Cosmic-ray and gamma-ray constraints on very-heavy dark matter decay

Decaying very-heavy dark matter (VHDM) particles at energies ? 10⁹ GeV can contribute to the ultrahigh-energy cosmic ray spectrum. However, the composition measurements by Pierre Auger Observatory indicate the presence of heavy-nuclei dominance at the highest energies. We constrain the flux of p + p¯ from Galactic and extragalactic VHDM using the latest spectrum and composition data and incorporating an astrophysical component. This provides improved limits on the VHDM decay timescale at ? 10¹² GeV. We also calculate the flux of UHE photons from VHDM decay and find that the constraints obtained using integral ?-ray flux upper limits from Auger are more stringent by a factor of ~ 10. This improves our limits to VHDM lifetime by a factor of two compared to earlier studies.