Several observatories designed to detect ultrahigh-energy neutrinos are planned for the next decade. The most imminent of these is the Payload for Ultrahigh Energy Observations (PUEO), a long-duration balloon-based experiment that will provide unprecedented sensitivity to neutrinos with energies in the range of $\sim 1–1000\mathrm{EeV}$. In this work, we assess the scientific reach of PUEO. In particular, we evaluate the sensitivity of this observatory to cosmogenic neutrinos and, in turn, to the proton fraction of the ultrahigh-energy cosmic-ray spectrum. We also consider the potential of PUEO to probe scenarios in which neutrinos are produced through the decays of ultraheavy dark matter particles or are radiated from cosmic strings. We find that PUEO will be able to constrain the proton composition of ultrahigh-energy cosmic rays in scenarios that feature very strong source evolution and in which protons are accelerated to extremely high energies. Although $\gamma$-ray observations are generally more sensitive to decaying particles than neutrino observations, PUEO is expected to set the strongest neutrino-detector constraints above ${10}^{19}\mathrm{eV}$. PUEO will also provide the strongest constraints on some models of cosmic strings.

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• Cosmic ray composition & spectra
• Cosmic strings & domain walls
• Extrasolar neutrino astronomy
• Particle dark matter