Updated and Projected Cosmic Microwave Background Bounds on WIMP Annihilation

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.