Determining the Mass Composition of Ultra-high Energy Cosmic Rays Using Air Shower Universality

Ultra-high energy cosmic rays are accelerated via the most energetic and powerful processes  in the Universe. For over a hundred years, the study of these particles has elicited great  interest. While our knowledge and theoretical models have vastly improved over the last  century, the exact sites at which and physical mechanisms by which the acceleration of these  charged nuclei occurs remain elusive. In order to elucidate their origins, it is critical for us  to better understand the energy spectrum and mass composition of cosmic rays. By doing  so, we can come to more fully understand the astrophysical conditions needed to accelerate  them and the interactions by which they are affected by during their propagation to Earth.  Human-made accelerators and low-energy cosmic ray experiments provide insight into  proposed acceleration and propagation models. Nevertheless, the most energetic ultra-high  energy cosmic rays have a flux of around 1 particle per km 2 per century at an energy of  around 10 20 eV. This energy is roughly a factor of one hundred more energetic than the  center-of-mass energies attainable at the Large Hadron Collider (and over a factor of a  thousand more energetic than the energies at which the charge and nuclear mass of a  cosmic ray may be directly measured). While models from the Large Hadron Collider may  be extrapolated to the highest energies, it is critical that large-scale detectors be used to  measure the macroscopic properties of cosmic rays.    The Pierre Auger Observatory (Auger), located in the Argentine Province of Mendoza, is  the largest ultra-high energy cosmic ray detector, extending over 3000 km^2 . As an ultra-high  energy cosmic ray traverses Earth’s atmosphere, it will interact with the atmospheric nuclei  to generate electromagnetic and hadronic cascades, which will continue to develop until the  remaining energy of a constituent particle is too small for further particle generation. Thus,  the Auger observatory uses the atmosphere as a calorimeter to measure the development of  an air shower cascade. The fluorescence detector measures the fluorescence light induced  by the interacting cascades (the longitudinal profile), and the surface detector samples  the footprint of the shower at the ground level (the lateral distribution). The depth at  which the cascade is fully developed may be determined from the longitudinal profile,  which is used to infer the primary mass. Due to its sensitivity to ambient light, the duty  cycle, however, of the fluorescence detector is limited to around 15 %, whereas the surface  detector is active around 100 % of the time. Thus, in order to measure enough events to test  astrophysical scenarios at the highest energy, the reconstruction of the surface detector must  be augmented to be able to infer the primary mass, which is not directly accessible from  the lateral distribution. This is possible with the air shower universality approach. Within  this method, the unique timing and signal distributions of different particle components  in the cosmic-ray-induced cascade are exploited to describe air showers as a function of  their primary energy, mass, and geometry. The universality approach is easily extendable  to other detector types and is of essential importance for the upgrade of Auger and future  analyses.    The major focus of this work is to determine the mass composition derived with the  universality approach. The results found are compatible with those found by the fluores-  cence detector and provide insight into the mass composition above 10^19.5 eV. At the highest  energies, the mass composition determined using the universality approach trends towards  a lighter composition, which is a promising signal for point-source anisotropy. To achieve  these results, a new reconstruction procedure was developed which exhibits minimal depen-  dence on the arrival direction, has an efficiency across all energies of more than 90 %, and  fully includes correlations between the reconstructed physics observables. Reconstructed  air shower simulations using contemporary hadronic interaction models were individually  studied and compared. Similarities and differences between reconstructed simulations and  data are highlighted throughout this work. The methods developed in this work are of  great interest for the data analysis of the forthcoming upgrade to Auger (AugerPrime).