Co-Decaying Dark Matter
  We propose a new mechanism for thermal dark matter freezeout, termed
Co-Decaying Dark Matter. Multi-component dark sectors with degenerate particles
and out-of-equilibrium decays can co-decay to obtain the observed relic
density. The dark matter density is exponentially depleted through the decay of
nearly degenerate particles, rather than from Boltzmann suppression. The relic
abundance is set by the dark matter annihilation cross-section, which is
predicted to be boosted, and the decay rate of the dark sector particles. The
mechanism is viable in a broad range of dark matter parameter space, with a
robust prediction of an enhanced indirect detection signal. Finally, we present
a simple model that realizes co-decaying dark matter.
