Zigfried Hampel-Arias
Published: 2017
Total Pages: 346
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Over the past two decades, a more detailed understanding of TeV-scale cosmic rays has emerged which appears to deviate from the isotropic, single power law description of the cosmic ray flux. This may be the result of the distribution of sources within the Galaxy, changes in source spectra, effects from the propagation of cosmic rays from their sources to Earth, or a combination of the three. Supernova remnants are thought to be the most likely source of Galactic cosmic rays, providing a natural power law source spectrum with sufficient power to generate the observed cosmic ray energy density. Yet, recent results from balloon-borne experiments hint at a possible change in the spectral index between 20−50 TeV. These direct detection apparatuses provide the most precise measurements of the cosmic ray flux up to ~30 TeV, beyond which they are limited by the combined effects of their physical dimensions, runtime durations, and a rapidly decreasing flux. Above ~100 TeV, the spectrum has been measured by ground based air shower arrays, with typical systematic uncertainties of order 10%. Despite having the combined measurements from various experimental techniques, their different energy scales and systematics imply that identifying finer structure between 10−100 TeV requires a single experimental method to span the entire range. Furthermore, as the nearest potential source is hundreds of parsecs away and the Larmor radius of TeV scale charged cosmic rays in the Galaxy is of order 10−3 parsecs, the previously observed anisotropy in arrival directions of cosmic rays is unexpected. In order to attain the statistical power necessary to observe TeV cosmic ray anisotropy at the 10−3 level and below, the long data taking periods required are only attainable by air shower arrays. This thesis presents a measurement of the cosmic ray energy spectrum and the energy dependence of the anisotropy on small scales O(10°) using data from the High Altitude Water Cherenkov (HAWC) Observatory, an air-shower array located near Puebla, Mexico that is sensitive to gamma rays and cosmic rays at TeV energies. The analyses in this work comprise data taking periods of order 1 yr containing ~1010 events. An analysis of the cosmic ray Moon shadow is first presented as a verification of the angular resolution and energy scale of the detector. Next, a measurement of the all-particle cosmic ray energy spectrum from 10−500 TeV is shown, with an indication of structure deviating from a single power law. The final results presented in this work show an improved spectral measurement of a particular region of cosmic ray excess at the 10−4 level, previously observed both in HAWC and in other experiments.