Modification of the Sommerfeld effect due to coannihilator decays*

Abstract In dark matter coannihilation scenarios, the Sommerfeld effect of coannihilators is usually important in the calculations of dark matter thermal relic abundance. Due to decays of coannihilators, two annihilating coannihilators can only approach each other from a finite initial separation. While conventional derivations of Sommerfeld factors essentially assume an infinite initial separation, Sommerfeld factors for a pair of annihilating coannihilators may be different. We find that the Sommerfeld factor for a pair of coannihilators is less prominent than that obtained without considering decays. Modification of the Sommerfeld factor may result in a change of the dark matter thermal relic abundance well beyond the percent level.