Inelastic freeze-in

Dark matter (DM) could be a nonthermal relic that freezes in from extremely weak, sub-Hubble annihilation and decay of Standard Model (SM) particles. The case of Dirac DM freezing in via a dark photon mediator is a well-studied benchmark for DM direct detection experiments. Here, we extend prior work to take into account the possibility that DM is pseudo-Dirac with a small mass splitting. If the mass splitting is greater than twice the electron mass but less than the dark photon mass, there will be distinct cosmological signatures. The excited state ₂ is initially produced in equal abundance to the ground state ₁. Due to the small couplings needed to explain the observed relic abundance, the excited state population can decay at late cosmological times, primarily via the three-body process ₂₁e^+e^-. This injects energetic electrons into the ambient environment, providing observable signatures involving big bang nucleosynthesis, cosmic microwave background spectral distortions and anisotropies, and the Lyman- forest. Furthermore, the ground state particles that are populated from the three-body decay receive a velocity kick, with implications for DM clustering on small scales. Inelastic freeze-in thus gives a motivated decaying DM candidate, where the long lifetimes are connected to the relic abundance mechanism.