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

Abstract

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).