Updated and Projected Cosmic Microwave Background Bounds on WIMP Annihilation

TL;DR

This paper updates constraints on annihilating dark matter from CMB data and forecasts future bounds, showing limited improvements from recent data but significant potential gains with upcoming satellite and ground-based experiments.

Contribution

It provides the latest CMB bounds on dark matter annihilation and forecasts the impact of future experiments, including B-mode polarization, on these constraints.

Findings

Recent ground-based data yield minor improvements (~10%) over Planck bounds.

A LiteBIRD-like satellite combined with ground observations could improve bounds by ~60%.

Future experiments including B-mode polarization can further tighten constraints.

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.