Study of the Highest Energy Cosmic Rays with the Pierre Auger Observatory

Measurements of ultra-high-energy cosmic rays (UHECRs) have provided an unprecedented view of the universe and access to astro-physics at energies beyond those accessible at terrestrial accelerators. Observations performed with the Pierre Auger Observatory (PAO), the largest cosmic ray detector array in the southern hemisphere, have advanced our understanding of UHECR sources, including how the particles are accelerated in the extreme environments. This award supports researchers at the Colorado School of Mines (Mines) and Case Western Reserve University (CWRU) to continue studies of the highest energy particles via major contributions to PAO instruments. The awarded activities will leverage innovative techniques to study the PAO detector and atmospheric systematic uncertainties, making it possible to couple the subtle manifestations of UHECR sources in the sky with the observed primary particle mass and charge. The team will also continue the investigations of cosmic ray energy-scale and flux discrepancies observed between PAO and the Telescope Array Observatory in the northern hemisphere via the micro-array detecter Auger@TA. The program includes research training in experimental methods for graduate and undergraduate students, including potential broader applications for atmospheric science and commercial GPS timing. The current PAO measurements of energy spectra, anisotropy, primary composition, neutrino and photon limits, and proton-air cross-section provide unique access to the high-energy universe. The influence of the atmosphere on the PAO fluorescence detector absolute and relative energy-scales remains a crucial element of science of the observatory, and the test-beam laser facilities managed by the team are central to the existing array and the on-going upgrade, AugerPrime. In parallel, the Auger@TA micro-array is currently under commissioning, with a planned two-year science run that will provide simultaneous and independent extensive air shower event data to cross-calibrate PAO and TA detectors. Doing so will permit a first direct comparison of the cosmic-ray energy-scales of the experiments to determine the origin of the observed energy-scale and flux discrepancies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.