Abstract : Ultrahigh-energy cosmic rays (UHECRs), whose origin is still mysterious, provide a unique
probe of the most extreme environments in the universe, of the intergalactic space and of particle physics
beyond the reach of terrestrial accelerators. The Pierre Auger Observatory started operating more than a
decade ago. Outperforming preceding experiments both in size and in precision, it has boosted forward the
field of UHECRs as witnessed by a wealth of results. These include the study of the energy spectrum beyond
1 EeV with its spectral suppression around 40 EeV, of the large-scale anisotropy, of the mass composition,
as well as stringent limits on photon and neutrino fluxes.
But any harvest of new results also calls for new questions: what is the true nature of the spectral suppression:
a propagation effect (so-called Greisen, Zatsepin and Kuz’min or GZK cutoff) or cosmic accelerators running
out of steam? What is the composition of UHECRs at the highest energies? In order to answer these
questions, the Auger Collaboration is undertaking a major upgrade program of its detectors, the AugerPrime
project. The science case and motivations, the technical strategy and the scientific prospects are presented.
