Sarazin, FredAnchordoqui, LuisBeatty, JamesBergman, DouglasCovault, CorbinFarrar, GlennysKrizmanic, JohnNitz, DavidOlinto, AngelaUnger, MichaelTinyakov, PeterWiencke, Lawrence2019-03-292019-03-292019-03-10Fred Sarazin, Luis Anchordoqui, et.al, What is the nature and origin of the highest-energy particles in the universe, Astrophysics , High Energy Astrophysical Phenomena, 2019, https://arxiv.org/abs/1903.04063?context=astro-phhttp://hdl.handle.net/11603/13278This white paper was submitted to the US Astronomy and Astrophysics Decadal Survey (Astro2020) and defines the science questions to be answered in the next decade in the field of Ultra-High Energy Cosmic-Rays. Following a review of the recent experimental and theoretical advances in the field, the paper outlines strategies and requirements desirable for the design of future experiments.18 pagesen-USThis item is likely protected under Title 17 of the U.S. Copyright Law. Unless on a Creative Commons license, for uses protected by Copyright Law, contact the copyright holder or the author.Ultra-High Energy Cosmic-Raysenergetic particlesThe UHECR paradigm shiftText