Measurement of the muon atmospheric production depth with the water cherenkov detectors of the Pierre Auger Observatory

Ultra-high-energy cosmic rays (UHECR) are particles of uncertain origin and composition, with energies above 1 EeV (1018 eV or 0.16 J). The measured flux of UHECR is a steeply decreasing function of energy. Above 1 J, we expect to collect one event per year per km2 per steradian. This low flux makes it impossible to detect them above the atmosphere. This kind of extremely energetic primary particle thus gives rise to huge shower containing billions of daughter particles. We have no first-hand access to the identity of the primary particle and are therefore forced to build huge arrays of particle detectors at the ground if we want to study the nature of this non-thermal sort of radiation that continuously bombards the Earth’s atmosphere. The fact that we can only record and study those secondary particles, customarily known as Extensive Air Showers (EAS), add extra difficulties in our quest to understand what are UHECR, where they are produced and what mechanisms are at work to deliver such extraordinary energies, which are far from being matched by any man-made particle accelerators. The field of UHECR is therefore not short in supply of unanswered questions. Not surprisingly it was and continues to be a very active field, where large international collaborations assemble to understand the physics behind this extreme manifestation of the non-thermal Universe. The largest and most sensitive apparatus built to date to record and study EAS is the Pierre Auger Observatory. Covering 3000km2 it was devised to reveal the nature of charged cosmic rays thanks to the simultaneous use of two detection techniques: the detection of fluorescence light and the sampling of the particles that reach the ground. It is thus a hybrid detector with improved capabilities since its calibration is data-driven and for this purpose it does not rely on cumbersome simulations affected by large uncertainties. The Pierre Auger Observatory has produced the largest and finest amount of data ever collected for UHECR. A broad physics program is being carried out covering all relevant topics of the field. Among them, one of the most interesting is the problem related to the estimation of the mass composition of cosmic rays in this energy range. Currently the best measurements of mass are those obtained by studying the longitudinal development of the electromagnetic part of the EAS with the Fluorescence Detector. However, the collected statistics is small, specially at energies above several tens of EeV. Although less precise, the volume of data gathered with the Surface Detector is nearly a factor ten larger than the fluorescence data. So new ways to study composition with data collected at the ground are under investigation. The subject of this thesis follows one of those new lines of research. Using preferentially the time information associated with the muons that reach the ground, we try to build observables related to the composition of the primaries that initiated the EAS. A simple phenomenological model relates the arrival times with the depths in the atmosphere where muons are produced. The experimental confirmation that the distributions of muon production depths (MPD) correlate with the mass of the primary particle was done in [1]. This opened the way to a variety of studies of which this thesis is a continuation of the original work with the aim of enlarging and improving its range of applicability. This document is organized as follows: chapter 1 contains introductory text to the most important milestones reached in cosmic ray physics. In chapter 2 the Pierre Auger Observatory and the main features of this hybrid detector are described. Since this thesis is based on the analysis of the data registered by the Surface Detector, we discuss in depth in chapter 3 how the properties of the primary cosmic rays are reconstructed using the information provided by the Water-Cherenkov Detectors (WCD) [2, 3]. In chapter 4 we revisit the phenomenological model which is at the root of the analysis and discuss a new way to improve some aspects of the model [4]. In chapter 5 we carried out a thorough revision of the original analysis with the aim to understand the different contributions to the total bias and resolution when building MPDs on an event-by-event basis [5, 6]. Chapter 6 is focused on an alternative way to build MPDs: we consider average MPDs for ensembles of air-showers with the aim of enlarging the range of applicability of this kind of analyses. Finally, in chapter 7 we analyze how different improvements in the WCD electronics and its internal configuration affect the resolution of the MPD [7]. We conclude summarizing the main results and discussing potential ways to improve MPD-based mass composition studies.Tesis Univ. Granada. Departamento de Física Teórica y del Cosmo