Ultra-High-Energy Cosmic Ray Physics in Earth-Based Experiments: The Pierre Auger Observatory
Faculty: G. Anastasi*, R. Caruso, A. Del Popolo
Primary cosmic rays are charged particles (nuclei ranging from protons to iron) that constantly reach Earth from the cosmos at energies between 109 and 1020 eV. When a highly energetic cosmic ray (E > 1019 eV) collides with nuclei in the upper layers of the Earth's atmosphere, it produces secondary particles which, through successive nuclear and electromagnetic interactions and collisions, create an avalanche or "cascade" of secondary cosmic rays called an "Extended Air Shower" (EAS), showering the Earth's surface with billions of particles over areas of thousands of m2. Inferring the energy, direction of arrival, and chemical composition of the primary cosmic ray that originates the EAS allows us to understand the mechanisms of production, acceleration, and propagation of primaries in the cosmos and provides valuable information on astrophysical sources, on the mechanisms involving fundamental interactions (at energies that cannot be reached at accelerators in terrestrial laboratories), and on the origin and nature of matter in the Universe. The search for ultra-high energy cosmic rays (UHECRs) is done through the indirect measurement of EAS with gigantic detector arrays on the Earth's surface.
Since 2000, DFA faculty members working on this line of research have been members of an international scientific collaboration (approximately 400 scientists from 17 countries) operating within the Pierre Auger Observatory. Currently, the Pierre Auger Observatory is the world's largest multi-hybrid observatory for the search for UHECRs, with 1,660 ground-based detectors (water Cherenkov detectors) spread over an area of approximately 3,000 km2 on a vast plateau (1,450 m above sea level) in the Argentine Pampas, close to the Andes, surrounded by 27 fluorescence telescopes.
Over the years, DFA faculty have actively contributed to the Observatory's foundation, detector design and installation, maintenance and data-taking schedules, and the implementation of an AugerPrime upgrade of the Observatory between 2015 and 2025. They have also contributed to simulations and data analysis, physics studies and results, and have achieved milestone scientific successes published in Science 318 (2007) 938; Science 357 (2017) 1266; and PRL 135 (2025) 241002, highlighted as a 2025 "Highlight" in Physics by APS.
(*) (external contribution to the development of machine learning approaches to data processing as part of the CN-HPC project)