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 characteristics, we consider two extreme cases: a homogeneous universe, and a local halo with uniform distribution. We show that in a homogeneous universe, the decay of UHDM with a mass \sim 10^15 GeV and a lifetime \sim a few times \tau_0 the age of the Universe, can not explain the flux of observed Ultra High Energy Cosmic Rays (UHECRs). This shows the importance of nearby sources, notably galactic halo. In a uniform clump with an over-density of \sim 200 extended to 100 kpc or more, the lifetime must be \sim 10 - 100 \tau_0 or the contribution in the DM must be proportionally smaller. We also compare our calculation with observed gamma-rays at E \sim 10^11 eV by EGRET and CASA-MIA limit at E \sim 10^15 eV. They are compatible with a UHDM with relatively short lifetime. 
Houri Ziaeepour