Exploring the ultra-high energy (>100 PeV) universe informs us of the most powerful particle accelerators that are known to exist. Using cosmic rays (CRs) as a lens, we now understand that the most energetic particles ever observed originate from outside of our Galaxy. At these energies, only "indirect detection" is possible, and observatories rely on measuring the particle cascades that develop in Earth's atmosphere. Making further progress on pinning down the exact source(s) of CRs requires a more accurate model of the physics that governs how these particle cascades develop, and is currently hindered by our understanding of hadronic interactions. Simultaneously, large efforts are underway to also enable studying the ultra-high energy universe using neutrinos as a cosmic messenger. It is expected that neutrinos and CRs, at these energies, are produced at the same sources and thus the two messengers are highly linked. However, CRs are also the main background for detecting neutrinos, and this background has not been studied in detail.
During my talk, I will describe the main challenges for observing neutrinos and CRs at ultra-high energies and how our understanding and observations are informed by (and inform) particle physics. Further, I will talk about my current and future research to overcome these challenges by using the current and next-generation technologies to unlock the ultra-high energy universe.