Lateral Distribution Function and Energy Spectrum for the 750 m Array of the Pierre Auger Observatory

There are many known sources in the universe, e.g. supernovae explosions, which produce and accelerate highly energetic charged particles. These so-called cosmic rays propagate through space where they are affected by magnetic fields and other propagation effects. By measuring the energy spectrum of cosmic rays upon their arrival at Earth, one can get not
only a better understanding of astrophysical objects and galactic magnetic fields but also an insight into particle physics at energies far beyond the energies achieved by modern particle colliders.
Large ground-based experiments like the Pierre Auger Observatory measure the energy spectrum in the range of the highest energies where the flux of cosmic rays is low by detecting the secondary particles originating from primary cosmic rays inducing a particle shower cascade in the atmosphere. While originally designed to measure the changes in the spectrum – so-called features – for energies above 1018 eV with a detector array of 1500 m spacing, a denser extension with 750 m spacing was designed to measure the spectrum at lower energies. In this thesis, I will present an update of the event reconstruction using new triggers to lower the overall energy threshold of the 750 m array. ... mehrI suggest a new approach to the likelihood used during the shower reconstruction procedure. New selection conditions for events where all necessary parameters can be fitted freely are introduced and used to find a stable parameterisation of the lateral distribution function of the shower. Therefore, the reconstruction of events detected between 2014 and 2021 by the 750 m array is performed, including low-energetic events that could not be reconstructed satisfyingly without this parameterisation. An updated correction for atmospheric effects using the constant intensity cut is provided as well as a new energy calibration using SD-FD hybrids. We will briefly explore the detector efficiency in a data-driven way to decide down to which energy threshold low-energetic events can be used for a spectrum analysis.
Including these low-energy events leads to an updated energy spectrum that allows us to investigate the spectrum properties in the energy range of a feature known as the second knee as reported by different cosmic ray experiments. I will argue that the second knee is measured by the 750 m array for the first time by showing that the fitted features of the spectrum are stable for different fit configurations.