Determining the Mass Composition of Ultra-high Energy Cosmic Rays Using Air Shower Universality

Ultra-high energy cosmic rays are accelerated via the most energetic and powerful processes
in the Universe. For over a hundred years, the study of these particles has elicited great
interest. While our knowledge and theoretical models have vastly improved over the last
century, the exact sites at which and physical mechanisms by which the acceleration of these
charged nuclei occurs remain elusive. In order to elucidate their origins, it is critical for us
to better understand the energy spectrum and mass composition of cosmic rays. By doing
so, we can come to more fully understand the astrophysical conditions needed to accelerate
them and the interactions by which they are affected by during their propagation to Earth.
Human-made accelerators and low-energy cosmic ray experiments provide insight into
proposed acceleration and propagation models. Nevertheless, the most energetic ultra-high
energy cosmic rays have a flux of around 1 particle per km 2 per century at an energy of
around 10 20 eV. This energy is roughly a factor of one hundred more energetic than the
center-of-mass energies attainable at the Large Hadron Collider (and over a factor of a
thousand more energetic than the energies at which the charge and nuclear mass of a
cosmic ray may be directly measured). ... mehrWhile models from the Large Hadron Collider may
be extrapolated to the highest energies, it is critical that large-scale detectors be used to
measure the macroscopic properties of cosmic rays.

The Pierre Auger Observatory (Auger), located in the Argentine Province of Mendoza, is
the largest ultra-high energy cosmic ray detector, extending over 3000 km^2 . As an ultra-high
energy cosmic ray traverses Earth’s atmosphere, it will interact with the atmospheric nuclei
to generate electromagnetic and hadronic cascades, which will continue to develop until the
remaining energy of a constituent particle is too small for further particle generation. Thus,
the Auger observatory uses the atmosphere as a calorimeter to measure the development of
an air shower cascade. The fluorescence detector measures the fluorescence light induced
by the interacting cascades (the longitudinal profile), and the surface detector samples
the footprint of the shower at the ground level (the lateral distribution). The depth at
which the cascade is fully developed may be determined from the longitudinal profile,
which is used to infer the primary mass. Due to its sensitivity to ambient light, the duty
cycle, however, of the fluorescence detector is limited to around 15 %, whereas the surface
detector is active around 100 % of the time. Thus, in order to measure enough events to test
astrophysical scenarios at the highest energy, the reconstruction of the surface detector must
be augmented to be able to infer the primary mass, which is not directly accessible from
the lateral distribution. This is possible with the air shower universality approach. Within
this method, the unique timing and signal distributions of different particle components
in the cosmic-ray-induced cascade are exploited to describe air showers as a function of
their primary energy, mass, and geometry. The universality approach is easily extendable
to other detector types and is of essential importance for the upgrade of Auger and future
analyses.

The major focus of this work is to determine the mass composition derived with the
universality approach. The results found are compatible with those found by the fluores-
cence detector and provide insight into the mass composition above 10^19.5 eV. At the highest
energies, the mass composition determined using the universality approach trends towards
a lighter composition, which is a promising signal for point-source anisotropy. To achieve
these results, a new reconstruction procedure was developed which exhibits minimal depen-
dence on the arrival direction, has an efficiency across all energies of more than 90 %, and
fully includes correlations between the reconstructed physics observables. Reconstructed
air shower simulations using contemporary hadronic interaction models were individually
studied and compared. Similarities and differences between reconstructed simulations and
data are highlighted throughout this work. The methods developed in this work are of
great interest for the data analysis of the forthcoming upgrade to Auger (AugerPrime).