Bounds on Long-Lived Relics from Diffuse Gamma Ray Observations

We place bounds on long-lived primordial relics using measurements of the diffuse gamma ray spectrum from EGRET and COMPTEL. Bounds are derived for both radiative and hadronic decays with stronger bounds applying for the latter decay mode. We present an exclusion plot in the relic density-lifetime plane that shows nontrivial dependence on the mass of the relic. The violations of scaling with mass are a consequence of the different possible scattering processes which lead to differing electromagnetic showering profiles. The tightest bounds for shorter lifetimes come from COMPTEL observations of the low energy part of the spectrum, while for longer lifetimes the highest observable energy at EGRET gives the tightest bounds. We discuss the implications of the bounds for dark matter candidates as well as relics that have a mass density substantially below the critical density. These bounds can be utilized to eliminate models that contain relics with lifetimes longer than $10^{-4}$ times the age of the universe.