Protohalo Constraints to the Resonant Annihilation of Dark Matter

TL;DR

This paper examines how resonantly enhanced dark matter annihilation in early protohalos could produce observable signals, and discusses constraints from cosmic background observations that challenge this explanation for cosmic ray excesses.

Contribution

It introduces constraints on low-velocity dark matter annihilation cross sections based on early universe photon and electron-positron emissions, questioning their role in cosmic ray anomalies.

Findings

Early protohalos produce detectable gamma-ray and electron-positron signals.

Energy injection constraints limit dark matter annihilation cross sections.

Reconciling these constraints with cosmic ray excess explanations is challenging.

Abstract

It has recently been argued that the PAMELA, ATIC and PPB-BETS data showing an anomalous excess of high-energy cosmic ray positrons and electrons might be explained by dark matter annihilating in the Galactic halo with a cross section resonantly enhanced compared to its value in the primeval plasma. We find that with a very large annihilation cross section the flash of energetic photons and electron-positron pairs expected from dark-matter annihilation in the first protohalos that form at redshift z~40 is likely substantial and observable. As a consequence, bounds on the allowed energy injection into the primordial gas and the energy density of the diffuse gamma-ray background give rise to limits on the low-velocity dark matter cross section that can be difficult to reconcile with this interpretation of the PAMELA, ATIC and PPB-BETS results.