Event Reconstruction with the AugerPrime Upgrade

Abstract

The origin and mass composition of ultra-high energy cosmic rays (UHECR) remains as one of the biggest unsolved mysteries in astrophysics. The Pierre Auger Observatory (Auger), adopts a hybrid detection technique which utilises fluorescence telescopes and surface detector stations which measure extensive air showers (EAS) produced by cosmic rays interactions with the atmosphere, allowing Auger to reconstruct the energy, arrival direction and mass composition of UHECRs. Auger has deployed scintillator surface detectors (SSD) on top of their existing surface detector stations, above the existing water-Cherenkov detectors (WCD). The calibration of the new SSD detector has been carefully defined, based on the existing WCD detector. The SSD signal is recorded as the number of minimum ionising particles (MIP), and the calibration has been defined based on simulations until the proposed measurement is performed in the field. A study into the different types of detector shielding has been performed. Through a custom mock simulation of the Telescope Array surface detector (TASD) incorporated within the Auger SSD simulation framework, the TASD has been simulated alongside the SSD. The results describe in detail how different types of detector shielding, such as stainless steel or aluminium, can significantly impact the quantity of signal measured, as well as change the proportions of signal measured from different electromagnetic particles. Furthermore, using this simulation, an analysis on changing the mass composition assumptions of cosmic rays during reconstruction procedures is performed, testing whether a significant alteration of cosmic ray spectra is seen. The SSD signal uncertainty model has been recalculated, using real data from SSD station multiplets, improving upon the original model which was defined with Monte Carlo simulations. The signal uncertainty analysis required an SSD Lateral Distribution Function (LDF) to be defined, and so, an SSD LDF parameterisation for the 750 m ground array, and 1500 m ground array has been performed, using the maximum likelihood. The residuals from the SSD LDF parameterised form compared with real data show that the fits perform well for distances from the shower core up to 2000 m, for primary particle energies greater than 1018.5 eV, at all zenith angles. The SSD and WCD will simultaneously collect data, allowing researchers to compare signals from the two detectors in a variety of ways. One particular use case is to compare to two signals to understand cosmic ray mass composition. An analysis is performed on the signal ratio (SSD/WCD), where the signal ratio for simulated proton and iron air showers is compared to the signal ratio from real data, outlining the issues that need to be tackled before accurate mass composition analysis can be achieved through study of signal ratios. Similarly, a matrix formalism analysis is performed on different sets of simulated data, to understand and outline some of the best case scenarios that could be achievable, when real data becomes abundant. The results of this thesis will help others understand the difficulties and challenges that need to be overcome when performing detailed SSD analysis.