The Highest Energy Astroparticle Physics

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The origin of ultrahigh energy cosmic rays (UHECRs) is one of the most puzzling questions in high energy particle astrophysics. Its solution may radically change our knowledge of the most extreme astrophysical objects in the Universe, of the magnetic fields which permeate it, or of particle physics. Several open questions remain - What is the origin of the observed features in the energy spectrum? What is the composition of the UHECRs? What are the sources of the UHECRs? In addition, UHECRs explore particle interactions at energies beyond the reach of current accelerators. Are particle interactions above 100 TeV center-of-mass energy what we expect?This award provides funds for the continued participation of the U. Chicago group with the Pierre Auger Observatory (PAO) and related particle astrophysics. With its 3000 km-squared area, it is the largest cosmic ray detector ever built. Particle showers produced by UHECRs are detected at the ground by a sparse array of surface detectors (SD), and in the atmosphere by fluorescence detector (FD) telescopes sensitive to the light from nitrogen fluorescence excited by the shower particles. Since its completion in 2008, the PAO has unequivocally established a steep flux suppression at the end of the energy spectrum, found hints of anisotropies in the UHECRs arrival directions and indications of a heavier UHECR composition at the highest energies. This group will address these questions by improving Auger measurements (spectrum, anisotropy, composition, proton-air cross section) with better statistics and data analysis methods, and by providing phenomenological interpretations of the data. Immersive research experiences, innovative detector laboratories, and partnerships with planetariums will bring UHECR science to formal and informal audiences. Research experiences developed in collaboration with programs focused on underrepresented groups, e.g. Fisk-Vanderbilt Bridge, will enhance diversity at the graduate and undergraduate levels. UHECR science will reach broad and diverse audiences through partnerships with the Auger Visitors Center, the Malargue Planetarium, the Adler Planetarium and videos. The Adler collaboration will include the high-impact Astronomy Conversations program, where scientists interact directly with museum visitors, exploring Spanish language programming, and developing a full dome UHECR show.