Matthew Meehan
Published: 2019
Total Pages: 156
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Although cosmic rays do not point back to their sources, the distribution of their arrival directions can be used to constrain propagation models, study the distribution of their sources, and probe the structure of the local interstellar environment. A small, part-per-mille, anisotropy in the arrival directions of cosmic rays has been measured at both large and small angular scales by many experiments. Diffusion theory predicts a dipole anisotropy, but decades of observations have resulted in measurements at least an order of magnitude below this naive prediction. Solving the cosmic-ray anisotropy problem has proven difficult, in part due to observational biases of ground-based measurements that render physical information about the anisotropy inaccessible. Spaced-based observatories can provide new information to help shed light on the origin of the anisotropy. In particular, they can offer all-sky measurements that constrain the full two-dimensional phase of the anisotropy, while ground-based experiments only measure the anisotropy along right ascension. The first ever search for cosmic-ray proton anisotropy using the Fermi Large Area Telescope (LAT) is presented in this thesis. The LAT's wide field of view, large effective area, and long exposure time have resulted in the largest data set of primary protons recorded by any instrument. A search for cosmic-ray proton anisotropy is performed using eight years of LAT data from 78 GeV to almost 10 TeV in energy. As the first analysis of its kind using LAT data, a custom event selection suitable for an anisotropy search was developed and systematics unique to this measurement were studied for the first time. The final data set, comprised of 179 million protons, is the largest full-sky, single instrument data set studying anisotropy at these energies to date, capable of probing dipole anisotropy below an amplitude of $10^{-3}$. It is also the most sensitive data set to the declination component of the anisotropy of any experiment to date. We measure a dipole amplitude $\delta = 3.9 \times 10^{-4}$ with a p-value of 0.01 (pre-trials) for protons with a minimum energy of 78 GeV. The weak statistical excess makes it difficult to rule out signal or systematic interpretation of the measurement. We therefore discuss various systematic effects that could give rise to the dipole excess and calculate upper limits on the dipole amplitude as a function of minimum energy. The 95\% CL upper limit on the dipole amplitude is $\delta_{UL}=1.3\times 10^{-3}$ for protons with a minimum energy of 78 GeV and $\delta_{UL}=1.2 \times 10^{-3}$ for protons with a minimum energy of 251 GeV.