From SIMP miracles to WIMP dead ends: navigating the freeze out of MeV-mass dark matter

We summarize here our studies Chu: 2022xuh, Chu: 2023jyb, Chu: 2024rrv on two distinct scenarios for MeV-mass thermal dark matter freeze-out. First, we determine the minimal viable mass for dark matter below tens of MeV, considering annihilation into Standard Model particles, including photons, electrons, and neutrinos. Using a full three-sector abundance calculation, we track heat transfer between sectors and provide accurate thermal annihilation cross sections, particularly for velocity-dependent cases. The results identify fine-tuned regions where neutrino final states permit otherwise excluded p-wave annihilation scenarios. Second, we examine dark matter freeze-out in strongly interacting theories, where the relic abundance can be regulated not only through standard 3 2 annihilation but also via bound-state formation X, enabling effective two-body processes XX and/or X. Together, these studies highlight complementary pathways to thermal MeV-mass dark matter.