Neutrinos with energies above 1017 eV are detectable with the Surface Detector Array of the Pierre Auger Observatory. The identification is efficiently performed for neutrinos of all flavors interacting in the atmosphere at large zenith angles, as well as for Earth-skimming τ neutrinos with nearly tangential trajectories relative to the Earth. No neutrino candidates were found in ∼ 14.7 years of data taken up to 31 August 2018. This leads to restrictive upper bounds on their flux. The 90% C.L. single-flavor limit to the diffuse flux of ultra-high-energy neutrinos with an Eν−2 spectrum in the energy range 1.0 × 1017 eV –2.5 × 1019 eV is E2 dNν/dEν < 4.4 × 10−9 GeV cm−2 s−1 sr−1, placing strong constraints on several models of neutrino production at EeV energies and on the properties of the sources of ultra-high-energy cosmic rays.

We report a measurement of the energy spectrum of cosmic rays for energies above 2.5 × 10 18 eV based on 215,030 events recorded with zenith angles below 60°. A key feature of the work is that the estimates of the energies are independent of assumptions about the unknown hadronic physics or of the primary mass composition. The measurement is the most precise made hitherto with the accumulated exposure being so large that the measurements of the flux are dominated by systematic uncertainties except at energies above 5 × 10 19 eV. The principal conclusions are (1) The flattening of the spectrum near 5 × 10 18 eV, the so-called "ankle," is confirmed. (2) The steepening of the spectrum at around 5 × 10 19 eV is confirmed. (3) A new feature has been identified in the spectrum: in the region above the ankle the spectral index γ of the particle flux (∝ E −γ) changes from 2.51 AE 0.03 ðstatÞ AE 0.05 ðsystÞ to 3.05 AE 0.05 ðstatÞ AE 0.10 ðsystÞ before changing sharply to 5.1 AE 0.3 ðstatÞ AE 0.1 ðsystÞ above 5 × 10 19 eV. (4) No evidence for any dependence of the spectrum on declination has been found other than a mild excess from the Southern Hemisphere that is consistent with the anisotropy observed above 8 × 10 18 eV.

We report a measurement of the energy spectrum of cosmic rays above 2.5 × 10 18 eV based on 215 030 events. New results are presented: at about 1.3 × 10 19 eV, the spectral index changes from Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.