Relic abundance in a secluded dark matter scenario with a massive mediator

The relic abundance of the dark matter (DM) particle d is studied in a secluded DM scenario, in which the d number decreasing process dominantly occurs not through the pair annihilation of d into the standard model particles, but via the dd -> mm scattering process with a subsequently decaying mediator particle m. It is pointed out that the cosmologically observed relic abundance of DM can be accomplished even with a massive mediator having a mass m(m) non-negligibly heavy compared with the DM particle mass m(d). In the degenerated d - m case (m(d) - m(m)), the DM relic abundance is realized by adjusting the dd. mm scattering amplitude large enough and by choosing an appropriate mediator particle lifetime. The DM evolution in the early universe exhibits characteristic terrace behavior, or two-step number density decreasing behavior, having a fake freeze-out at the first step. Based on these observations, a novel possibility of the DM model buildings is introduced in which the mediator particle m is unified with the DM particle d in an approximate dark symmetry multiplet. A pionic DM model is proposed to illustrate this idea in a renormalizable field theory framework.