The ⟨Δ⟩ Method: An estimator for the mass composition of ultra-high-energy cosmic rays

This thesis is organized as follows. Chapter 1 gives an overview of cosmic rays. In chapter 2 the main features of the Pierre Auger Observatory are described in detail. Chapter 3 is dedicated exclusively to the experimental determination of the risetime. In chapter 4 we introduce the specific method used in this thesis to study mass composition: the ⟨Δ⟩ method. In chapters 5 and 6 we apply the ⟨Δ⟩ method to data collected with the Pierre Auger Observatory. In chapter 5 we use the data collected with the array of surface detectors whose separation is 1500 m while in chapter 6 we use a smaller array with a separation between detectors amounting to 750 m. The difference between both data sets comes from the different energy ranges. The 1500 m array is fully efficient for above 3 EeV while the 750 m array provides data at lower energies. The chapter 7 shows the combination of the results obtained in chapters 5 and 6. Finally, in chapter 8 we conclude this thesis using the observable ⟨Δ⟩ for an additional purpose: to assess the level of concordance between data and the predictions provided by hadronic interactions models tuned to reproduced LHC data.Tesis Univ. Granada. Programa Oficial de Doctorado en: Física y Ciencias del Espaci