Analysis of Data from the Telescope Array Observatory

The Telescope Array (TA) Observatory is the largest detector of ultra-high energy cosmic rays (UHECRs) in the Northern Hemisphere. A combination of surface detectors (SD) and florescence detectors (FD) provides the ability to observe and characterize cosmic ray events at the highest energies. This award renews support for the TA Observatory data analysis activities of the cosmic ray group at the University of Utah, including graduate student researchers, conducting studies of the ultra-high energy cosmic ray spectrum and composition and the tentative TA “hotspot” regions. The team will also continue to develop the ASPIRE outreach program that includes web lessons and direct interactions with the local community. The TA observatory measurements have provided indications of possible differences in the cosmic ray spectral shape at the highest energies when compared to the Pierre Auger Observatory, the largest UHECR detector in the southern hemisphere. In addition, a “hotspot” of UHECRs above 57 EeV in the northern hemisphere has emerged at 3.5 sigma significance. Advanced studies of the hotspot region, for which the potential source remains unknown, show initial indications of differences in the cosmic ray spectral shape for events within compared to outside, as well as an apparent declination dependence of the spectrum at the highest energies. Additional data collection underway with the TAx4 array, combined with the energy calibrations realized with the FD, will improve the sensitivity of the observatory toward resolving these open questions in the understanding of the highest energy particles in the Universe. 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.