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Updated and Projected Cosmic Microwave Background Bounds on WIMP Annihilation

Charlotte Myers, Dominic Agius, Daniele Gaggero, Angelo Ricciardone

TL;DR

This paper reevaluates and tightens Cosmic Microwave Background bounds on WIMP annihilation by integrating Planck with ACT/SPT measurements and DESI BAO, focusing on the model-independent parameter $p_{ m ann} \equiv f_{ m eff}\langle\sigma v\rangle/m_\chi$. It demonstrates that the dominant constraining power arises from large-scale $E$-mode polarization, with low-$\ell$ data effectively setting the limits and high-$\ell$ data offering only modest gains once those large scales are constrained. Forecasts for future surveys show that a LiteBIRD-like satellite, when combined with high-resolution ground-based observations (e.g., CMB-S4 or Simons Observatory), nearly saturates the cosmic-variance limit, achieving bounds down to $p_{ m ann} \lesssim 1.3\times10^{-28}$ cm$^{3}$ s$^{-1}$ GeV$^{-1}$ and approaching the CVL floor. Mapping these bounds to the $(m_\chi, \langle \sigma v\rangle)$ plane indicates that for hadronic channels the improvements remain below current astrophysical limits, while the $\mu^+\mu^-$ channel could probe up to $\sim10$ TeV, though the thermal WIMP cross-section remains out of reach above $\sim30$ GeV. Overall, the study reinforces that future progress hinges on advancing large-scale polarization control and systematics, with BB data offering limited independent leverage in two-point analyses.

Abstract

We derive updated Cosmic Microwave Background (CMB) constraints on annihilating dark matter, and present forecasts for upcoming CMB surveys. We show that the addition of recent temperature, polarization, and lensing data from ground-based experiments yields only minor improvements ($\approx 10\%$) compared to Planck bounds, confirming that the sensitivity remains dominated by the large-scale E-mode polarization. Forecasts, using a LiteBIRD-like setup, indicate that pairing a low-noise, wide-sky satellite at $\ell < 200$ with high-resolution ground observations nearly saturates the cosmic-variance limit, improving bounds by $\approx 60\%$, where our derived 95th percentile limit is $p_{\rm ann} < 1.27{\times}10^{-28}\,\mathrm{cm^{3}\,s^{-1}\,GeV^{-1}}$. We also consider the inclusion of B-mode polarization for a realistic future experiment.

Updated and Projected Cosmic Microwave Background Bounds on WIMP Annihilation

TL;DR

This paper reevaluates and tightens Cosmic Microwave Background bounds on WIMP annihilation by integrating Planck with ACT/SPT measurements and DESI BAO, focusing on the model-independent parameter . It demonstrates that the dominant constraining power arises from large-scale -mode polarization, with low- data effectively setting the limits and high- data offering only modest gains once those large scales are constrained. Forecasts for future surveys show that a LiteBIRD-like satellite, when combined with high-resolution ground-based observations (e.g., CMB-S4 or Simons Observatory), nearly saturates the cosmic-variance limit, achieving bounds down to cm s GeV and approaching the CVL floor. Mapping these bounds to the plane indicates that for hadronic channels the improvements remain below current astrophysical limits, while the channel could probe up to TeV, though the thermal WIMP cross-section remains out of reach above GeV. Overall, the study reinforces that future progress hinges on advancing large-scale polarization control and systematics, with BB data offering limited independent leverage in two-point analyses.

Abstract

We derive updated Cosmic Microwave Background (CMB) constraints on annihilating dark matter, and present forecasts for upcoming CMB surveys. We show that the addition of recent temperature, polarization, and lensing data from ground-based experiments yields only minor improvements () compared to Planck bounds, confirming that the sensitivity remains dominated by the large-scale E-mode polarization. Forecasts, using a LiteBIRD-like setup, indicate that pairing a low-noise, wide-sky satellite at with high-resolution ground observations nearly saturates the cosmic-variance limit, improving bounds by , where our derived 95th percentile limit is . We also consider the inclusion of B-mode polarization for a realistic future experiment.

Paper Structure

This paper contains 23 sections, 11 equations, 4 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Constraining Power
  3. Updated Bounds
  4. Datasets
  5. Methodology
  6. Results
  7. Projected Bounds
  8. Future CMB Surveys
  9. LiteBIRD.
  10. CMB-S4 (concept).
  11. Simons Observatory.
  12. Methodology
  13. Noise Modeling
  14. Fisher Forecasting
  15. Forecasting via MCMC Sampling
  16. ...and 8 more sections

Figures (4)

  • Figure 1: Top: Fractional impact of varying $p_{\mathrm{ann}}$ on the EE and TT spectra. Outer shaded bands indicate cosmic variance + instrumental noise, and inner shaded bands indicate cosmic variance alone; both are computed for the highest value of $p_{\mathrm{ann}}$ ($1.0 \times 10^{-27}\,\mathrm{cm}^3\,\mathrm{s}^{-1}\,\mathrm{GeV}^{-1}$). Middle: Same as middle panel but for the TT power spectrum. Bottom: Forecasted $2\sigma$ upper limits on $p_{\mathrm{ann}}$ for different experimental configurations and multipole ranges included in the Fisher analysis. Solid curves include TT+EE; dashed curves include EE-only. A Planck-like experiment with $\ell_\text{max} = 2500$ is taken as a baseline (marked by star).
  • Figure 2: Projected $2\sigma$ upper bounds on the DM annihilation cross section $\langle\sigma v\rangle$ as a function of DM mass $m_\chi$ for three annihilation channels: $W^+W^-$ (top), $b\bar{b}$ (middle), and $\mu^+\mu^-$ (bottom). The channel-dependent $f_{\mathrm{eff}}$ curves used to convert constraints on $p_{\mathrm{ann}}$ to $\langle\sigma v\rangle$ are taken from Ref. Madhavacheril:2013cna. Solid lines show current CMB constraints derived in this work for Planck and Planck + SPT; dashed lines show projected bounds for CMB-S4 + Planck, CMB-S4 + LiteBIRD, and a CVL experiment. Dotted lines indicate complementary limits from indirect detection, extrapolated from Cirelli:2024ssz (see also references therein). We also show the thermal cross section as a function of DM mass Steigman:2012nb.
  • Figure 3: Fisher-forecasted $2\sigma$ upper limit on $p_{\mathrm{ann}}$ as a function of the minimum multipole $\ell_{\min}$ included in the analysis. For $\ell > 200$, forecasts assume either a CMB-S4–like experiment (solid lines), or a Simons Observatory–like configuration for Simons + LiteBIRD (purple dashed); see Table \ref{['tab:forecast_specs']} for configuration details. For $\ell < 200$, each forecast uses the low-$\ell$ experiment indicated in the legend. The CVL forecast (dotted) assumes coverage of $f_{\mathrm{sky}} = 0.7$ and sensitivity over the entire multipole range up to $\ell = 5000$, without imposing any $\ell_{\min}$ cut. Horizontal dashed lines refer to bounds computed in this work (see Table \ref{['tab:dm_ann_limits']}).
  • Figure 4: Joint posteriors for $\{p_{\rm ann},\,n_s,\,\tau_{\rm reio},\,\log(10^{10}A_s),\,\Omega_bh^2,\,\Omega_ch^2,\,H_0\}$ from updated data combinations (see Table \ref{['tab:dm_ann_limits']}). Shaded regions denote 68% and 95% credible intervals; diagonal panels show the corresponding 1D marginals.