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The Probe Of Extreme Multi-Messenger Astrophysics (POEMMA) is a probe Class B mission designed to observe ultra-high 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 and cosmic neutrinos above 20 PeV over the entire sky.
POEMMA is comprised of two identical observatories flying in formation to detect EASs in mono and stereo modes. Each observatory is composed of a 4-meter photometer designed with Schmidt wide (45◦) field-of-view (FoV) optics (see Figure) and a spacecraft bus. The photometer focal surface has a hybrid design for two complementary capabilities: a fast (1 μs) ultraviolet camera to observe fluorescence signals and an ultra-fast (10 ns) optical camera to detect Cherenkov signals. EASs from UHECRs and cosmic neutrinos are observed from an orbit altitude of 525 km and a range of attitudes in the dark sky. POEMMA will point from close to the nadir, to optimize stereo fluorescence observations, to about 47◦ from the nadir to monitor the Earth's limb (located at 67.5◦) for Cherenkov emission from EASs from cosmic neutrinos (below the limb) and UHECRs (∼2◦ above the limb).

POEMMA will provide a significant increase in the statistics of observed UHECRs at the highest energies over the entire sky and will have a target of opportunity (ToO) follow-up program for cosmic neutrinos from extremely energetic transient astrophysical events.

POEMMA will:

• Discover the nature and origin of the highest-energy particles in the universe. Where do UHECRs come from? What are these extreme cosmic accelerators and how do they accelerate to such high energies? What is the UHECR composition at the highest energies? What are the magnetic fields in the galactic and extragalactic media? How do UHECRs interact in the source, in galactic and extragalactic space, and in the atmosphere of the Earth?
• Discover neutrino emission above 20 PeV for extreme astrophysical transients. What is the high-energy neutrino emission of gravitational wave events? Do binaries with black holes, white dwarfs, and neutron stars produce high-energy neutrinos when they coalesce? Neutrino observations will elucidate the underlying dynamics of blazar flares, gravitational wave events, gamma-ray bursts, newborn pulsars, tidal disruption events, and other transient events as seen by neutrinos?
• Probe particle interactions at extreme energies. POEMMA can test models with physics beyond the Standard Model (BSM) through cosmic neutrino observations from hundreds of PeVs to tens of ZeVs;
• Observe Transient Luminous Events and study the dynamics of the Earth's atmosphere, including extreme storms;
• Observe Meteors, thereby contributing to the understanding of the dynamics ofmeteors in the Solar System;
• Search for Exotic particles such as nuclearites.

POEMMA will provide new Multi-Messenger Windows onto the most energetic environments and events in the universe, enabling the study of new astrophysics and particle physics at these otherwise inaccessible energies.