CMB bounds on dark matter annihilation: Nucleon energy-losses after recombination

TL;DR

This paper investigates how protons and anti-protons from dark matter annihilation lose energy in the early universe, refining bounds on dark matter properties from cosmic microwave background observations.

Contribution

It provides a detailed numerical analysis of proton and anti-proton energy losses after recombination, improving the accuracy of CMB bounds on dark matter annihilation.

Findings

Protons and anti-protons carry about 20% of the energy from dark matter annihilation.

Approximately half of their energy is transferred to photons and electrons/positrons.

A simple method is proposed to update CMB bounds considering these energy losses.

Abstract

We consider the propagation and energy losses of protons and anti-protons produced by dark matter annihilation at redshifts 100<z<~2000. In the case of dark matter annihilations into quarks, gluons and weak gauge bosons, protons and anti-protons carry about 20% of the energy injected into e^\pm and \gamma's, but their interactions are normally neglected when deriving cosmic microwave background bounds from altered recombination histories. Here, we follow numerically the energy-loss history of typical protons/antiprotons in the cosmological medium. We show that about half of their energy is channeled into photons and e^\pm, and we present a simple prescription to estimate the corresponding strengthening of the cosmic microwave background bounds on the dark matter annihilation cross section.