Probing superheavy dark matter through lunar radio observations of ultrahigh-energy neutrinos

Ultrahigh-energy neutrinos (UHEνs) can be used as a valuable probe of superheavy dark matter above ~ 10⁹ GeV, the latter being difficult to probe with collider and direct detection experiments due to the feebly interacting nature. Searching for radio emissions originating from the interaction of UHEνs with the lunar regolith enables us to explore energies beyond 10¹² GeV, which astrophysical accelerators cannot achieve. Taking into account the interaction of UHEνs with the cosmic neutrino background and resulting standard neutrino cascades to calculate the neutrino flux on Earth, we investigate sensitivities of such lunar radio observations to very heavy dark matter. We show that the proposed ultra-long wavelength lunar radio telescope, as well as the existing low-frequency array, can provide the most stringent constraints on decaying or annihilating superheavy dark matter with masses at 10¹² GeV. The limits are complementary to or even stronger than those from other UHEν detectors, such as the IceCube-Gen2 radio array and GRAND.