Monocular measurement of the ultra-high energy cosmic ray spectrum
Creators: name: Shah, Priti Dhanesh
Publisher: name: University of Utah
Year: 2012
Subjects: ankle, Cosmic Rays, Energy Spectrum, fluorescence detector, GZK cutoff, Telescope Array

description: The Telescope Array Project was designed to observe cosmic rays with energies greater than 1018 eV. Its goals are to study the physics of cosmic rays by measuring their anisotropy, composition, and energy spectrum. This work makes a monocular measurement of the ultra high energy cosmic ray spectrum and analyzes the physics produced from that spectrum. The flux of cosmic rays observed on Earth follows a power law over 12 decades in energy and 32 decades in flux. At the highest energies, the spectrum has detailed structure. Studying these features can tell us about the astrophysics of the production and propagation of cosmic rays. First, it can tell us about the sources of cosmic rays such as they capable of producing a power law spectrum and the maximum energy of cosmic rays that they can produce. Second, the acceleration mechanisms that can boost cosmic rays to ultra high energies can be studied. Third, the spectral features themselves can tell us about their possible cause for formation. For example, the ankle feature in the ultra high energy regime can tell us if it is the galactic-extragalactic transition or if it is due to e+e− pair production. Fourth, the energy losses that cosmic rays incur can tell us about their physical interactions during propagation. Studying the physics of the cosmic ray spectrum in the ultra high energy regime with data from the Telescope Array Project is the goal of this analysis. The Telescope Array Project consists of three fluorescence detectors overlooking an array of 507 scintillation surface detectors. Due to their extremely low flux at these energies, cosmic rays can only be observed indirectly via an extensive air shower produced when they collide with the nucleus of an atom in the Earth's atmosphere. These charged secondary particles produce fluorescence light. The array of surface detectors observes the lateral footprint of the extensive air shower when it reaches the ground. The fluorescence detectors observe the longitudinal profile of this fluorescence light. This thesis analyzes the data from one of the fluorescence detectors, Middle Drum, using a different geometry reconstruction technique, the Time versus Angle geometry. The results of this analysis show an ultra high energy cosmic ray spectrum that is consistent with the results previously published by the High Resolution Fly's Eye (HiRes) experiment, the Telescope Array surface detectors, and other experiments in this energy region. Due to insufficient statistics at this date, the GZK cutoff cannot be confirmed in this analysis, but a fit shows the cutoff to be at log10 E (E/eV) = 19.56 ± 0.36, with a spectral index after the cutoff of -3.86 ± 2.0. This is within the range determined previously by other measurements. This analysis shows that the feature known as the ankle occurs at log10 E (E/eV) = 18.63 ± 0.09, with a spectral index of -3.27 ± 0.07 before the ankle and a spectral index of -2.81 ± 0.10 after the ankle. The normalized log likelihood per degree of freedom is 0.90. The ankle is observed at the 4−5! confidence level. The fit to the ankle is also in excellent agreement with previous measurements, and even more remarkable given that some other measurements use different techniques. While this study cannot tell us information about the sources or the acceleration mechanisms of cosmic rays, it does show us a feature and tell us about energy losses during propagation. The dip at the ankle is clearly visible in the spectrum. The results of this study are consistent with the energy loss model of extragalactic protons interacting with the cosmic microwave background radiation and supports the idea that the ankle is excavated due to e+e− pair production. The location of the ankle at a threshold greater than for e+e− pair production supports that the ankle is a composite feature where the redshift energy losses begin to dominate the e+e− pair production losses. The location of the ankle also implies that sources at larger distances than the GZK cutoff contribute to its formation.