The science of ultra-high energy cosmic rays after more than 15 years of operation of the Pierre Auger Observatory

Abstract: Ultra-high energy cosmic rays (UHECRs) have been studied with the data of the Pierre Auger Observatory for more than fifteen years. An essential feature of the Observatory is its hybrid design: UHECRs are detected through the observation of the associated extensive air showers (EASs) with different and complementary techniques. The analyses of the multi-detector data have enabled high-statistics and high-precision studies of the UHECR energy spectrum, mass composition and distribution of arrival directions. The resulting science of UHECRs is summarized in this contribution. While no discrete source of UHECRs has been identified so far, the extragalactic origin of the particles has been recently confirmed from the arrival directions above 8~EeV, and the ring is closing around nearby astrophysical sites at higher energies. Also, the established upper limits on fluxes of UHE neutrinos and photons have implications on multi-messenger studies and beyond-the-Standard-Model (BSM) physics.