophysics detectors takes centre stage in this rapidly evolving field.

    This thesis presents a study of the atmospheric neutrino flux in an energy range between 5.6~GeV and 180~GeV reconstructed energy. Particular attention is paid to measurements of the kaon-to-pion parent meson contributions to the flux, providing a direct measure of the production mechanisms of the neutrinos in the atmospheric interactions. The study utilizes the world's largest accumulated data set of atmospheric neutrinos, detected with the cubic-kilometre-scale IceCube Neutrino Observatory located at South Pole Station, Antarctica.

    The results of the analysis are the first atmospheric neutrino energy spectrum measurements from IceCube's DeepCore low-energy detector array. The data set extends the previous measured IceCube atmospheric neutrino energy spectrum from 100~GeV in $\nu{\mu}$~\cite{NuMuprev} and 80~GeV in $\nue$~\cite{NuEprev}, down to 5.6~GeV; providing the highest precision to date in much of the considered energy range. A series of 19 leading atmospheric neutrino flux models were tested against the IceCube-DeepCore data; the best fit of the atmospheric models was rejected at a level of 3.79$\sigma$. Due to limited statistics in the data sample above $\sim$80~GeV, it was found that this analysis had only limited sensitivity to the kaon-to-pion ratio and provided a measurement of predominately pion parent mesons in the sample. We note that the methods used in this analysis are flux preserving, unlike previous IceCube-DeepCore oscillation results, and we find the extracted oscillation parameters to be within the 90\% confidence limits of the previous results~\cite{josh}.

    The results from this study directly impact the on-going and future measurements and modelling of atmospheric neutrinos and their production processes.
    It is rather remarkable that, after providing one of the key measurements that launched the field of measuring neutrino mixing parameters, the atmospheric neutrino oscillation parameters are now some of the least well known. One of the primary limitations to improving this scenario has been a precise knowledge of the source of atmospheric neutrinos at energies neighbouring those where oscillation measurements are prevalent.

    The presented analysis investigates a crucial (overlap) region for the atmospheric neutrino flux energy spectrum that complements previous measurements at lower-energies ({\it e.g.} by the Super-Kamiokande experiment in Japan) and the significantly higher energy regime opened by the IceCube detector. This overlap region has largely been lacking in experimental measurements, affecting the historical precision of estimating the potential flux contributions at these energies. The direct measurements provided here augment the active modelling of the atmospheric neutrino predictions at these energies, providing a path to directly improve the current and future atmospheric neutrino oscillation programs.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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