Constraints on Dark Matter annihilations from reionization and heating of the intergalactic gas

TL;DR

This paper constrains dark matter annihilation models by analyzing their effects on cosmic reionization and intergalactic gas heating, ruling out certain particle masses and cross sections based on observational data.

Contribution

It provides new bounds on dark matter annihilation parameters using reionization and gas temperature data, impacting models explaining cosmic ray excesses.

Findings

Large annihilation cross sections into leptons are ruled out for TeV-scale dark matter.

Low mass (<10 GeV) dark matter particles are disfavored due to excessive heating.

Optical depth constraints are robust and largely independent of structure formation history.

Abstract

Dark Matter annihilations after recombination and during the epoch of structure formation deposit energy in the primordial intergalactic medium, producing reionization and heating. We investigate the constraints that are imposed by the observed optical depth of the Universe and the measured temperature of the intergalactic gas. We find that the bounds are significant, and have the power to rule out large portions of the `DM mass/cross section' parameter space. The optical depth bound is generally stronger and does not depend significantly on the history of structure formation. The temperature bound can be competitive in some cases for small masses or the hadronic annihilation channels (and is affected somewhat by the details of structure formation). We find in particular that DM particles with a large annihilation cross section into leptons and a few TeV mass, such as those needed to explain the PAMELA and FERMI+HESS cosmic ray excesses in terms of Dark Matter, are ruled out as they produce too many free electrons. We also find that low mass particles (<~ 10 GeV) tend to heat too much the gas and are therefore disfavored.