Abstract We revisit constraints on sub-GeV dark matter (DM) annihilation via s -wave, p -wave, and resonance processes using current and future CMB data from Planck, FIRAS, and upcoming experiments such as LiteBIRD, CMB-S4, PRISTINE, and PIXIE. For s -wave annihilation, we provide updated limits for both e + e - and ππ channels, with the profile likelihood method yielding stronger constraints than the marginal posterior method. In the p -wave case, we comprehensively present a model-independent inequality for the 95% upper limits from FIRAS, PRISTINE, and PIXIE, with future experiments expected to surpass current BBN limits. For resonance annihilation, we report — for the first time — the 95% upper limits on the decay branching ratio of the mediator particle, when the resonance peaks during the recombination epoch and at higher redshifts. Overall, our study highlights the complementary strengths of spectral distortions and CMB anisotropies in probing sub-GeV DM annihilation.
Wang et al. (Fri,) studied this question.
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