A number of conundrums endure regarding the very highest energy cosmic rays measured at Earth. For one, their sources remain shrouded in mystery with attempts at back-tracing these particles to their origins confounded by deflections in galactic and extra-galactic magnetic fields. The region in energy for the emergence of the extra-galactic contribution remains unclear, and each hypothesis is accompanied by an additional set of complications to address. The suppression of the flux at the highest energies, once explainable by photo-pion production through interactions with the cosmic microwave background, has been placed under scrutiny by measurements of an increasingly heavier composition above the ankle, a region which was thought to be dominated by protons just a little over a decade ago. Relatedly, the relative roles of photo-disintegration and the maximum rigidity achievable by nearby extragalactic sources in defining features of the energy spectrum are unclear. A deficit in the number of muons predicted by simulations of extensive air showers using the LHC-tuned hadronic interaction models also remains unexplained. Knowledge of the energy dependencies of the elemental abundances of cosmic rays would be highly beneficial in answering each of these questions and numerous others.
The Pierre Auger Observatory is the largest of the two experiments currently measuring ultra-high-energy cosmic rays and aims to address the aforementioned outstanding issues in the field. In this pursuit, it is performing a large-scale detector upgrade, known as AugerPrime, in order to reconstruct the mass of cosmic rays on a per event basis by means of measuring the correlated magnitude of the muonic component of extensive air showers. The placement of a scintillator atop each water-Cherenkov detector is the highlight feature of the detector upgrade with the principle concept being the exploitation of the differing responses of the two sub-detectors to the electromagnetic and muonic shower components. The work presented in this dissertation includes (i) analyses used to inform the design of the scintillator, (ii) the upgrades to the observatory's software framework in order to accommodate the detector upgrade, (iii) the development and tuning of comprehensive scintillator surface detector simulations, (iv) the assessment of a matrix-based algorithm used to reconstruct the mass of cosmic rays from measurements of the two surface sub-detectors, and (v) an application of this algorithm in selecting protons from the background of two different astrophysical scenarios for the purpose of performing charged particle astronomy.
The tools and methods developed in this work are of pivotal importance for future analyses in the context of AugerPrime.