Final bound-state formation effect on dark matter annihilation *

Abstract If two annihilation products of dark matter (DM) particles are non-relativistic and couple to a light force mediator, their plane wave functions modified due multiple exchanges the mediator. This gives rise final state Sommerfeld (FSS) effect. It is also possible that form bound state. Both FSS effect bound-state (FBS) need be considered in calculation DM relic abundance. The can if masses comparable those annihilating particles. We study FBS effects mass-degenerate region using specific models. models serve illustrate different partial-wave contributions calculations effects. find when strongly with Those significantly modify