Indirect Dark Matter Signatures in the Cosmic Dark Ages I. Generalizing the Bound on s-wave Dark Matter Annihilation from Planck

TL;DR

This paper generalizes the constraints on s-wave dark matter annihilation from Planck CMB data by providing a method to calculate effective annihilation parameters for arbitrary models, enabling broader application of these bounds.

Contribution

It introduces a new approach to compute $f_ ext{eff}$ for any dark matter annihilation spectrum, extending Planck constraints to diverse models with s-wave annihilation.

Findings

New $f_ ext{eff}$ values for photons and $e^+e^-$ pairs across keV-TeV energies.

Robust constraints on dark matter models, excluding those explaining cosmic ray positron excess.

Validation of the method using principal component analysis.

Abstract

Recent measurements of the cosmic microwave background (CMB) anisotropies by Planck provide a sensitive probe of dark matter annihilation during the cosmic dark ages, and specifically constrain the annihilation parameter $f_\mathrm{eff} \langle \sigma v \rangle/m_\chi$. Using new results (Paper II) for the ionization produced by particles injected at arbitrary energies, we calculate and provide $f_\mathrm{eff}$ values for photons and $e^+e^-$ pairs injected at keV-TeV energies; the $f_\mathrm{eff}$ value for any dark matter model can be obtained straightforwardly by weighting these results by the spectrum of annihilation products. This result allows the sensitive and robust constraints on dark matter annihilation presented by the Planck Collaboration to be applied to arbitrary dark matter models with $s$-wave annihilation. We demonstrate the validity of this approach using principal component analysis. As an example, we integrate over the spectrum of annihilation products for a range of Standard Model final states to determine the CMB bounds on these models as a function of dark matter mass, and demonstrate that the new limits generically exclude models proposed to explain the observed high-energy rise in the cosmic ray positron fraction. We make our results publicly available at http://nebel.rc.fas.harvard.edu/epsilon.