Analysis of the first data of the AugerPrime detector upgrade

Despite their discovery almost one hundred years ago, the physics of cosmic rays is still a mystery. Huge efforts have been employed over the last decades in order to understand their origin and propagation through the cosmos until they reach the Earth. Several astrophysical scenarios were developed aiming to explain how cosmic rays can be accelerated up to such high energies, which are orders of magnitude above the ones achieved by man-made accelerators. The different features observed in the measured energy spectrum such as the flux suppression at the highest energies or the region thought to be a transition between galactic and extra-galactic origin remain unclear. The need of models for the magnetic fields at the source environment and acting on the cosmic rays during propagation adds a further complication. The interpretation of measurements from extensive air showers induced by cosmic rays often requires the usage of simulations based on LHC-tuned models for the hadronic interactions. The observation of a deficit in the number of muons predicted by these models is also an unclear issue. Knowing the mass composition of cosmic rays at the top of the atmosphere is of key importance in order to answer these questions. The Pierre Auger Observatory is currently the largest experiment dedicated to the measurement of ultra-high-energy cosmic rays. Covering more than 3000 km${}^2$ with ground-based detectors, it employs a hybrid detection technique of air showers. The Fluorescence Detector uses telescopes for measuring the longitudinal profile of showers as they develop in the atmosphere. The Surface Detector measures the footprint of showers at the ground by sampling lateral distributions of particles with water-Cherenkov detectors (WCDs). This allows us to estimate the energy and the mass composition of the primary particle. However, the reduced duty cycle of the Fluorescence Detector limits the statistics of mass-sensitive measurements at the highest energies. In order to achieve the aforementioned goals, the observatory is currently undergoing a detector upgrade, named AugerPrime, which aims to enhance the mass-sensitivity of the Surface Detector by placing a plastic scintillator on top of each of the water-Cherenkov detectors. This Scintillator Surface Detector (SSD) provides with a complementary measurement thus allowing for the separation of the electromagnetic and muonic shower components. By the end of 2016, twelve Surface Detector stations were upgraded as part of an engineering phase. In addition, seventy-seven SSDs were deployed during March 2019 increasing the wealth of showers measured at high energies. The analysis of the data provided by these detectors is the final goal of this work. The work presented in this dissertation includes methods developed for the reconstruction of air showers with the SSDs and the analysis of the first data recorded. 1. Studies of the calibration of the SSD by means of simulating the flux of secondary particles from low energy showers. 2. Inclusion of the SSD into the standard reconstruction methods with the Surface Detector. This implies the study of the lateral distribution of the SSD signals. 3. Development of algorithms for reconstructing the mass composition using information from both the WCD and the SSD reconstructions. 4. Analysis of first data delivered by upgraded detectors during the engineering phase. 5. First estimate of the mass composition using data from AugerPrime detectors. The tools developed in this work will serve as a precedent for future analyses carried out in the context of mass composition with AugerPrime.