Conservative Constraints on Dark Matter from the Fermi-LAT Isotropic Diffuse Gamma-Ray Background Spectrum

TL;DR

This paper derives conservative constraints on WIMP dark matter annihilation using Fermi-LAT gamma-ray background data, ruling out many dark matter explanations for observed cosmic-ray positron excesses.

Contribution

It provides robust, conservative limits on dark matter annihilation cross-sections into various final states, incorporating local and extragalactic contributions and considering different halo models.

Findings

Constraints reach the thermal relic cross-section for low-mass WIMPs.

Dark matter annihilation into two leptons is ruled out as explanation for PAMELA positron excess.

Annihilation into four leptons remains a viable, unconstrained channel.

Abstract

We examine the constraints on final state radiation from Weakly Interacting Massive Particle (WIMP) dark matter candidates annihilating into various standard model final states, as imposed by the measurement of the isotropic diffuse gamma-ray background by the Large Area Telescope aboard the Fermi Gamma-Ray Space Telescope. The expected isotropic diffuse signal from dark matter annihilation has contributions from the local Milky Way (MW) as well as from extragalactic dark matter. The signal from the MW is very insensitive to the adopted dark matter profile of the halos, and dominates the signal from extragalactic halos, which is sensitive to the low mass cut-off of the halo mass function. We adopt a conservative model for both the low halo mass survival cut-off and the substructure boost factor of the Galactic and extragalactic components, and only consider the primary final state radiation. This provides robust constraints which reach the thermal production cross-section for low mass WIMPs annihilating into hadronic modes. We also reanalyze limits from HESS observations of the Galactic Ridge region using a conservative model for the dark matter halo profile. When combined with the HESS constraint, the isotropic diffuse spectrum rules out all interpretations of the PAMELA positron excess based on dark matter annihilation into two lepton final states. Annihilation into four leptons through new intermediate states, although constrained by the data, is not excluded.