Abstract
This PhD thesis investigates ultra-high energy cosmic rays: particles from space containing the highest energy levels ever measured. These particles can tell us a lot about the universe and about particle physics. Unfortunately, the sources of these particles and the way in which they obtain their energy are still unknown.
Cosmic rays comprise atomic nuclei that collide with the atmosphere with enormous energy. This collision creates new particles that again create new particles in turn, resulting in a shower of secondary particles. The Pierre Auger Observatory captures these showers and uses telescopes and water tanks to detect them.
This PhD thesis develops a new method that uses a time-dependent signal measured by the water detectors. This method can be used to determine the point in the atmosphere where the number of particles is the greatest (Xmax), which is a measure of the mass of the particle. The method reveals that at the very highest energies, the average mass of the cosmic particles increases with energy.
The accurate Xmax measurement using the water detectors is a good step in the direction towards identifying the type of cosmic particle. This development will contribute towards localising and understanding the astrophysical sources of ultra-high energy cosmic rays.
