Auger@TA: Towards a Better Understanding of the Full UHECR Sky

The highest-energy cosmic rays are particles from outer space that provide a unique opportunity to study the most extreme particle acceleration processes in the universe. Some may originate from astrophysical sources millions of light-years away. The world's two largest cosmic-ray observatories, located in Argentina and Utah, have collected more than 20 years of data on these rare particles. However, they report significant differences in their measurements. Determining whether these differences arise from the cosmic-ray sources themselves or from differences in how the observatories measure the extensive air showers produced in Earth's atmosphere is essential for developing a consistent picture of the highest-energy universe. This understanding will also provide a framework for cross-calibrating future cosmic-ray observatories. The Auger@TA project carries out a direct, in-situ cross calibration of the Pierre Auger Observatory (Auger) and the Telescope Array (TA) experiments using an Auger-like hexagonal array of water-Cherenkov detectors embedded within the TA footprint in Utah. During this funding period, the array will be maintained and upgraded with scintillator surface detectors identical to those deployed in the Auger Prime upgrade, enabling simultaneous measurement of the same cosmic-ray extensive air showers by both detector types. The primary scientific goal is to measure the relative energy scale of the two observatories as a function of energy using same-shower events, directly testing the reported ~20% per-decade energy-dependent discrepancy. A secondary objective will exploit the combined water-Cherenkov and scintillator response of the detectors using deep neural networks to reconstruct shower maximum and test whether any observed energy-scale tension exhibits composition dependency. The results will help clarify long-standing tensions between the two observatories and demonstrate methodologies for cross-calibrating future large-scale cosmic-ray detector arrays. 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.