Performance and science reach of the Probe of Extreme Multimessenger Astrophysics for ultrahigh-energy particles

The Probe of Extreme Multimessenger Astrophysics (POEMMA) is a potential NASA Astrophysics Probe-class mission designed to observe ultrahigh-energy cosmic rays (UHECRs) and cosmic neutrinos from space. POEMMA will monitor colossal volumes of the Earth's atmosphere to detect extensive air showers (EASs) produced by extremely energetic cosmic messengers: UHECRs above 20 EeV over the full sky and cosmic neutrinos above 20 PeV. We focus most of this study on the impact of POEMMA for UHECR science by simulating the detector response and mission performance for EAS from UHECRs. We show that POEMMA will provide a significant increase in the statistics of observed UHECRs at the highest energies over the entire sky. POEMMA will be the first UHECR fluorescence detector deployed in space that will provide high-quality stereoscopic observations of the longitudinal development of air showers. Therefore it will be able to provide event-by-event estimates of the calorimetric energy and nuclear mass of UHECRs. The particle physics in the interactions limits the interpretation of the shower maximum on an event-by-event basis. In contrast, the calorimetric energy measurement is significantly less sensitive to the different possible final states in the early interactions. POEMMA will increase by a factor of 30 fluorescence observations, with accurate measurements of the shower maximum. We study the prospects to discover the origin and nature of UHECRs using expectations for measurements of the energy spectrum, the distribution of arrival direction, and the atmospheric column depth at which the EAS longitudinal development reaches maximum. We also explore supplementary science capabilities of POEMMA through its sensitivity to particle interactions at extreme energies and its ability to detect ultrahigh-energy neutrinos and photons produced by top-down models including cosmic strings and superheavy dark matter particle decay in the halo of the Milky Way.