Sensitivities of ultra-high energy neutrino experiments to dark matter direct annihilation and decay to neutrinos

The evidence of dark matter is compelling from Galactic to cosmological scales, but its nature remains elusive. Recent Fermi-LAT observations weaken the promising weakly interacting massive particle scenario, and alternative scenarios involving superheavy dark matter (SHDM), possibly coupled to neutrinos, are being considered. Ultra-high energy neutrino detectors can uniquely probe the properties of SHDM annihilating or decaying into neutrinos. In the mass range $m_\chi = 10^7-10^{15}\,{\rm GeV}$, we evaluate the sensitivities of the future observatories POEMMA and GRAND, compared with the ones of IceCube, Auger and ANITA, to dark matter thermally averaged annihilation cross section and dark matter lifetime. We show that a ground-based radio detector such as GRAND can achieve high sensitivities due to its high duty cycle in radio quiet areas. Space-based Cherenkov detectors such as POEMMA have the advantage of full-sky coverage and rapid slewing, enabling an optimized SHDM observation strategy focusing on the Galactic Center. Moreover, POEMMA’s fluorescence observation mode will achieve state-of-the-art sensitivity to SHDM properties.