Cosmological implications of photon-flux upper limits at ultrahigh energies in scenarios of Planckian-interacting massive particles for dark matter

Using the data of the Pierre Auger Observatory, we report on a search for signatures that would be suggestive of super-heavy particles decaying in the Galactic halo. From the lack of signal, we present upper limits for different energy thresholds above $\ensuremath{\gtrsim}{10}^{8}\text{ }\text{ }\mathrm{GeV}$ on the secondary by-product fluxes expected from the decay of the particles. Assuming that the energy density of these super-heavy particles matches that of dark matter observed today, we translate the upper bounds on the particle fluxes into tight constraints on the couplings governing the decay process as a function of the particle mass. Instantons, which are nonperturbative solutions to Yang-Mills equations, can give rise to decay channels otherwise forbidden and transform stable particles into metastable ones. Assuming such instanton-induced decay processes, we derive a bound on the reduced coupling constant of gauge interactions in the dark sector: ${\ensuremath{\alpha}}_{X}\ensuremath{\lesssim}0.09$, for ${10}^{9}\ensuremath{\lesssim}{M}_{X}/\mathrm{GeV}<{10}^{19}$. Conversely, we obtain that, for instance, a reduced coupling constant ${\ensuremath{\alpha}}_{X}=0.09$ excludes masses ${M}_{X}\ensuremath{\gtrsim}3\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }\mathrm{GeV}$. In the context of dark matter production from gravitational interactions alone during the reheating epoch, we derive constraints on the parameter space that involves, in addition to ${M}_{X}$ and ${\ensuremath{\alpha}}_{X}$, the Hubble rate at the end of inflation, the reheating efficiency, and the nonminimal coupling of the Higgs with curvature.

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