Revisiting ultrahigh-energy constraints on decaying superheavy dark matter

Abstract

We revisit constraints on decaying very heavy dark matter (VHDM) using the latest ultrahigh-energy cosmic-ray (UHECR; $E\ensuremath{\gtrsim}{10}^{18}\text{ }\text{ }\mathrm{eV}$) data and ultrahigh-energy (UHE) $\ensuremath{\gamma}$-ray flux upper limits, measured by the Pierre Auger Observatory. We present updated limits on the VHDM lifetime (${\ensuremath{\tau}}_{\ensuremath{\chi}}$) for masses up to $\ensuremath{\sim}{10}^{15}\text{ }\text{ }\mathrm{GeV}$, 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 $\ensuremath{\lesssim}{10}^{12}\text{ }\text{ }\mathrm{GeV}$ because VHDM decay does not produce UHECR nuclei. We also show that the constraints from the UHE $\ensuremath{\gamma}$-ray upper limits are $\ensuremath{\sim}10$ 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 $\ensuremath{\gtrsim}{10}^{12}\text{ }\text{ }\mathrm{GeV}$.