Rekindling s-Wave Dark Matter Annihilation Below 10GeV with Breit-Wigner Effects

Abstract: Velocity-independent (s-wave) annihilation of thermal Dark Matter is ruled out by CMB data for masses below 10GeV, effectively ruling out the possibility of indirectly detecting it in this mass range. We demonstrate in a model-independent framework that Breit-Wigner effects from very narrow resonances can circumvent CMB constraints, thereby reviving the potential to detect s-wave DM annihilation in the present Universe. The density of resonant s-wave Dark Matter continues to evolve long after chemical decoupling, leading to a scenario we refer to as belated freeze-out, where kinetic decoupling plays a significant role in determing the relic density.