Exploring sub-GeV dark matter via $s$-wave, $p$-wave, and resonance annihilation with CMB data

TL;DR

This paper assesses how current and future CMB observations constrain sub-GeV dark matter annihilation through s-wave, p-wave, and resonance processes, providing updated limits and highlighting the complementary roles of different measurements.

Contribution

It offers the first limits on mediator decay branching ratios during recombination and compares the effectiveness of various CMB data analysis methods for constraining dark matter.

Findings

Stronger constraints from profile likelihood than Bayesian methods for s-wave annihilation.

Future experiments will improve limits on p-wave annihilation beyond BBN constraints.

First-time limits on mediator decay branching ratios during recombination.

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 $\pi\pi$ 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. Overall, our study highlights the complementary strengths of $\mu$-distortion and CMB anisotropies in probing sub-GeV DM annihilation.