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Nahee Park
University of Wisconsin-Madison

Abstract

In 1912, Austrian physicist Victor Hess discovered, with a high-altitude balloon experiment, a flux of highly energetic particles coming from outer space - for which he won the Nobel Prize. Now known as cosmic rays, these particles have been the topic of numerous studies ever since their discovery. Because of their deflection by magnetic fields and their interactions with particles and radiation in interstellar and intergalactic space, cosmic rays arriving at Earth carry little information about their sources. Instead, observations of the neutral particles, such as gamma rays and neutrinos, produced during the interactions experienced by cosmic rays have been studied in order to search for their elusive source sites. Observations of neutrinos provide a key element in these studies, as neutrinos can probe source environments that, due to their distance or obscuration, are inaccessible to gamma-ray observatories. Recently, the IceCube experiment located at the South Pole has revealed the first neutrino view of the cosmos, opening a new window to explore the sources of high-energy cosmic rays in our Universe. I will highlight the role that these high-energy neutrino observations play in the emerging discipline of multi-messenger astrophysics, focusing on the recent IceCube neutrino alert from a flaring blazar, TXS 0506+056. I will also discuss what we expect to learn in the future with the next-generation neutrino observatory, IceCube-Gen2, and summarize how these observations will allow us to explore fundamental physics, including searches for decaying dark matter throughout the Universe.