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Mass composition of ultra-high-energy cosmic rays at the Pierre Auger ObservatoryThomas Fitoussi,
Andrej Filipčič,
Jon Paul Lundquist,
Shima Ujjani Shivashankara,
Samo Stanič,
Serguei Vorobiov,
Danilo Zavrtanik,
Marko Zavrtanik, 2025, published scientific conference contribution
Abstract: For the past 20 years, the Pierre Auger Observatory has collected the largest dataset of
ultra-high-energy cosmic rays (UHECRs) ever achieved using a hybrid detector. The study
of this dataset has led to numerous unexpected discoveries that enhance our understanding
of the origins of UHECRs. One of the key points in this study is their mass composition.
In this work, we will present the most recent results regarding the mass composition of UHECRs at the Pierre Auger Observatory. In particular, we will focus on the measurement of the depth
of the maximum of air-shower profiles, denoted as Xmax . This determination has been achieved through both direct measurements from the Fluorescence Detector data and the application of machine learning for estimating Xmax on an event-by-event basis using the Surface Detector data. The latter has allowed us to extend the measurement to energies up
to 100 EeV and indicates a correlation between changes in composition and three features
of the energy spectrum (ankle, instep, steepening). Moreover, the results provide evidence
of a heavy and nearly pure primary beam for energies greater than 50 EeV that is independent
of the hadronic interaction model. The implications of these findings for astrophysics and
for modelling hadronic interactions will be discussed.
Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, extensive air showers, UHECR mass composition
Published in RUNG: 28.03.2025; Views: 415; Downloads: 9
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