Searching the footprint of WIMPZILLAs

We constrain mass, lifetime and contribution of a very slowly decaying ultra heavy dark matter (UHDM) by simulating the cosmological evolution of its remnants. Most of interactions which participate in energy dissipation are included in the numerical solution of the Boltzmann equation. Cross-sections are calculated either analytically or by using pythia Monte Carlo program. This paper describes in detail our simulation. To show the importance of the distribution of matter in constraining WIMPZILLA [D. Chung et al., hep-ph/9810361] characteristics, we consider two extreme cases: a homogeneous universe, and a local halo with uniform distribution. In a homogeneous universe, the decay of the UHDM with a mass ∼1015 GeV and a lifetime as short as a few times of the age of the universe, cannot explain the flux of the observed ultra high energy cosmic rays even if the whole dark matter (DM) is composed of a decaying UHDM. If our simple halo model is reliable, in a uniform clump with an over-density of ∼200 extended to ∼100 kpc, the lifetime can be ∼10–100τ0, again assuming that DM is a decaying UHDM. We also compare our calculation with observed γ-rays at E∼1011 eV by EGRET and CASA-MIA limit at E∼1015 eV. They are compatible with a UHDM with relatively short lifetime.