Revisiting ultrahigh-energy constraints on decaying super-heavy dark matter

We revisit constraints on decaying very heavy dark matter (VHDM) using the latest ultrahigh-energy cosmic-ray (UHECR; $E\gtrsim {10}^{18}$ eV) data and ultrahigh-energy (UHE) $\gamma$-ray flux upper limits, measured by the Pierre Auger Observatory. We present updated limits on the VHDM lifetime (${\tau }_{\chi }$) for masses up to $\sim {10}^{15}$~GeV, considering decay into quarks, leptons, and massive bosons. In particular, we consider not only the UHECR spectrum but their composition data that favors heavier nuclei. Such a combined analysis improves the limits at $\lesssim {10}^{12}$ GeV because VHDM decay does not produce UHECR nuclei. We also show that the constraints from the UHE $\gamma$-ray upper limits are $\sim 10$ more stringent than that obtained from cosmic rays, for all of the Standard Model final states we consider. The latter improves our limits to VHDM lifetime by a factor of two for dark matter mass $\gtrsim {10}^{12}$ GeV.