Ultrahigh-energy cosmic rays

Abstract: Ultrahigh-energy cosmic rays (UHECRs) arrive at Earth from the most energetic astrophysical accelerators in the universe. They collide with atoms in the upper atmosphere with energies about ten times higher than any man-made accelerator, and produce gigantic cascades of secondary particles, called extensive air showers (EAS). Extensive air showers can be detected spreading particle detectors over a large area to record the interactions of secondary particles. The Pierre Auger Observatory has been designed to investigate the origin and nature of UHECRs using the combination of information from a surface array, measuring the lateral distributions of secondary particles at the ground, and fluorescence telescopes, observing the longitudinal profile of the electromagnetic component of EAS, providing an enhanced reconstruction capability. In this contribution, the status and prospects of understanding the physics of UHECRs will be reviewed, focusing on the progress made thanks to the measurements of the Pierre Auger Observatory. Physics results from the ultrahigh-energy cosmic ray data collected with the Pierre Auger Observatory opened new perspectives and motivated an upgrade of the Observatory, AugerPrime, whose main characteristics are also presented.